Jigo M, Tavdy D & Carrasco M (preprint).
Cortical magnification underlies differences across but not around the visual field.
Hung SC & Carrasco M (2022).
Microsaccades as a long-term oculomotor correlate in visual perceptual learning.
Psychonomic Bulletin & Review.
[Perceptual learning, microsaccades]
Carrasco M, Roberts M, Myers C & Shukla L (2022).
Visual field asymmetries vary between children and adults.
[Performance fields, behavior]
Himmelberg MM, Winawer J & Carrasco M (2022).
Linking individual differences in human primary visual cortex to contrast sensitivity around the visual field.
[Cortical magnification, contrast sensitivity, individual differences]
Cavanaugh M, Tadin D, Carrasco M* & Huxlin KR* (2022). * equal author contribution
Benefits of endogenous spatial attention during visual double-training in cortically-blinded fields.
Frontiers in Neuroscience.
[Spatial attention, cortical blindness, perceptual learning]
Hanning NM, Himmelberg MM & Carrasco M (2022).
Presaccadic attention enhances contrast sensitivity, but not at the upper vertical meridian.
[Presaccadic attention, performance fields, spatial attention]
Roberts M & Carrasco M (2022).
Exogenous attention generalizes perceptual learning in adults with amblyopia.
[Perceptual learning, exogenous attention, amblyopia]
Kupers E, Benson N, Carrasco M & Winawer J (2022).
Radial asymmetries around the visual field: from retina to cortex to behavior.
PLOS Computational Biology.
[Performance fields, modeling, behavior]
Fernandez A, Okun S & Carrasco M (2022).
Differential effects of endogenous and exogenous attention on sensory tuning.
Journal of Neuroscience.
[Reverse correlation, spatial attention]
Jigo M, Heeger DJ & Carrasco M (2021).
An image-computable model on how endogenous and exogenous attention differentially alter visual perception.
[Modeling, spatial attention, texture segmentation]
Lin YJ, Shukla L, Dugué L, Valero-Cabré & Carrasco M (2021).
Transcranial magnetic stimulation entrains alpha oscillatory activity in occipital cortex.
Himmelberg MM*, Kurzawski J*, Benson N, Pelli D, Carrasco M & Winawer J (2021). * equal author contribution
Cross-dataset reproducibility of human retinotopic maps.
[Performance fields, retinotopy]
Benson N, Kupers E, Barbot A, Carrasco M* & Winawer J* (2021). * equal author contribution
Cortical magnification in human visual cortex parallels task performance around the visual field.
[Performance fields, retinotopic mapping]
Li HH, Pan J & Carrasco M (2021).
Different computations underlie overt presaccadic attention and covert spatial attention.
Nature Human Behavior.
Perception and action are tightly coupled: visual responses at the saccade target are enhanced right before saccade onset. This phenomenon, presaccadic attention, is a form of overt attention—deployment of visual attention with concurrent eye movements. Presaccadic attention is well-documented, but its underlying computational process remains unknown. This is in stark contrast to covert attention—deployment of visual attention without concurrent eye movements—for which the computational processes are well characterized by a normalization model. Here, a series of psychophysical experiments reveal that presaccadic attention modulates visual performance only via response gain changes. A response gain change was observed even when attention field size increased, violating the predictions of a normalization model of attention. Our empirical results and model comparisons reveal that the perceptual modulations by overt presaccadic and covert spatial attention are mediated through different computations.
[Presaccadic attention, spatial attention, eye movements]
Denison R, Carrasco M & Heeger DJ (2021).
A dynamic normalization model of temporal attention.
Nature Human Behavior.
Vision is dynamic, handling a continuously changing stream of input, yet most models of visual attention are static. Here, we develop a dynamic normalization model of visual temporal attention and constrain it with new psychophysical human data. We manipulated temporal attention–the prioritization of visual information at specific points in time–to a sequence of two stimuli separated by a variable time interval. Voluntary temporal attention improved perceptual sensitivity only over a specific interval range. To explain these data, we modeled voluntary and involuntary attentional gain dynamics. Voluntary gain enhancement took the form of a limited resource over short time intervals, which recovered over time. Taken together, our theoretical and experimental results formalize and generalize the idea of limited attentional resources across space at a single moment to limited resources across time at a single location.
[Temporal attention, modeling]
Hung SC & Carrasco M (2021).
Feature-based attention enables robust, long-lasting transfer in human perceptual learning.
Visual perceptual learning (VPL) is typically specific to the trained location and feature. However, the degree of specificity depends upon particular training protocols. Manipulating covert spatial attention during training facilitates learning transfer to other locations. Here we investigated whether feature-based attention (FBA), which enhances the representation of particular features throughout the visual field, facilitates VPL transfer, and how long such an effect would last. To do so, we implemented a novel task in which observers discriminated a stimulus orientation relative to two reference angles presented simultaneously before each block. We found that training with FBA enabled remarkable location transfer, reminiscent of its global effect across the visual field, but preserved orientation specificity in VPL. Critically, both the perceptual improvement and location transfer persisted after 1 year. Our results reveal robust, long-lasting benefits induced by FBA in VPL, and have translational implications for improving generalization of training protocols in visual rehabilitation.
[Perceptual learning, feature-based attention]
Purokayastha S*, Roberts M* & Carrasco M (2021). * equal author contribution
Voluntary attention improves performance similarly around the visual field.
Attention Perception and Psychophysics.
Performance as a function of polar angle at isoeccentric locations across the visual field is known as a performance field (PF) and is characterized by two asymmetries: the HVA (Horizontal-Vertical Anisotropy) and VMA (Vertical Meridian Asymmetry). Exogenous (involuntary) spatial attention does not affect the shape of the PF, improving performance similarly across polar angle. Here we investigated whether endogenous (voluntary) spatial attention, a flexible mechanism, can attenuate these perceptual asymmetries. Twenty participants performed an orientation discrimination task while their endogenous attention was either directed to the target location or distributed across all possible locations. The effects of attention were assessed either using the same stimulus contrast across locations, or equating difficulty across locations using individually-titrated contrast thresholds. In both experiments, endogenous attention similarly improved performance at all locations, maintaining the canonical PF shape. Thus, despite its voluntary nature, like exogenous attention, endogenous attention cannot alleviate perceptual asymmetries at isoeccentric locations.
Barbot A*, Xue S* & Carrasco M (2021). * equal author contribution
Asymmetries in visual acuity around the visual field.
Journal of Vision.
Human vision is heterogeneous around the visual field. At a fixed eccentricity, performance is better along the horizontal than the vertical meridian and along the lower than the upper vertical meridian. These asymmetric patterns, termed performance fields, have been found in numerous visual tasks, including those mediated by contrast sensitivity and spatial resolution. However, it is unknown whether spatial resolution asymmetries are confined to the cardinal meridians or whether and how far they extend into the upper and lower hemifields. Here, we measured visual acuity at isoeccentric peripheral locations (10 deg eccentricity), every 15° of polar angle. On each trial, observers judged the orientation (± 45°) of one of four equidistant, suprathreshold grating stimuli varying in spatial frequency (SF). On each block, we measured performance as a function of stimulus SF at 4 of 24 isoeccentric locations. We estimated the 75%-correct SF threshold, SF cutoff point (i.e., chance-level), and slope of the psychometric function for each location. We found higher SF estimates (i.e., better acuity) for the horizontal than the vertical meridian and for the lower than the upper vertical meridian. These asymmetries were most pronounced at the cardinal meridians and decreased gradually as the angular distance from the vertical meridian increased. This gradual change in acuity with polar angle reflected a shift of the psychometric function without changes in slope. The same pattern was found under binocular and monocular viewing conditions. These findings advance our understanding of visual processing around the visual field and help constrain models of visual perception.
Dugué L, Merriam EP, Heeger DJ & Carrasco M (2020).
Differential impact of endogenous and exogenous activity in human visual cortex.
How do endogenous (voluntary) and exogenous (involuntary) attention modulate activity in visual cortex? Using ROI-based fMRI analysis, we measured fMRI activity for valid and invalid trials (target at cued/un-cued location, respectively), pre- or post-cueing endogenous or exogenous attention, while participants performed the same orientation discrimination task. We found stronger modulation in contralateral than ipsilateral visual regions, and higher activity in valid- than invalid-trials. For endogenous attention, modulation of stimulus-evoked activity due to a pre-cue increased along the visual hierarchy, but was constant due to a post-cue. For exogenous attention, modulation of stimulus-evoked activity due to a pre-cue was constant along the visual hierarchy, but was not modulated due to a post-cue. These findings reveal that endogenous and exogenous attention distinctly modulate activity in visuo-occipital areas during orienting and reorienting; endogenous attention facilitates both the encoding and the readout of visual information whereas exogenous attention only facilitates the encoding of information.
[fMRI; spatial attention]
Carrasco M & Hanning NM (2020).
Visual perception: Attending beyond the eyes' reach.
It has been long debated whether visual attention can shift covertly, decoupled from programming eye movements. Now we know that patients with gaze paralysis show conventional benefits of exogenous (involuntary) attention, confirming that covert attention is not driven by oculomotor programming.
Fernández A & Carrasco M (2020).
Extinguishing exogenous attention via transcranial magnetic stimulation.
Orienting covert exogenous (involuntary) attention to a target location improves performance in many visualtasks. It is unknown whether early visual cortical areas are necessary for this improvement. To establisha causal link between these areas and attentional modulations, we used transcranial magnetic stimulation(TMS) to briefly alter cortical excitability and determine whether early visual areas mediate the effect of exogenous attention on performance. Observers performed an orientation discrimination task. After a peripheralvalid, neutral, or invalid cue, two cortically magnified gratings were presented, one in the stimulated regionand the other in the symmetric region in the opposite hemifield. Observers received two successive TMSpulses around their occipital pole while the stimuli were presented. Shortly after, a response cue indicatedthe grating whose orientation observers had to discriminate. The response cue either matched—targetstimulated—or did not match—distractor stimulated—the stimulated side. Grating contrast was varied tomeasure contrast response functions (CRF) for all combinations of attention and TMS conditions. Whenthe distractor was stimulated, exogenous attention yielded response gain—performance benefits in thevalid-cue condition and costs in the invalid-cue condition compared with the neutral condition at the highcontrast levels. Crucially, when the target was stimulated, this response gain was eliminated. Therefore,TMS extinguished the effect of exogenous attention. These results establish a causal link between early visual areas and the modulatory effect of exogenous attention on performance.
[TMS; spatial attention]
Himmelberg MM, Winawer J & Carrasco M (2020).
Stimulus-dependent contrast sensitivity asymmetries around the visual field.
Journal of Vision.
Asymmetries in visual performance at isoeccentric locations are well-documented and functionally important. At a fixed eccentricity, visual performance is best along the horizontal, intermediate along the lower vertical, and poorest along the upper vertical meridian. These performance fields are pervasive across a range of visual tasks, including those mediated by contrast sensitivity. However, contrast performance fields have not been characterized with a systematic manipulation of stimulus spatial frequency, eccentricity, and size; three parameters that constrain contrast sensitivity. Further, individual differences in performance fields measurements have not been assessed. Here, we use an orientation discrimination task to characterize the pattern of contrast sensitivity across four isoeccentric locations along the cardinal meridians, and to examine whether and how this asymmetry pattern changes with systematic manipulation of stimulus spatial frequency (4 cpd to 8 cpd), eccentricity (4.5 degrees to 9 degrees), and size (3 degrees visual angle to 6 degrees visual angle). Our data demonstrate that contrast sensitivity is highest along the horizontal, intermediate along the lower vertical, and poorest along the upper vertical meridian. This pattern is consistent across stimulus parameter manipulations, even though they cause profound shifts in contrast sensitivity. Eccentricity-dependent decreases in contrast sensitivity can be compensated for by scaling stimulus size alone. Moreover, we find that individual variability in the strength of performance field asymmetries is consistent across conditions. This study is the first to systematically and jointly manipulate, and compare, contrast performance fields across spatial frequency, eccentricity, and size, and to address individual variability in performance fields.
Badde S, Myers C, Yuval-Greenberg S & Carrasco M (2020).
Oculomotor freezing reflects tactile temporal expectation and aids tactile perception.
The oculomotor system keeps the eyes steady in expectation of visual events. Here, recording microsaccades while people performed a tactile, frequency discrimination task enabled us to test whether the oculomotor system shows an analogous preparatory response for unrelated tactile events. We manipulated the temporal predictability of tactile targets using tactile cues, which preceded the target by either constant (high predictability) or variable (low predictability) time intervals. We find that microsaccades are inhibited prior to tactile targets and more so for constant than variable intervals, revealing a tight crossmodal link between tactile temporal expectation and oculomotor action. These findings portray oculomotor freezing as a marker of crossmodal temporal expectation. Moreover, microsaccades occurring around the tactile target presentation are associated with reduced task performance, suggesting that oculomotor freezing mitigates potential detrimental, concomitant effects of microsaccades and revealing a crossmodal coupling between tactile perception and oculomotor action.
[microsaccades, tactile perception, temporal expectation]
Abeles D, Amit R, Tal-Perry N, Carrasco M & Yuval-Greenberg S (2020).
Oculomotor inhibition precedes temporally expected auditory targets.
Eye movements are inhibited prior to the onset of temporally-predictable visual targets. This oculomotor inhibition effect could be considered a marker for the formation of temporal expectations and the allocation of temporal attention in the visual domain. Here we show that eye movements are also inhibited before predictable auditory targets. In two experiments, we manipulate the period between a cue and an auditory target to be either predictable or unpredictable. The findings show that although there is no perceptual gain from avoiding gaze-shifts in this procedure, saccades and blinks are inhibited prior to predictable relative to unpredictable auditory targets. These findings show that oculomotor inhibition occurs prior to auditory targets. This link between auditory expectation and oculomotor behavior reveals a multimodal perception action coupling, which has a central role in temporal expectations.
[microsaccades, tactile perception, temporal expectation]
Jigo M & Carrasco M (2020).
Differential impact of exogenous and endogenous attention on the contrast sensitivity function across eccentricity.
Journal of Vision.
Both exogenous and endogenous covert spatial attention enhance contrast sensitivity, a fundamental measure of visual function that depends substantially on the spatial frequency and eccentricity of a stimulus. Whether and how each type of attention systematically improves contrast sensitivity across spatial frequency and eccentricity are fundamental to our understanding of visual perception. Previous studies have assessed the effects of spatial attention at individual spatial frequencies and, separately, at different eccentricities, but this is the first study to do so parametrically with the same task and observers. Using an orientation discrimination task, we investigated the effect of attention on contrast sensitivity over a wide range of spatial frequencies and eccentricities. Targets were presented alone or among distractors to assess signal enhancement and distractor suppression mechanisms of spatial attention. At each eccentricity, we found that exogenous attention preferentially enhanced spatial frequencies higher than the peak frequency in the baseline condition. In contrast, endogenous attention similarly enhanced a broad range of lower and higher spatial frequencies. The presence or absence of distractors did not alter the pattern of enhancement by each type of attention. Our findings reveal how the two types of covert spatial attention differentially shape how we perceive basic visual dimensions across the visual field.
[modeling; spatial attention, contrast sensitivity, eccentricity]
Donovan I, Shen A, Tortarolo C, Barbot A & Carrasco M (2020).
Exogenous attention facilitates perceptual learning in visual acuity to untrained stimulus locations and features.
Journal of Vision.
Visual perceptual learning (VPL) refers to the improvement in performance on a visual task due to practice. A hallmark of VPL is specificity, as improvements are often confined to the trained retinal locations or stimulus features. We have previously found that exogenous (involuntary, stimulus-driven) and endogenous (voluntary, goal-driven) spatial attention can facilitate the transfer of VPL across locations in orientation discrimination tasks mediated by contrast sensitivity. Here, we investigated whether exogenous spatial attention can facilitate such transfer in acuity tasks that have been associated with higher specificity. We trained observers for 3 days (days 2–4) in a Landolt acuity task (Experiment 1) or a Vernier hyperacuity task (Experiment 2), with either exogenous precues (attention group) or neutral precues (neutral group). Importantly, during pre-tests (day 1) and post-tests (day 5), all observers were tested with neutral precues; thus, groups differed only in their attentional allocation during training. For the Landolt acuity task, we found evidence of location transfer in both the neutral and attention groups, suggesting weak location specificity of VPL. For the Vernier hyperacuity task, we found evidence of location and feature specificity in the neutral group, and learning transfer in the attention group—similar improvement at trained and untrained locations and features. Our results reveal that, when there is specificity in a perceptual acuity task, exogenous spatial attention can overcome that specificity and facilitate learning transfer to both untrained locations and features simultaneously with the same training. Thus, in addition to improving performance, exogenous attention generalizes perceptual learning across locations and features.
[spatial attention, perceptual learning]
Denison R, Parker J & Carrasco M (2020).
Modeling pupil responses to rapid sequential events.
Behavior Research Methods.
Pupil size is an easily accessible, noninvasive online indicator of various perceptual and cognitive processes. Pupil measurements have the potential to reveal continuous processing dynamics throughout an experimental trial, including anticipatory responses. However, the relatively sluggish (∼2 s) response dynamics of pupil dilation make it challenging to connect changes in pupil size to events occurring close together in time. Researchers have used models to link changes in pupil size to specific trial events, but such methods have not been systematically evaluated. Here we developed and evaluated a general linear model (GLM) pipeline that estimates pupillary responses to multiple rapid events within an experimental trial. We evaluated the modeling approach using a sample dataset in which multiple sequential stimuli were presented within 2-s trials. We found: (1) Model fits improved when the pupil impulse response function (puRF) was fit for each observer. PuRFs varied substantially across individuals but were consistent for each individual. (2) Model fits also improved when pupil responses were not assumed to occur simultaneously with their associated trial events, but could have non-zero latencies. For example, pupil responses could anticipate predictable trial events. (3) Parameter recovery confirmed the validity of the fitting procedures, and we quantified the reliability of the parameter estimates for our sample dataset. (4) A cognitive task manipulation modulated pupil response amplitude. We provide our pupil analysis pipeline as open-source software (Pupil Response Estimation Toolbox: PRET) to facilitate the estimation of pupil responses and the evaluation of the estimates in other datasets.
[modeling; temporal attention, eye movements]
Yashar A, Wu X, Chen J & Carrasco M (2019).
Crowding and binding: Not all feature-dimensions behave in the same way.
Humans often fail to identify a target because of nearby flankers. The nature and stages at which this crowding occurs are unclear, and whether crowding operates via a common mechanism across visual dimensions is unknown. Usinga dual-estimation report (N = 42), we quantitatively assessed the processing of features alone and in conjunction with another feature both within and between dimensions. Under crowding, observers misreported colors andorientations (i.e., reported a flanker value instead of the target’s value) but averaged the target’s and flankers’ spatialfrequencies (SFs). Interestingly, whereas orientation and color errors were independent, orientation and SF errorswere interdependent. These qualitative differences of errors across dimensions revealed a tight link between crowdingand feature binding, which is contingent on the type of feature dimension. These results and a computational modelsuggest that crowding and misbinding are due to pooling across a joint coding of orientations and SFs but not of colors.
Kupers ER, Carrasco M & Winawer J (2019).
Modeling visual performance differences 'around' the visual field: A computational observer approach.
PLOS Computational Biology.
Visual performance depends on polar angle, even when eccentricity is held constant; on many psychophysical tasks observers perform best when stimuli are presented on the horizontal meridian, worst on the upper vertical, and intermediate on the lower vertical meridian. This variation in performance ‘around’ the visual field can be as pronounced as that of doubling the stimulus eccentricity. The causes of these asymmetries in performance are largely unknown. Some factors in the eye, e.g. cone density, are positively correlated with the reported variations in visual performance with polar angle. However, the question remains whether these correlations can quantitatively explain the perceptual differences observed ‘around’ the visual field. To investigate the extent to which the earliest stages of vision–optical quality and cone density–contribute to performance differences with polar angle, we created a computational observer model. The model uses the open-source software package ISETBIO to simulate an orientation discrimination task for which visual performance differs with polar angle. The model starts from the photons emitted by a display, which pass through simulated human optics with fixational eye movements, followed by cone isomerizations in the retina. Finally, we classify stimulus orientation using a support vector machine to learn a linear classifier on the photon absorptions. To account for the 30% increase in contrast thresholds for upper vertical compared to horizontal meridian, as observed psychophysically on the same task, our computational observer model would require either an increase of ~7 diopters of defocus or a reduction of 500% in cone density. These values far exceed the actual variations as a function of polar angle observed in human eyes. Therefore, we conclude that these factors in the eye only account for a small fraction of differences in visual performance with polar angle. Substantial additional asymmetries must arise in later retinal and/or cortical processing.
[modeling; performance fields]
Fernández A, Li HH & Carrasco M (2019).
How exogenous spatial attention affects visual representation.
Journal of Vision.
Orienting covert spatial attention to a target location enhances visual sensitivity and benefits performance in many visual tasks. How these attention-related improvements in performance affect the underlying visual representation of low-level visual features is not fully understood. Here we focus on characterizing how exogenous spatial attention affects the feature representations of orientation and spatial frequency. We asked observers to detect a vertical grating embedded in noise and performed psychophysical reverse correlation. Doing so allowed us to make comparisons with previous studies that utilized the same task and analysis to assess how endogenous attention and presaccadic modulations affect visual representations. We found that exogenous spatial attention improved performance and enhanced the gain of the target orientation without affecting orientation tuning width. Moreover, we found no change in spatial frequency tuning. We conclude that covert exogenous spatial attention alters performance by strictly boosting gain of orientation-selective filters, much like covert endogenous spatial attention.
[reverse correlation; spatial attention]
Donovan I, Zhou YJ & Carrasco M (2019).
In search of exogenous feature-based attention.
Attention, Perception & Psychophysics.
Visual attention prioritizes the processing of sensory information at specific spatial locations (spatial attention; SA) or with specific feature values (feature-based attention; FBA). SA is well characterized in terms of behavior, brain activity, and temporal dynamics—for both top-down (endogenous) and bottom-up (exogenous) spatial orienting. FBA has been thoroughly studied in terms of top-down endogenous orienting, but much less is known about the potential of bottom-up exogenous influences of FBA. Here, in four experiments, we adapted a procedure used in two previous studies that reported exogenous FBA effects, with the goal of replicating and expanding on these findings, especially regarding its temporal dynamics. Unlike the two previous studies, we did not find significant effects of exogenous FBA. This was true (1) whether accuracy or RT was prioritized as the main measure, (2) with precues presented peripherally or centrally, (3) with cue-to-stimulus ISIs of varying durations, (4) with four or eight possible target locations, (5) at different meridians, (6) with either brief or long stimulus presentations, (7) and with either fixation contingent or noncontingent stimulus displays. In the last experiment, a postexperiment participant questionnaire indicated that only a small subset of participants, who mistakenly believed the irrelevant color of the precue indicated which stimulus was the target, exhibited benefits for valid exogenous FBA precues. Overall, we conclude that with the protocol used in the studies reporting exogenous FBA, the exogenous stimulus-driven influence of FBA is elusive at best, and that FBA is primarily a top-down, goal-driven process.
Waite SA, Grigorian A, Alexander RG, Macknik SL, Carrasco M, Heeger D & Martinez-Conde S (2019).
Analysis of Perceptual Expertise in Radiology – Current Knowledge and a New Perspective.
Frontiers in Human Neuroscience.
Radiologists rely principally on visual inspection to detect, describe, and classify findings in medical images. As most interpretive errors in radiology are perceptual in nature, understanding the path to radiologic expertise during image analysis is essential to educate future generations of radiologists. We review the perceptual tasks and challenges in radiologic diagnosis, discuss models of radiologic image perception, consider the application of perceptual learning methods in medical training, and suggest a new approach to understanding perceptional expertise. Specific principled enhancements to educational practices in radiology promise to deepen perceptual expertise among radiologists with the goal of improving training and reducing medical error.
[review; attention, eye movements, visual search, special populations]
Li HH, Pan J & Carrasco M (2019).
Presaccadic attention improves or impairs performance by enhancing sensitivity to higher spatial frequencies.
Right before we move our eyes, visual performance and neural responses for the saccade target are enhanced. This effect, presaccadic attention, is considered to prioritize the saccade target and to enhance behavioral performance for the saccade target. Recent evidence has shown that presaccadic attention modulates the processing of feature information. Hitherto, it remains unknown whether presaccadic modulations on feature information are flexible, to improve performance for the task at hand, or automatic, so that they alter the featural representation similarly regardless of the task. Using a masking procedure, here we report that presaccadic attention can either improve or impair performance depending on the spatial frequency content of the visual input. These counterintuitive modulations were significant at a time window right before saccade onset. Furthermore, merely deploying covert attention within the same temporal interval without preparing a saccade did not affect performance. This study reveals that presaccadic attention not only prioritizes the saccade target, but also automatically modifies its featural representation.
Vetter P*, Badde S*, Phelps EA & Carrasco M (2019). * equal author contribution
Emotional faces guide the eyes in the absence of awareness.
The ability to act quickly to a threat is a key skill for survival. Under awareness, threat-related emotional information, such as an angry or fearful face, has not only perceptual advantages but also guides rapid actions such as eye movements. Emotional information that is suppressed from awareness still confers perceptual and attentional benefits. However, it is unknown whether suppressed emotional information can directly guide actions, or whether emotional information has to enter awareness to do so. We suppressed emotional faces from awareness using continuous flash suppression and tracked eye gaze position. Under successful suppression, as indicated by objective and subjective measures, gaze moved towards fearful faces, but away from angry faces. Our findings reveal that: (1) threat-related emotional stimuli can guide eye movements in the absence of visual awareness; (2) threat-related emotional face information guides distinct oculomotor actions depending on the type of threat conveyed by the emotional expression.
[eye movements, emotion]
Cavanaugh MR, Barbot A, Carrasco M* & Huxlin KR* (2019). * equal author contribution
Feature-based attention potentiates recovery of fine direction discrimination in cortically blind patients.
Training chronic, cortically-blind (CB) patients on a coarse [left-right] direction discrimination and integration (CDDI) task recovers performance on this task at trained, blind field locations. However, fine direction difference (FDD) thresholds remain elevated at these locations, limiting the usefulness of recovered vision in daily life. Here, we asked if this FDD impairment can be overcome by training CB subjects with endogenous, feature-based attention (FBA) cues. Ten CB subjects were recruited and trained on CDDI and FDD with an FBA cue or FDD with a neutral cue. After completion of each training protocol, FDD thresholds were re-measured with both neutral and FBA cues at trained, blind-field locations and at corresponding, intact-field locations. In intact portions of the visual field, FDD thresholds were lower when tested with FBA than neutral cues. Training subjects in the blind field on the CDDI task improved FDD performance to the point that a threshold could be measured, but these locations remained impaired relative to the intact field. FDD training with neutral cues resulted in better blind field FDD thresholds than CDDI training, but thresholds remained impaired relative to intact field levels, regardless of testing cue condition. Importantly, training FDD in the blind field with FBA lowered FDD thresholds relative to CDDI training, and allowed the blind field to reach thresholds similar to the intact field, even when FBA trained subjects were tested with a neutral rather than FBA cue. Finally, FDD training appeared to also recover normal integration thresholds at trained, blind-field locations, providing an interesting double dissociation with respect to CDDI training. In summary, mechanisms governing FBA appear to function normally in both intact and impaired regions of the visual field following V1 damage. Our results mark the first time that FDD thresholds in CB fields have been seen to reach intact field levels of performance. Moreover, FBA can be leveraged during visual training to recover normal, fine direction discrimination and integration performance at trained, blind-field locations, potentiating visual recovery of more complex and precise aspects of motion perception in cortically-blinded fields.
[feature-based attention, cortical blindness, perceptual learning]
Carrasco M & Barbot A (2019).
Spatial attention alters visual appearance.
Current Opinion in Psychology.
It is well established that attention improves performance on many visual tasks. However, for more than 100 years, psychologists, philosophers, and neurophysiologists have debated its phenomenology—whether attention actually changes one’s subjective experience. Here, we show that it is possible to objectively and quantitatively investigate the effects of attention on subjective experience. First, we review evidence showing that attention alters the appearance of many static and dynamic basic visual dimensions, which mediate changes in appearance of higher-level perceptual aspects. Then, we summarize current views on how attention alters appearance. These findings have implications for our understanding of perception and attention, illustrating that attention affects not only how we perform in visual tasks, but actually alters our experience of the visual world.
[spatial attention; appearance]
Michel M, Beck D, Block N, Blumenfeld H, Brown R, Carmel D, Carrasco M, Chirimuuta M, Chun M, Cleeremans A, Dehaene S, Fleming SM, Frith C, Haggard P, He B, Heyes C, Goodale MA, Irvine L, Kawato M, Kentridge R, King J-R, Knight RT, Kouider S, Lamme V, Lamy D, Lau H, Laureys S, LeDoux J, Lin Y-T, Liu K, Macknik SL, Martinez-Conde S, Mashour GA, Melloni L, Miracchi L, Mylopoulos M, Naccache L, Owen AM, Passingham RE, Pessoa L, Peters MAK, Rahnev D, Ro T, Rosenthal D, Sasaki Y, Sergent C, Solovey G, Schiff ND, Seth A, Tallon-Baudry C, Tamietto M, Tong F, van Gaal S, Vlassova A, Watanabe T, Weisberg J, Yan K, Yoshida M (2019).
Opportunities and challenges for a maturing science of consciousness.
Nature Human Behavior.
[commentary, attention, consciousness]
Carrasco M (2019).
Attention alters appearance.
In Blockheads!: Essays on Ned Block's Philosophy of Mind and Consciousness. A. Pautz and D Stoljar (Eds.) Cambridge, MA: MIT Press.
[book, attention, appearance]
Amit R, Abeles D, Carrasco M & Yuval-Greenberg S (2019).
Oculomotor inhibition reflects temporal expectations.
The accurate extraction of signals out of noisy environments is a major challenge of the perceptual system. Forming temporal expectations and continuously matching them with perceptual input can facilitate this process. In humans, temporal expectations are typically assessed using behavioral measures, which provide only retrospective but no real-time estimates during target anticipation, or by using electrophysiological measures, which require extensive preprocessing and are difficult to interpret. Here we show a new correlate of temporal expectations based on oculomotor behavior. Observers performed an orientation-discrimination task on a central grating target, while their gaze position and EEG were monitored. In each trial, a cue preceded the target by a varying interval ("foreperiod"). In separate blocks, the cue was either predictive or non-predictive regarding the timing of the target. Results showed that saccades and blinks were inhibited more prior to an anticipated regular target than a less-anticipated irregular one. This consistent oculomotor inhibition effect enabled a trial-by-trial classification according to interval-regularity. Additionally, in the regular condition the slope of saccade-rate and drift were shallower for longer than shorter foreperiods, indicating their adjustment according to temporal expectations. Comparing the sensitivity of this oculomotor marker with those of other common predictability markers (e.g. alpha-suppression) showed that it is a sensitive marker for cue-related anticipation. In contrast, temporal changes in conditional probabilities (hazard-rate) modulated alpha-suppression more than cue-related anticipation. We conclude that pre-target oculomotor inhibition is a correlate of temporal predictions induced by cue-target associations, whereas alpha-suppression is more sensitive to conditional probabilities across time.
[EEG; temporal expectation, eye movements]
Fernández A, Denison RN & Carrasco M (2019).
Temporal attention improves perception similarly at foveal and parafoveal locations.
Journal of Vision.
Temporal attention, the prioritization of information at a specific point in time, improves visual performance, but it is unknown whether it does so to the same extent across the visual field. This knowledge is necessary to establish whether temporal attention compensates for heterogeneities in discriminability and speed of processing across the visual field. Discriminability and rate of information accrual depend on eccentricity as well as on polar angle, a characteristic known as performance fields. Spatial attention improves speed of processing more at locations at which discriminability is lower and information accrual is slower, but it improves discriminability to the same extent across isoeccentric locations. Here we asked whether temporal attention benefits discriminability in a similar or differential way across the visual field. Observers were asked to report the orientation of one of two targets presented at different points in time at the same spatial location (fovea, right horizontal meridian, or upper vertical meridian, blocked). Temporal attention improved discriminability and shortened reaction times at the foveal and each parafoveal location similarly. These results provide evidence that temporal attention is similarly effective at multiple locations in the visual field. Consequently, at the tested locations, performance fields are preserved with temporal orienting of attention.
[temporal attention, performance fields]
Denison R, Yuval-Greenberg S & Carrasco M (2019).
Directing voluntary temporal attention increases fixational stability.
Journal of Neuroscience.
Our visual input is constantly changing, but not all moments are equally relevant. Visual temporal attention, the prioritization of visual information at specific points in time, increases perceptual sensitivity at behaviorally relevant times. The dynamic processes underlying this increase are unclear. During fixation, humans make small eye movements called microsaccades, and inhibiting microsaccades improves perception of brief stimuli. Here, we investigated whether temporal attention changes the pattern of microsaccades in anticipation of brief stimuli. Human observers (female and male) judged stimuli presented within a short sequence. Observers were given either an informative precue to attend to one of the stimuli, which was likely to be probed, or an uninformative (neutral) precue. We found strong microsaccadic inhibition before the stimulus sequence, likely due to its predictable onset. Critically, this anticipatory inhibition was stronger when the first target in the sequence (T1) was precued (task-relevant) than when the precue was uninformative. Moreover, the timing of the last microsaccade before T1 and the first microsaccade after T1 shifted such that both occurred earlier when T1 was precued than when the precue was uninformative. Finally, the timing of the nearest pre- and post-T1 microsaccades affected task performance. Directing voluntary temporal attention therefore affects microsaccades, helping to stabilize fixation at the most relevant moments over and above the effect of predictability. Just as saccading to a relevant stimulus can be an overt correlate of the allocation of spatial attention, precisely timed gaze stabilization can be an overt correlate of the allocation of temporal attention.SIGNIFICANCE STATEMENT We pay attention at moments in time when a relevant event is likely to occur. Such temporal attention improves our visual perception, but how it does so is not well understood. Here, we discovered a new behavioral correlate of voluntary, or goal-directed, temporal attention. We found that the pattern of small fixational eye movements called microsaccades changes around behaviorally relevant moments in a way that stabilizes the position of the eyes. Microsaccades during a brief visual stimulus can impair perception of that stimulus. Therefore, such fixation stabilization may contribute to the improvement of visual perception at attended times. This link suggests that, in addition to cortical areas, subcortical areas mediating eye movements may be recruited with temporal attention.
[temporal attention, microsaccades]
Pham A, Carrasco M & Kiorpes L (2018).
Endogenous attention improves perception in amblyopic macaques.
Journal of Vision.
Amblyopia, a developmental disorder of vision, affects many aspects of spatial vision as well as motion perception and some cognitive skills. Current models of amblyopic vision based on known neurophysiological deficiencies have yet to provide an understanding of the wide range of amblyopic perceptual losses. Visual spatial attention is known to enhance performance in a variety of detection and discrimination tasks in visually typical humans and nonhuman primates. We investigated whether and how voluntary spatial attention affected psychophysical performance in amblyopic macaques. Full-contrast response functions for motion direction discrimination were measured for each eye of six monkeys: five amblyopic and one control. We assessed whether the effect of a valid spatial cue on performance corresponded to a change in contrast gain, a leftward shift of the function, or response gain, an upward scaling of the function. Our results showed that macaque amblyopes benefit from a valid spatial cue. Performance with amblyopic eyes viewing showed enhancement of both contrast and response gain whereas fellow and control eyes’ performance showed only contrast gain. Reaction time analysis showed no speed accuracy trade- off in any case. The valid spatial cue improved contrast sensitivity for the amblyopic eye, effectively eliminating the amblyopic contrast sensitivity deficit. These results suggest that engaging endogenous spatial attention may confer substantial benefit to amblyopic vision.
[spatial attention, amblyopia, macaque, motion discrimination, contrast]
Cutrone EK, Heeger DJ & Carrasco M (2018).
On spatial attention and its field size on the repulsion effect.
Journal of Vision.
We investigated the attentional repulsion effect—stimuli appear displaced further away from attended locations—in three experiments: one with exogenous (involuntary) attention, and two with endogenous (voluntary) attention with different attention-field sizes. It has been proposed that differences in attention-field size can account for qualitative differences in neural responses elicited by attended stimuli. We used psychophysical comparative judgments and manipulated either exogenous attention via peripheral cues or endogenous attention via central cues and a demanding rapid serial visual presentation task. We manipulated the attention field size of endogenous attention by presenting streams of letters at two specific locations or at two of many possible locations during each block. We found a robust attentional repulsion effect in all three experiments: with endogenous and exogenous attention and with both attention-field sizes. These findings advance our understanding of the influence of spatial attention on the perception of visual space and help relate this repulsion effect to possible neurophysiological correlates.
[spatial attention, attention field size]
Barbot A & Carrasco M (2018).
Emotion and anxiety potentiate the way attention alters visual appearance.
The ability to swiftly detect and prioritize the processing of relevant information around us is critical for the way we interact with our environment. Selective attention is a key mechanism that serves this purpose, improving performance in numerous visual tasks. Reflexively attending to sudden information helps detect impeding threat or danger, a possible reason why emotion modulates the way selective attention affects perception. For instance, the sudden appearance of a fearful face potentiates the effects of exogenous (involuntary, stimulus-driven) attention on performance. Internal states such as trait anxiety can also modulate the impact of attention on early visual processing. However, attention does not only improve performance; it also alters the way visual information appears to us, e.g. by enhancing perceived contrast. Here we show that emotion potentiates the effects of exogenous attention on both performance and perceived contrast. Moreover, we found that trait anxiety mediates these effects, with stronger influences of attention and emotion in anxious observers. Finally, changes in performance and appearance correlated with each other, likely reflecting common attentional modulations. Altogether, our findings show that emotion and anxiety interact with selective attention to truly alter how we see.
[spatial attention, emotion, appearance, contrast]
Jigo M & Carrasco M (2018).
Attention alters spatial resolution by modulating second-order processing.
Journal of Vision.
Endogenous and exogenous visuospatial attention both alter spatial resolution, but they operate via distinct mechanisms. In texture segmentation tasks, exogenous attention inflexibly increases resolution even when detrimental for the task at hand and does so by modulating second-order processing. Endogenous attention is more flexible and modulates resolution to benefit performance according to task demands, but it is unknown whether it also operates at the second-order level. To answer this question, we measured performance on a second-order texture segmentation task while independently manipulating endogenous and exogenous attention. Observers discriminated a second- order texture target at several eccentricities. We found that endogenous attention improved performance uniformly across eccentricity, suggesting a flexible mechanism that can increase or decrease resolution based on task demands. In contrast, exogenous attention improved performance in the periphery but impaired it at central retinal locations, consistent with an inflexible resolution enhancement. Our results reveal that endogenous and exogenous attention both alter spatial resolution by differentially modulating second-order processing.
[spatial attention, texture segmentation, spatial resolution]
Hilo-Merkovich R, Carrasco M & Yuval-Greenberg S (2018).
Task performance in covert, but not overt, attention correlates with early laterality of visual evoked potentials.
Attention affects visual perception at target locations via the amplification of stimuli signal strength, perceptual performance and perceived contrast. Behavioral and neural correlates of attention can be observed when attention is both covertly and overtly oriented (with or without accompanying eye movements). Previous studies have demonstrated that at the grand-average level, lateralization of Event Related Potentials (ERP) is associated with attentional facilitation at cued, relative to uncued locations. Yet, the correspondence between ERP lateralization and behavior has not been established at the single-subject level. Specifically, it is an open question whether inter-individual differences in the neural manifestation of attentional orienting can predict differences in perception. Here, we addressed this question by examining the correlation between ERP lateralization and visual sensitivity at attended locations. Participants were presented with a cue indicating where a low-contrast grating patch target will appear, following a delay of varying durations. During this delay, while participants were waiting for the target to appear, a task-irrelevant checkerboard probe was presented briefly and bilaterally. ERP was measured relative to the onset of this probe. In separate blocks, participants were requested to report detection of a low-contrast target either by making a fast eye-movement toward the target (overt orienting), or by pressing a button (covert orienting). Results show that in the covert orienting condition, ERP lateralization of individual participants was positively correlated with their mean visual sensitivity for the target. But, no such correlation was found in the overt orienting condition. We conclude that ERP lateralization of individual participants can predict their performance on a covert, but not an overt, target detection task.
Carrasco M (2018).
How visual spatial attention alters perception.
Visual attention is essential for visual perception. Spatial attention allows us to grant priority in processing and selectively process information at a given location. In this paper, I explain how two kinds of spatial attention: covert (allocated to the target location, without accompanying eye movements) and presaccadic (allocated to the location of the upcoming saccade’s target) affect performance and alter appearance. First, I highlight some behavioral and neuroimaging research on covert attention, which alters performance and appearance in many basic visual tasks. Second, I review studies showing that presaccadic attention improves performance and alters appearance at the saccade target location. Further, these modulations change the processing of feature information automatically, even when it is detrimental to the task at hand. We propose that saccade preparation may support transsaccadic integration. Systematically investigating the common and differential characteristics of covert attention and presaccadic attention will continue to further our understanding of the pervasive selective processing of information, which enables us to make sense of our complex visual world.
[review; spatial attention]
Barbot A, Liu S, Kimchi R & Carrasco M (2018).
Attention enhances apparent perceptual organization.
Psychonomic Bulletin & Review.
Perceptual organization and selective attention are two crucial processes that influence how we perceive visual information. The former structures complex visual inputs into coherent units, whereas the later selects relevant information. Attention and perceptual organization can modulate each other, affecting visual processing and performance in various tasks and conditions. Here, we tested whether attention can alter the way multiple elements appear to be perceptually organized. We manipulated covert spatial attention using a rapid serial visual presentation task, and measured perceptual organization of two multielements arrays organized by luminance similarity as rows or columns, at both the attended and unattended locations. We found that the apparent perceptual organization of the multielement arrays is intensified when attended and attenuated when unattended. We ruled out response bias as an alternative explanation. These findings reveal that attention enhances the appearance of perceptual organization, a midlevel vision process, altering the way we perceive our visual environment.
[spatial attention, appearance, perceptual organization]
Denison RN, Adler WT, Carrasco M & Ma WJ (2018).
Humans incorporate attention-dependent uncertainty into perceptual decisions and confidence.
Perceptual decisions are better when they take uncertainty into account. Uncertainty arises not only from the properties of sensory input but also from cognitive sources, such as different levels of attention. However, it is unknown whether humans appropriately adjust for such cognitive sources of uncertainty during perceptual decision-making. Here we show that, in a task in which uncertainty is relevant for performance, human categorization and confidence decisions take into account uncertainty related to attention. We manipulated uncertainty in an orientation categorization task from trial to trial using only an attentional cue. The categorization task was designed to disambiguate decision rules that did or did not depend on attention. Using formal model comparison to evaluate decision behavior, we found that category and confidence decision boundaries shifted as a function of attention in an approximately Bayesian fashion. This means that the observer’s attentional state on each trial contributed probabilistically to the decision computation. This responsiveness of an observer’s decisions to attention-dependent uncertainty should improve perceptual decisions in natural vision, in which attention is unevenly distributed across a scene.
[spatial attention, confidence, perceptual decision]
Eckstein MP, Yu C, Sagi D, Carrasco M & Lu Z-L (2018).
Introduction to special issue on perceptual learning.
[editorial; perceptual learning]
Rolfs M, Murray-Smith N & Carrasco M (2018).
Perceptual learning while preparing saccades.
Traditional perceptual learning protocols rely almost exclusively on long periods of uninterrupted fixation. Taking a first step towards understanding perceptual learning in natural vision, we had observers report the orientation of a briefly flashed stimulus (clockwise or counterclockwise from a reference orientation) presented strictly during saccade preparation at a location offset from the saccade target. For each observer, the saccade direction, stimulus location, and orientation remained the same throughout training. Subsequently, we assessed performance during fixation in three transfer sessions, either at the trained or at an untrained location, and either using an untrained (Experiment 1) or the trained (Experiment 2) stimulus orientation. We modeled the evolution of contrast thresholds (i.e., the stimulus contrast necessary to discriminate its orientation correctly 75% of the time) as an exponential learning curve, and quantified departures from this curve in transfer sessions using two new, complementary measures of transfer costs (i.e., performance decrements after the transition into the Transfer phase). We observed robust perceptual learning and associated transfer costs for untrained locations and orientations. We also assessed if spatial transfer costs were reduced for the remapped location of the presaccadic stimulus—the location the stimulus would have had (but never had) after the saccade. Although the pattern of results at that location differed somewhat from that at the control location, we found no clear evidence for perceptual learning at remapped locations. Using novel, model-based ways to assess learning and transfer costs, our results show that location and feature specificity, hallmarks of perceptual learning, subsist if the target stimulus is presented strictly during saccade preparation throughout training.
[modeling; eye movements, perceptual learning, saccade preparation]
Donovan I & Carrasco M (2018).
Endogenous spatial attention during perceptual learning facilitates location transfer.
Journal of Vision.
Covert attention and perceptual learning enhance perceptual performance. The relation between these two mechanisms is largely unknown. Previously, we showed that manipulating involuntary, exogenous spatial attention during training improved performance at trained and untrained locations, thus overcoming the typical location specificity. Notably, attention-induced transfer only occurred for high stimulus contrasts, at the upper asymptote of the psychometric function (i.e., via response gain). Here, we investigated whether and how voluntary, endogenous attention, the top-down and goal-based type of covert visual attention, influences perceptual learning. Twenty-six participants trained in an orientation discrimination task at two locations: half of participants received valid endogenous spatial precues (attention group), while the other half received neutral precues (neutral group). Before and after training, all participants were tested with neutral precues at two trained and two untrained locations. Within each session, stimulus contrast varied on a trial basis from very low (2%) to very high (64%). Performance was fit by a Weibull psychometric function separately for each day and location. Performance improved for both groups at the trained location, and unlike training with exogenous attention, at the threshold level (i.e., via contrast gain). The neutral group exhibited location specificity: Thresholds decreased at the trained locations, but not at the untrained locations. In contrast, participants in the attention group showed significant location transfer: Thresholds decreased to the same extent at both trained and untrained locations. These results indicate that, similar to exogenous spatial attention, endogenous spatial attention induces location transfer, but influences contrast gain instead of response gain.
[modeling; spatial attention, perceptual learning, orientation discrimination]
Barbot A & Carrasco M (2017).
Attention modifies spatial resolution according to task demands.
How does visual attention affect spatial resolution? In texture-segmentation tasks, exogenous (involuntary) attention automatically increases resolution at the attended location, which improves performance where resolution is too low (at the periphery) but impairs performance where resolution is already too high (at central locations). Conversely, endogenous (voluntary) attention improves performance at all eccentricities, which suggests a more flexible mechanism. Here, using selective adaptation to spatial frequency, we investigated the mechanism by which endogenous attention benefits performance in resolution tasks. Participants detected a texture target that could appear at several eccentricities. Adapting to high or low spatial frequencies selectively affected performance in a manner consistent with changes in resolution. Moreover, adapting to high, but not low, frequencies mitigated the attentional benefit at central locations where resolution was too high; this shows that attention can improve performance by decreasing resolution. Altogether, our results indicate that endogenous attention benefits performance by modulating the contribution of high-frequency information in order to flexibly adjust spatial resolution according to task demands.
[adaptation; spatial attention, texture segmentation, spatial resolution, spatial frequency]
Denison RN, Heeger DJ & Carrasco M (2017).
Attention flexibly trades off across points in time.
Psychonomic Bulletin & Review.
Sensory signals continuously enter the brain, raising the question of how perceptual systems handle this constant flow of input. Attention to an anticipated point in time can prioritize visual information at that time. However, how we voluntarily attend across time when there are successive task-relevant stimuli has been barely investigated. We developed a novel experimental protocol that allowed us to assess, for the first time, both the benefits and costs of voluntary temporal attention when perceiving a short sequence of two or three visual targets with predictable timing. We found that when humans directed attention to a cued point in time, their ability to perceive orientation was better at that time but also worse earlier and later. These perceptual tradeoffs across time are analogous to those found across space for spatial attention. We concluded that voluntary attention is limited, and selective, across time.
Dankner Y, Shalev L, Carrasco M & Yuval-Greenberg S (2017).
Prestimulus inhibition of saccades in adults with and without ADHD as an index for temporal expectation.
Knowing when to expect important events to occur is critical for preparing context-appropriate behavior. However, anticipation is inherently complicated to assess because conventional measurements of behavior, such as accuracy and reaction time, are available only after the predicted event has occurred. Anticipatory processes, which occur prior to target onset, are typically measured only retrospectively by these methods. In this study, we utilized a novel approach for assessing temporal expectations through the dynamics of prestimulus saccades. Results showed that saccades of neurotypical participants were inhibited prior to the onset of stimuli that appeared at predictable compared with less predictable times. No such inhibition was found in most participants with attention-deficit/hyperactivity disorder (ADHD), and particularly not in those who experienced difficulties in sustaining attention over time. These findings suggest that individuals with ADHD, especially those with sustained-attention deficits, have diminished ability to benefit from temporal predictability, and this could account for some of their context-inappropriate behaviors.
[temporal expectation, eye movements, saccades, ADHD]
Mikulak A (2017).
Eye movements reveal temporal expectation deficits in ADHD.
Association for Psychological Science.
Dugué L, Xue AM & Carrasco M (2017).
Distinct perceptual rhythms for feature and conjunction searches.
Journal of Vision.
Feature and conjunction searches are widely used to study attentional deployment. However, the spatiotemporal behavior of attention integration in these tasks remains under debate. Are multiple search stimuli processed in parallel or sequentially? Does sampling of visual information and attentional deployment differ between these two types of search? If so, how? We used an innovative methodology to estimate the distribution of attention on a single-trial basis for feature and conjunction searches. Observers performed feature- and conjunction-search tasks. They had to detect and discriminate a tilted low-spatial-frequency grating among three low-spatial-frequency vertical gratings (feature search) or low-spatial-frequency vertical gratings and high-spatial-frequency tilted gratings (conjunction search). After a variable delay, two probes were flashed at random locations. Performance in reporting the probes was used to infer attentional deployment to those locations. By solving a second-degree equation, we determined the probability of probe report at the most (P1) and least (P2) attended locations on a given trial. Were P1 and P2 equal, we would conclude that attention had been uniformly distributed across all four locations. Otherwise, we would conclude that visual information sampling and attentional deployment had been nonuniformly distributed. Our results show that processing was nonuniformly distributed across the four locations in both searches, and was modulated periodically over time at ;5 Hz for the conjunction search and ;12 Hz for the feature search. We argue that the former corresponds to the periodicity of attentional deployment during the search, whereas the latter corresponds to ongoing sampling of visual information. Because different locations were not simultaneously processed, this study rules out a strict parallel model for both search types.
[spatial attention, visual search]
Dugué L, Merriam EP, Heeger DJ & Carrasco M (2017).
Specific visual subregions of TPJ mediate reorienting of spatial attention.
The temporo-parietal junction (TPJ) has been associated with various cognitive and social functions, and is critical for attentional reorienting. Attention affects early visual processing. Neuroimaging studies dealing with such processes have thus far concentrated on striate and extrastriate areas. Here, we investigated whether attention orienting or reorienting modulate activity in visually driven TPJ subregions. For each observer we identified 3 visually responsive subregions within TPJ: 2 bilateral (vTPJant and vTPJpost) and 1 right lateralized (vTPJcent). Cortical activity in these subregions was measured using fMRI while observers performed a 2-alternative forced-choice orientation discrimination task. Covert spatial endogenous (voluntary) or exogenous (involuntary) attention was manipulated using either a central or a peripheral cue with task, stimuli and observers constant. Both endogenous and exogenous attention increased activity for invalidly cued trials in right vTPJpost; only endogenous attention increased activity for invalidly cued trials in left vTPJpost and in right vTPJcent; and neither type of attention modulated either right or left vTPJant. These results demonstrate that vTPJpost and vTPJcent mediate the reorientation of covert attention to task relevant stimuli, thus playing a critical role in visual attention. These findings reveal a differential reorienting cortical response after observers’ attention has been oriented to a given location voluntarily or involuntarily.
[fMRI; spatial attention, reorienting]
Yashar A, White AL, Fang W & Carrasco M (2017).
Feature singletons attract spatial attention independently of feature priming.
Journal of Vision.
People perform better in visual search when the target feature repeats across trials (intertrial feature priming [IFP]). Here, we investigated whether repetition of a feature singleton’s color modulates stimulus-driven shifts of spatial attention by presenting a probe stimulus immediately after each singleton display. The task alternated every two trials between a probe discrimination task and a singleton search task. We measured both stimulus-driven spatial attention (via the distance between the probe and singleton) and IFP (via repetition of the singleton’s color). Color repetition facilitated search performance (IFP effect) when the set size was small. When the probe appeared at the singleton’s location, performance was better than at the opposite location (stimulus-driven attention effect). The magnitude of this attention effect increased with the singleton’s set size (which increases its saliency) but did not depend on whether the singleton’s color repeated across trials, even when the previous singleton had been attended as a search target. Thus, our findings show that repetition of a salient singleton’s color affects performance when the singleton is task relevant and voluntarily attended (as in search trials). However, color repetition does not affect performance when the singleton becomes irrelevant to the current task, even though the singleton does capture attention (as in probe trials). Therefore, color repetition per se does not make a singleton more salient for stimulus- driven attention. Rather, we suggest that IFP requires voluntary selection of color singletons in each consecutive trial.
[spatial attention, visual search, feature singletons]
Li H-H, Rankin J, Rinzel J, Carrasco M & Heeger DJ (2017).
Attention model of binocular rivalry.
When the corresponding retinal locations in the two eyes are presented with incompatible images, a stable percept gives way to perceptual alternations in which the two images compete for perceptual dominance. As perceptual experience evolves dynamically under constant external inputs, binocular rivalry has been used for studying intrinsic cortical computations and for understanding how the brain regulates competing inputs. Converging behavioral and EEG results have shown that binocular rivalry and attention are intertwined: binocular rivalry ceases when attention is diverted away from the rivalry stimuli. In addition, the competing image in one eye suppresses the target in the other eye through a pattern of gain changes similar to those induced by attention. These results require a revision of the current computational theories of binocular rivalry, in which the role of attention is ignored. Here, we provide a computational model of binocular rivalry. In the model, competition between two images in rivalry is driven by both attentional modulation and mutual inhibition, which have distinct selectivity (feature vs. eye of origin) and dynamics (relatively slow vs. relatively fast). The proposed model explains a wide range of phenomena reported in rivalry, including the three hallmarks: (i) binocular rivalry requires attention; (ii) various perceptual states emerge when the two images are swapped between the eyes multiple times per second; (iii) the dominance duration as a function of input strength follows Levelt’s propositions. With a bifurcation analysis, we identified the parameter space in which the model’s behavior was consistent with experimental results.
[modeling; spatial attention, binocular rivalry]
Poletti M, Rucci M & Carrasco M (2017).
Selective attention within the foveola.
Efficient control of attentional resources and high-acuity vision are both fundamental for survival. Shifts in visual attention are known to covertly enhance processing at locations away from the center of gaze, where visual resolution is low. It is unknown, however, whether selective spatial attention operates where the observer is already looking—that is, within the high-acuity foveola, the small yet disproportionally important rod-free region of the retina. Using new methods for precisely controlling retinal stimulation, here we show that covert attention flexibly improves and speeds up both detection and discrimination at loci only a fraction of a degree apart within the foveola. These findings reveal a surprisingly precise control of attention and its involvement in fine spatial vision. They show that the commonly studied covert shifts of attention away from the fovea are the expression of a global mechanism that exerts its action across the entire visual field.
[spatial attention, eye movements, microsaccades]
Roberts M, Ashinoff BK, Castellanos FX & Carrasco M (2017).
When attention is intact in adults with ADHD.
Psychonomic Bulletin & Review.
Is covert visuospatial attention—selective processing of information in the absence of eye movements—preserved in adults with attention-deficit/hyperactivity disorder (ADHD)? Previous findings are inconclusive due to inconsistent terminology and suboptimal methodology. To settle this question, we used well-established spatial cueing protocols to investigate the perceptual effects of voluntary and involuntary attention on an orientation discrimination task for a group of adults with ADHD and their neurotypical age-matched and gender-matched controls. In both groups, voluntary attention significantly improved accuracy and decreased reaction times at the relevant location, but impaired accuracy and slowed reaction times at irrelevant locations, relative to a distributed attention condition. Likewise, involuntary attention improved accuracy and speeded responses. Critically, the magnitudes of all these orienting and reorienting attention effects were indistinguishable between groups. Thus, these counterintuitive findings indicate that spatial covert attention remains functionally intact in adults with ADHD.
[spatial attention, special populations]
Li H-H, Barbot A & Carrasco M (2016).
Saccade preparation reshapes sensory tuning.
Human observers make large rapid eye movements— saccades—to bring behaviorally relevant information into the fovea, where spatial resolution is high. In some visual tasks [1–4], performance at the location of a saccade target improves before the eyes move. Although these findings provide evidence that extraretinal signals evoked by saccades can enhance visual perception, it remains unknown whether and how presaccadic modulations change the processing of feature information and thus modulate visual representations. To answer this question, one must go beyond the use of methods that only probe performance accuracy (d') in different tasks. Here, using a psychophysical reverse correlation approach [5–8], we investigated how saccade preparation influences the processing of orientation and spatial frequency— two building blocks of early vision. We found that saccade preparation selectively enhanced the gain of high spatial frequency information and narrowed orientation tuning at the upcoming saccade landing position. These modulations were time locked to saccade onset, peaking right before the eyes moved (-50–0 ms). Moreover, merely deploying covert attention within the same temporal interval without preparing a saccade did not alter performance. The observed presaccadic tuning changes may correspond to the presaccadic enhancement [9–11] and receptive field shifts reported in neurophysiological studies [12–14]. Saccade preparation may support transaccadic integration by reshaping the representation of the saccade target to be more fovea-like just before the eyes move. The presaccadic modulations on spatial frequency and orientation processing illustrate a strong perception-action coupling by revealing that the visual system dynamically reshapes feature selectivity contingent upon eye movements.
[reverse correlation; spatial attention, eye movements, orientation tuning, spatial frequency]
Roberts M, Cymerman R, Smith RT, Kiorpes L & Carrasco M (2016).
Covert spatial attention is functionally intact in amblyopic human adults.
Journal of Vision.
Certain abnormalities in behavioral performance and neural signaling have been attributed to a deficit of visual attention in amblyopia, a neurodevelopmental disorder characterized by a diverse array of visual deficits following abnormal binocular childhood experience. Critically, most have inferred attention’s role in their task without explicitly manipulating and measuring its effects against a baseline condition. Here, we directly investigate whether human amblyopic adults benefit from covert spatial attention—the selective processing of visual information in the absence of eye movements—to the same degree as neurotypical observers. We manipulated both involuntary (Experiment 1) and voluntary (Experiment 2) attention during an orientation discrimination task for which the effects of covert spatial attention have been well established in neurotypical and special populations. In both experiments, attention significantly improved accuracy and decreased reaction times to a similar extent (a) between the eyes of the amblyopic adults and (b) between the amblyopes and their age- and gender- matched controls. Moreover, deployment of voluntary attention away from the target location significantly impaired task performance (Experiment 2). The magnitudes of the involuntary and voluntary attention benefits did not correlate with amblyopic depth or severity. Both groups of observers showed canonical performance fields (better performance along the horizontal than vertical meridian and at the lower than upper vertical meridian) and similar effects of attention across locations. Despite their characteristic low-level vision impairments, covert spatial attention remains functionally intact in human amblyopic adults.
[spatial attention, special populations, amblyopia, orientation discrimination]
Yashar A & Carrasco M (2016).
Rapid and long-lasting learning of feature binding.
How are features integrated (bound) into objects and how can this process be facilitated? Here we investigated the role of rapid perceptual learning in feature binding and its long-lasting effects. By isolating the contributions of individual features from their conjunctions between training and test displays, we demonstrate for the first time that training can rapidly and substantially improve feature binding. Observers trained on a conjunction search task consisting of a rapid display with one target-conjunction, then tested with a new target-conjunction. Features were the same between training and test displays. Learning transferred to the new target when its conjunction was presented as a distractor, but not when only its component features were presented in different conjunction distractors during training. Training improvement lasted for up to 16 months, but, in all conditions, it was specific to the trained target. Our findings suggest that with short training observers’ ability to bind two specific features into an object is improved, and that this learning effect can last for over a year. Moreover, our findings show that while the short-term learning effect reflects activation of presented items and their binding, long-term consolidation is task specific.
[perceptual learning, visual search, feature binding]
Dugué L, Roberts M & Carrasco M (2016).
Attention reorients periodically.
Reorienting of voluntary attention enables the processing of stimuli at previously unattended locations. Although studies have identified a ventral frontoparietal network underlying attention [1, 2], little is known about whether and how early visual areas are involved in involuntary [3, 4] and even less in voluntary  reorienting, and their temporal dynamics are unknown. We used transcranial magnetic stimulation (TMS) over the occipital cortex to interfere with attentional reorienting and study its role and temporal dynamics in this process. Human observers performed an orientation discrimination task, with either valid or invalid attention cueing, across a range of stimulus contrasts. Valid cueing induced a behavioral response gain increase, higher asymptotic performance for attended than unattended locations. During subsequent TMS sessions, observers performed the same task, with high stimulus contrast. Based on phosphene mapping, TMS double pulses were applied at one of various delays to a consistent brain location in retinotopic areas (V1/V2), corresponding to the evoked signal of the target or distractor, in a valid or invalid trial. Thus, the stimulation was identical for the four experimental conditions (valid/invalid cue condition 3 target/distractor-stimulated). TMS modulation of the target and distractor were both periodic (5 Hz, theta) and out of phase with respect to each other in invalid trials only, when attention had to be disengaged from the distractor and reoriented to the target location. Reorientation of voluntary attention periodically involves V1/V2 at the theta frequency. These results suggest that TMS probes theta phase-reset by attentional reorienting and help link periodic sampling in time and attention reorienting in space.
[TMS; spatial attention, orientation discrimination, reorienting, contrast, temporal dynamics]
Yashar A, Chen J & Carrasco M (2015).
Rapid and long-lasting reduction of crowding through training.
Journal of Vision.
Crowding is the failure to identify an object in the peripheral visual field in the presence of nearby objects. Recent studies have shown that crowding can be alleviated after several days of training, but the processes underlying this improvement are still unclear. Here we tested whether a few hundred trials within a short period of training can alleviate crowding, whether the learning is location specific, and whether the improvement reflects facilitation by target enhancement or flankers suppression. Observers were asked to identify the orientation of a letter in the periphery surrounded by two flanker letters. Observers were tested before (pretest) and after (posttest) training (600 trials). In Experiment 1 we tested whether learning is location specific or can transfer to a different location; the training and test occurred at the same or different hemifields. In a control experiment, we ruled out alternative explanations for the learning effect in Experiment 1. In Experiment 2, we assessed different components of feature selection by training with either the same flanker polarity as the pre/posttest but opposite polarity group (flanker polarity group) or the same target polarity as the pre/posttest but opposite flanker polarity (target polarity group). Following training, overall performance increased in all four conditions, but only the same-location group (Experiment 1) and the same flanker polarity (Experiment 2) showed a significant reduction in crowding as assessed by the distance at which the flankers no longer interfere with target identification, that is, the critical spacing. These results show that training can rapidly reduce crowding and that improvement primarily reflects learning to ignore the irrelevant flankers. Remarkably, in the two conditions in which training significantly reduced crowding, the benefit of short training persisted for up to a year.
[perceptual learning, crowding, contrast]
White AL, Rolfs M & Carrasco M (2015).
Stimulus competition mediates the joint effects of spatial and feature-based attention.
Journal of Vision.
Distinct attentional mechanisms enhance the sensory processing of visual stimuli that appear at task-relevant locations and have task-relevant features. We used a combination of psychophysics and computational modeling to investigate how these two types of attention—spatial and feature based—interact to modulate sensitivity when combined in one task. Observers monitored overlapping groups of dots for a target change in color saturation, which they had to localize as being in the upper or lower visual hemifield. Pre-cues indicated the target’s most likely location (left/ right), color (red/green), or both location and color. We measured sensitivity (d') for every combination of the location cue and the color cue, each of which could be valid, neutral, or invalid. When three competing saturation changes occurred simultaneously with the target change, there was a clear interaction: The spatial cueing effect was strongest for the cued color, and the color cueing effect was strongest at the cued location. In a second experiment, only the target dot group changed saturation, such that stimulus competition was low. The resulting cueing effects were statistically independent and additive: The color cueing effect was equally strong at attended and unattended locations. We account for these data with a computational model in which spatial and feature-based attention independently modulate the gain of sensory responses, consistent with measurements of cortical activity. Multiple responses then compete via divisive normalization. Sufficient competition creates interactions between the two cueing effects, although the attentional systems are themselves independent. This model helps reconcile seemingly disparate behavioral and physiological findings.
[modeling; spatial attention, feature based attention]
Li H-H, Carrasco M & Heeger DJ (2015).
Deconstructing interocular suppression: Attention and divisive normalization.
PLOS Computational Biology.
In interocular suppression, a suprathreshold monocular target can be rendered invisible by a salient competitor stimulus presented in the other eye. Despite decades of research on interocular suppression and related phenomena (e.g., binocular rivalry, flash suppression, continuous flash suppression), the neural processing underlying interocular suppression is still unknown. We developed and tested a computational model of interocular suppression. The model included two processes that contributed to the strength of interocular suppression: divisive normalization and attentional modulation. According to the model, the salient competitor induced a stimulus-driven attentional modulation selective for the location and orientation of the competitor, thereby increasing the gain of neural responses to the competitor and reducing the gain of neural responses to the target. Additional suppression was induced by divisive normalization in the model, similar to other forms of visual masking. To test the model, we conducted psychophysics experiments in which both the size and the eye-of-origin of the competitor were manipulated. For small and medium competitors, behavioral performance was consonant with a change in the response gain of neurons that responded to the target. But large competitors induced a contrast-gain change, even when the competitor was split between the two eyes. The model correctly predicted these results and outperformed an alternative model in which the attentional modulation was eye specific. We conclude that both stimulus-driven attention (selective for location and feature) and divisive normalization contribute to interocular suppression.
Donovan I, Szpiro S & Carrasco M (2015).
Exogenous attention facilitates location transfer of perceptual learning.
Journal of Vision.
Perceptual skills can be improved through practice on a perceptual task, even in adulthood. Visual perceptual learning is known to be mostly specific to the trained retinal location, which is considered as evidence of neural plasticity in retinotopic early visual cortex. Recent findings demonstrate that transfer of learning to untrained locations can occur under some specific training procedures. Here, we evaluated whether exogenous attention facilitates transfer of perceptual learning to untrained locations, both adjacent to the trained locations (Experiment 1) and distant from them (Experiment 2). The results reveal that attention facilitates transfer of perceptual learning to untrained locations in both experiments, and that this transfer occurs both within and across visual hemifields. These findings show that training with exogenous attention is a powerful regime that is able to overcome the major limitation of location specificity.
[spatial attention, perceptual learning, orientation discrimination]
Szpiro SFA & Carrasco M (2015).
Exogenous attention enables perceptual learning.
Practice can improve visual perception, and these improvements are considered to be a form of brain plasticity. Training-induced learning is time-consuming and requires hundreds of trials across multiple days. The process of learning acquisition is understudied. Can learning acquisition be potentiated by manipulating visual attentional cues? We developed a protocol in which we used task-irrelevant cues for between-groups manipulation of attention during training. We found that training with exogenous attention can enable the acquisition of learning. Remarkably, this learning was maintained even when observers were subsequently tested under neutral conditions, which indicates that a change in perception was involved. Our study is the first to isolate the effects of exogenous attention and to demonstrate its efficacy to enable learning. We propose that exogenous attention boosts perceptual learning by enhancing stimulus encoding.
[spatial attention, perceptual learning, spatial frequency, orientation]
Cavanaugh MR, Zhang R, Melnick MD, Das A, Roberts M, Tadin D, Carrasco M* & Huxlin KR* (2015).
Visual recovery in cortical blindness is limited by high internal noise.
Journal of Vision.
* equal author contribution
Damage to the primary visual cortex typically causes cortical blindness (CB) in the hemifield contralateral to the damaged hemisphere. Recent evidence indicates that visual training can partially reverse CB at trained locations. Whereas training induces near-complete recovery of coarse direction and orientation discriminations, deficits in fine motion processing remain. Here, we systematically disentangle components of the perceptual inefficiencies present in CB fields before and after coarse direction discrimination training. In seven human CB subjects, we measured threshold versus noise functions before and after coarse direction discrimination training in the blind field and at corresponding intact field locations. Threshold versus noise functions were analyzed within the framework of the linear amplifier model and the perceptual template model. Linear amplifier model analysis identified internal noise as a key factor differentiating motion processing across the tested areas, with visual training reducing internal noise in the blind field. Differences in internal noise also explained residual perceptual deficits at retrained locations. These findings were confirmed with perceptual template model analysis, which further revealed that the major residual deficits between retrained and intact field locations could be explained by differences in internal additive noise. There were no significant differences in multiplicative noise or the ability to process external noise. Together, these results highlight the critical role of altered internal noise processing in mediating training-induced visual recovery in CB fields, and may explain residual perceptual deficits relative to intact regions of the visual field.
[modeling; cortical blindness]
Carrasco M & Barbot A (2015).
How attention affects spatial resolution.
Cold Spring Harbor Symposia on Quantitative Biology.
We summarize and discuss a series of psychophysical studies on the effects of spatial covert attention on spatial resolution, our ability to discriminate fine patterns. Heightened resolution is beneficial in most, but not all, visual tasks. We show how endogenous attention (voluntary, goal driven) and exogenous attention (involuntary, stimulus driven) affect performance on a variety of tasks mediated by spatial resolution, such as visual search, crowding, acuity, and texture segmentation. Exogenous attention is an automatic mechanism that increases resolution regardless of whether it helps or hinders performance. In contrast, endogenous attention flexibly adjusts resolution to optimize performance according to task demands. We illustrate how psychophysical studies can reveal the underlying mechanisms of these effects and allow us to draw linking hypotheses with known neurophysiological effects of attention.
[review, spatial attention, spatial resolution]
Spering M & Carrasco M (2015).
Acting without seeing: Eye movements reveal visual processing without awareness.
Trends in Neurosciences.
Visual perception and eye movements are considered to be tightly linked. Diverse fields, ranging from developmental psychology to computer science, utilize eye tracking to measure visual perception. However, this prevailing view has been challenged by recent behavioral studies. Here, we review converging evidence revealing dissociations between the contents of perceptual awareness and different types of eye movement. Such dissociations reveal situations in which eye movements are sensitive to particular visual features that fail to modulate perceptual reports. We also discuss neurophysiological, neuroimaging, and clinical studies supporting the role of subcortical pathways for visual processing without awareness. Our review links awareness to perceptual-eye movement dissociations and furthers our understanding of the brain pathways underlying vision and movement with and without awareness.
[review, eye movements]
Szinte M, Carrasco M, Cavanagh P & Rolfs M (2015).
Attentional trade-offs maintain the tracking of moving objects across saccades.
Journal of Neurophysiology.
In many situations like playing sports or driving a car, we keep track of moving objects, despite the frequent eye movements that drastically interrupt their retinal motion trajectory. Here we report evidence that transsaccadic tracking relies on trade-offs of attentional resources from a tracked object’s motion path to its remapped location. While participants covertly tracked a moving object, we presented pulses of coherent motion at different locations to probe the allocation of spatial attention along the object’s entire motion path. Changes in the sensitivity for these pulses showed that during fixation attention shifted smoothly in anticipation of the tracked object’s displacement. However, just before a saccade, attentional resources were withdrawn from the object’s current motion path and reflexively drawn to the retinal location the object would have after saccade. This finding demonstrates the predictive choice the visual system makes to maintain the tracking of moving objects across saccades.
[spatial attention, eye movements]
Cutrone EK, Heeger DJ & Carrasco M (2014).
Attention enhances contrast appearance via increased input baseline of neural responses.
Journal of Vision.
Covert spatial attention increases the perceived contrast of stimuli at attended locations, presumably via enhancement of visual neural responses. However, the relation between perceived contrast and the underlying neural responses has not been characterized. In this study, we systematically varied stimulus contrast, using a two-alternative, forced-choice comparison task to probe the effect of attention on appearance across the contrast range. We modeled performance in the task as a function of underlying neural contrast-response functions. Fitting this model to the observed data revealed that an increased input baseline in the neural responses accounted for the enhancement of apparent contrast with spatial attention.
[modeling; spatial attention, appearance, contrast]
Grubb MA, White AL, Heeger DJ & Carrasco M (2014).
Interactions between voluntary and involuntary attention modulate the quality and temporal dynamics of visual processing.
Psychonomic Bulletin & Review.
Successfully navigating a dynamic environment requires the efficient distribution of finite neural resources. Voluntary (endogenous) covert spatial attention selectively allocates those processing resources to goal-relevant locations in the visual scene in the absence of eye movements. However, the allocation of spatial attention is not always voluntary; abrupt onsets in the visual periphery automatically enhance processing of nearby stimuli (exogenous attention). In dynamic environments, exogenous events and internal goals likely compete to determine the distribution of attention, but how such competition is resolved is not well understood. To investigate how exogenous events interact with the con- current allocation of voluntary attention, we used a speed– accuracy trade-off (SAT) procedure. SAT conjointly measures the rate of information accrual and asymptotic discriminability, allowing us to measure how attentional interactions unfold over time during stimulus processing. We found that both types of attention sped information accrual and improved discriminability. However, focusing endogenous attention at the target location reduced the effects of exogenous cues on the rate of information accrual and rendered negligible their effects on asymptotic discriminability. We verified the robustness of these findings in four additional experiments that targeted specific, critical response delays. In conclusion, the speed and quality of visual processing depend conjointly on internally and externally driven attentional states, but it is possible to voluntarily diminish distraction by irrelevant events in the periphery.
[speed accuracy tradeoff; spatial attention]
Szpiro SFA, Wright BA & Carrasco M (2014).
Learning one task by interleaving practice with another task.
Perceptual learning is a sustainable improvement in performance on a perceptual task following training. A hallmark of perceptual learning is task specificity – after participants have trained on and learned a particular task, learning rarely transfers to another task, even with identical stimuli. Accordingly, it is assumed that performing a task throughout training is a requirement for learning to occur on that specific task. Thus, interleaving training trials of a target task, with those of another task, should not improve performance on the target task. However, recent findings in audition show that interleaving two tasks during training can facilitate perceptual learning, even when the training on neither task yields learning on its own. Here we examined the role of cross-task training in the visual domain by training 4 groups of human observers for 3 consecutive days on an orientation comparison task (target task) and/or spatial–frequency comparison task (interleaving task). Interleaving small amounts of training on each task, which were ineffective alone, not only enabled learning on the target orientation task, as in audition, but also surpassed the learning attained by training on that task alone for the same total number of trials. This study illustrates that cross-task training in visual perceptual learning can be more effective than single-task training. The results reveal a comparable learning principle across modalities and demonstrate how to optimize training regimens to maximize perceptual learning.
[perceptual learning, orientation, spatial frequency]
Szpiro SFA, Spering M & Carrasco M (2014).
Perceptual learning modifies untrained pursuit eye movements.
Journal of Vision.
Perceptual learning improves detection and discrimination of relevant visual information in mature humans, revealing sensory plasticity. Whether visual perceptual learning affects motor responses is unknown. Here we implemented a protocol that enabled us to address this question. We tested a perceptual response (motion direction estimation, in which observers overestimate motion direction away from a reference) and a motor response (voluntary smooth pursuit eye movements). Perceptual training led to greater overestimation and, remarkably, it modified untrained smooth pursuit. In contrast, pursuit training did not affect overestimation in either pursuit or perception, even though observers in both training groups were exposed to the same stimuli for the same time period. A second experiment revealed that estimation training also improved discrimination, indicating that overestimation may optimize perceptual sensitivity. Hence, active perceptual training is necessary to alter perceptual responses, and an acquired change in perception suffices to modify pursuit, a motor response.
[eye movements, perceptual learning]
Kim S, Al-Haj M, Fuller S, Chen S, Jain U, Carrasco M & Tannock R (2014).
Color vision in ADHD: Part 2 - Does attention influence color perception?
Behavioral and Brain Functions.
Background: To investigate the impact of exogenous covert attention on chromatic (blue and red) and achromatic visual perception in adults with and without Attention Deficit Hyperactivity Disorder (ADHD). Exogenous covert attention, which is a transient, automatic, stimulus-driven form of attention, is a key mechanism for selecting relevant information in visual arrays. Methods: 30 adults diagnosed with ADHD and 30 healthy adults, matched on age and gender, performed a psychophysical task designed to measure the effects of exogenous covert attention on perceived color saturation (blue, red) and contrast sensitivity. Results: The effects of exogenous covert attention on perceived blue and red saturation levels and contrast sensitivity were similar in both groups, with no differences between males and females. Specifically, exogenous covert attention enhanced the perception of blue saturation and contrast sensitivity, but it had no effect on the perception of red saturation. Conclusion: The findings suggest that exogenous covert attention is intact in adults with ADHD and does not account for the observed impairments in the perception of chromatic (blue and red) saturation.
[spatial attention, ADHD, color, contrast sensitivity]
Kim S, Al-Haj M, Chen S, Fuller S, Jain U, Carrasco M & Tannock R (2014).
Colour vision in ADHD: Part 1 - Testing the retinal dopaminergic hypothesis.
Behavioral and Brain Functions.
Objectives: To test the retinal dopaminergic hypothesis, which posits deficient blue color perception in ADHD, resulting from hypofunctioning CNS and retinal dopamine, to which blue cones are exquisitely sensitive. Also, purported sex differences in red color perception were explored. Methods: 30 young adults diagnosed with ADHD and 30 healthy young adults, matched on age and gender, performed a psychophysical task to measure blue and red color saturation and contrast discrimination ability. Visual function measures, such as the Visual Activities Questionnaire (VAQ) and Farnsworth-Munsell 100 hue test (FMT), were also administered. Results: Females with ADHD were less accurate in discriminating blue and red color saturation relative to controls but did not differ in contrast sensitivity. Female control participants were better at discriminating red saturation than males, but no sex difference was present within the ADHD group. Conclusion: Poorer discrimination of red as well as blue color saturation in the female ADHD group may be partly attributable to a hypo-dopaminergic state in the retina, given that color perception (blue-yellow and red-green) is based on input from S-cones (short wavelength cone system) early in the visual pathway. The origin of female superiority in red perception may be rooted in sex-specific functional specialization in hunter-gather societies. The absence of this sexual dimorphism for red colour perception in ADHD females warrants further investigation.
[spatial attention, ADHD, color, contrast sensitivity]
Carrasco M (2014).
Spatial Attention: Perceptual modulation.
The Oxford Handbook of Attention.
[book chapter; spatial attention]
Grubb MA, Behrmann M, Egan R, Minshew NJ, Heeger DJ & Carrasco M (2013).
Exogenous spatial attention: Evidence for intact functioning in adults with autism spectrum disorder.
Journal of Vision.
Abstract Deficits or atypicalities in attention have been reported in individuals with autism spectrum disorder (ASD), yet no consensus on the nature of these deficits has emerged. We conducted three experiments that paired a peripheral precue with a covert discrimination task, using protocols for which the effects of covert exogenous spatial attention on early vision have been well established in typically developing populations. Experiment 1 assessed changes in contrast sensitivity, using orientation discrimination of a contrast-defined grating; Experiment 2 evaluated the reduction of crowding in the visual periphery, using discrimination of a letter-like figure with flanking stimuli at variable distances; and Experiment 3 assessed improvements in visual search, using discrimination of the same letter-like figure with a variable number of distractor elements. In all three experiments, we found that exogenous attention modulated visual discriminability in a group of high-functioning adults with ASD and that it did so in the same way and to the same extent as in a matched control group. We found no evidence to support the hypothesis that deficits in exogenous spatial attention underlie the emergence of core ASD symptomatology.
[spatial attention, ASD]
Anton-Erxleben K & Carrasco M (2013).
Attentional enhancement of spatial resolution: Linking behavioral and neurophysiological evidence.
Nature Reviews Neuroscience.
Attention allows us to select relevant sensory information for preferential processing. Behaviourally, it improves performance in various visual tasks. One prominent effect of attention is the modulation of performance in tasks that involve the visual system’s spatial resolution. Physiologically, attention modulates neuronal responses and alters the profile and position of receptive fields near the attended location. Here, we develop a hypothesis linking the behavioural and electrophysiological evidence. The proposed framework seeks to explain how these receptive field changes enhance the visual system’s effective spatial resolution and how the same mechanisms may also underlie attentional effects on the representation of spatial information.
[review; spatial attention, spatial resolution]
Ferneyhough E, Kim MK, Phelps EA & Carrasco M (2013).
Anxiety modulates the effects of motion and attention on early vision.
Cognition & Emotion.
At attended locations emotion and attention interact to benefit contrast sensitivity, a basic visual dimension. Whether there are associated costs at unattended locations is unknown. Furthermore, emotion and attention affect response time, and anxiety modulates these effects. We investigated how trait-anxiety influences the interaction of emotion and attention on contrast sensitivity. On each trial, non-predictive pre-cues (neutral or fearful faces) directed exogenous attention to four contrast- varying, tilted stimuli (Gabor patches). Attention was cued toward the target (valid), a distracter (invalid), or distributed over all locations. Observers discriminated target orientation, and completed self-report measures of anxiety. Effects of fearful expressions were mediated by trait anxiety. Only high-trait-anxious individuals showed decreased target contrast sensitivity after attention was diverted to a distracter by a fearful cue, and anxiety score correlated with degree of impairment across participants. This indicates that increasing anxiety exacerbates threat-related attentional costs to visual perception, hampering processing at non-threat-related locations.
[spatial attention, emotion, contrast sensitivity, orientation discrimination]
Grubb MA, Behrmann M, Egan R, Minshew NJ, Carrasco M & Heeger DJ (2013).
Endogenous spatial attention: Evidence for intact functioning in adults with autism.
Rapid manipulation of the attention field (i.e. the location and spread of visual spatial attention) is a critical aspect of human cognition, and previous research on spatial attention in individuals with autism spectrum disorders (ASD) has produced inconsistent results. In a series of three psychophysical experiments, we evaluated claims in the literature that individuals with ASD exhibit a deficit in voluntarily controlling the deployment and size of the spatial attention field. We measured the spatial distribution of performance accuracies and reaction times to quantify the sizes and locations of the attention field, with and without spatial uncertainty (i.e. the lack of predictability concerning the spatial position of the upcoming stimulus). We found that high-functioning adults with autism exhibited slower reaction times overall with spatial uncertainty, but the effects of attention on performance accuracies and reaction times were indistinguishable between individuals with autism and typically developing individuals in all three experiments. These results provide evidence of intact endogenous spatial attention function in high-functioning adults with ASD, suggesting that atypical endogenous attention cannot be a latent characteristic of autism in general.
[spatial attention, ASD]
White AL, Rolfs M & Carrasco M (2013).
Adaptive deployment of spatial and feature-based attention before saccades.
What you see depends not only on where you are looking but also on where you will look next. The pre-saccadic attention shift is an automatic enhancement of visual sensitivity at the target of the next saccade. We investigated whether and how perceptual factors independent of the oculomotor plan modulate pre-saccadic attention within and across trials. Observers made saccades to one (the target) of six patches of moving dots and discriminated a brief luminance pulse (the probe) that appeared at an unpredictable location. Sensitivity to the probe was always higher at the target’s location (spatial attention), and this attention effect was stronger if the previous probe appeared at the previous target’s location. Furthermore, sensitivity was higher for probes moving in directions similar to the target’s direction (feature- based attention), but only when the previous probe moved in the same direction as the previous target. Therefore, implicit cognitive processes permeate pre-saccadic attention, so that–contingent on recent experience–it flexibly distributes resources to potentially relevant locations and features.
[spatial attention, feature based attention, presaccadic attention, eye movements, motion perception]
Carrasco M, Eckstein M, Krauzlis R & Verghese (2013).
Attentional modulation: Target selection, active search and cognitive processing.
Rolfs M, Lawrence BM & Carrasco M (2013).
Reach preparation enhances visual performance and appearance.
Philosophical Transactions of the Royal Society B.
We investigated the impact of the preparation of reach movements on visual perception by simultaneously quantifying both an objective measure of visual sensitivity and the subjective experience of apparent contrast. Using a two-by-two alternative forced choice task, observers compared the orientation (clockwise or counterclockwise) and the contrast (higher or lower) of a Standard Gabor and a Test Gabor, the latter of which was presented during reach preparation, at the reach target location or the opposite location. Discrimination performance was better overall at the reach target than at the opposite location. Perceived contrast increased continuously at the target relative to the opposite location during reach preparation, that is, after the onset of the cue indicating the reach target. The finding that performance and appearance do not evolve in parallel during reach preparation points to a distinction with saccade preparation, for which we have shown previously there is a parallel temporal evolution of performance and appearance. Yet akin to saccade preparation, this study reveals that overall reach preparation enhances both visual performance and appearance.
[reach, appearance, orientation discrimination, contrast]
Pomplun M, Garaas TW & Carrasco M (2013).
The effects of task difficulty on visual search strategy in virtual 3D displays.
Journal of Vision.
Analyzing the factors that determine our choice of visual search strategy may shed light on visual behavior in everyday situations. Previous results suggest that increasing task difficulty leads to more systematic search paths. Here we analyze observers’ eye movements in an ‘‘easy’’ conjunction search task and a ‘‘difficult’’ shape search task to study visual search strategies in stereoscopic search displays with virtual depth induced by binocular disparity. Standard eye-movement variables, such as fixation duration and initial saccade latency, as well as new measures proposed here, such as saccadic step size, relative saccadic selectivity, and xy target distance, revealed systematic effects on search dynamics in the horizontal-vertical plane throughout the search process. We found that in the ‘‘easy’’ task, observers start with the processing of display items in the display center immediately after stimulus onset and subsequently move their gaze outwards, guided by extrafoveally perceived stimulus color. In contrast, the ‘‘difficult’’ task induced an initial gaze shift to the upper- left display corner, followed by a systematic left-right and top-down search process. The only consistent depth effect was a trend of initial saccades in the easy task with smallest displays to the items closest to the observer. The results demonstrate the utility of eye- movement analysis for understanding search strategies and provide a first step toward studying search strategies in actual 3D scenarios.
[eye movements, visual search]
White AL, Lunau R & Carrasco M (2013).
The attentional effects of single cues and color singletons on visual sensitivity.
Journal of Experimental Psychology: Human Perception and Performance.
Sudden changes in the visual periphery can automatically draw attention to their locations. For example, the brief flash of a single object (a “cue”) rapidly enhances contrast sensitivity for subsequent stimuli in its vicinity. Feature singletons (e.g., a red circle among green circles) can also capture attention in a variety of tasks. Here, we evaluate whether a peripheral cue that enhances contrast sensitivity when it appears alone has a similar effect when it appears as a color singleton, with the same stimuli and task. In four experiments we asked observers to report the orientation of a target Gabor stimulus, which was preceded by an uninformative cue array consisting either of a single disk or of 16 disks containing a color or luminance singleton. Accuracy was higher and contrast thresholds lower when the single cue appeared at or near the target’s location, compared with farther away. The color singleton also modulated performance but to a lesser degree and only when it appeared exactly at the target’s location. Thus, this is the first study to demonstrate that cueing by color singletons, like single cues, can enhance sensory signals at an early stage of processing.
[spatial attention, contrast sensitivity]
Anton-Erxleben K, Herrmann K & Carrasco M (2012).
Independent effects of adaptation and attention on perceived speed.
Adaptation and attention are two mechanisms by which sensory systems manage limited bioenergetic resources: Whereas adaptation decreases sensitivity to stimuli just encountered, attention increases sensitivity to behaviorally relevant stimuli. In the visual system, these changes in sensitivity are accompanied by a change in the appearance of different stimulus dimensions, such as speed. Adaptation causes an underestimation of speed, whereas attention leads to an overestimation of speed. In the two experiments reported here, we investigated whether the effects of these mechanisms interact and how they affect the appearance of stimulus features. We tested the effects of adaptation and the subsequent allocation of attention on perceived speed. A quickly moving adaptor decreased the perceived speed of subsequent stimuli, whereas a slow adaptor did not alter perceived speed. Attention increased perceived speed regardless of the adaptation effect, which indicates that adaptation and attention affect perceived speed independently. Moreover, the finding that attention can alter perceived speed after adaptation indicates that adaptation is not merely a by-product of neuronal fatigue.
[spatial attention, adaptation, appearance, motion]
Rolfs M & Carrasco M (2012).
Rapid simulations enhancement of visual sensitivity and perceived contrast during saccade preparation.
The Journal of Neuroscience.
Humans and other animals with foveate vision make saccadic eye movements to prioritize the visual analysis of behaviorally relevant information. Even before movement onset, visual processing is selectively enhanced at the target of a saccade, presumably gated by brain areas controlling eye movements. Here we assess concurrent changes in visual performance and perceived contrast before saccades, and show that saccade preparation enhances perception rapidly, altering early visual processing in a manner akin to increasing the physical contrast of the visual input. Observers compared orientation and contrast of a test stimulus, appearing briefly before a saccade, to a standard stimulus, presented previously during a fixation period. We found simultaneous progressive enhancement in both orientation discrimination performance and perceived contrast as time approached saccade onset. These effects were robust as early as 60 ms after the eye movement was cued, much faster than the voluntary deployment of covert attention (without eye movements), which takes 300 ms. Our results link the dynamics of saccade preparation, visual performance, and subjective experience and show that upcoming eye movements alter visual processing by increasing the signal strength.
[presaccadic attention, eye movements, orientation discrimination, contrast, appearance]
Perini F, Cattaneo L, Carrasco M & Schwarzbach JV (2012).
Occipital transcranial magnetic stimulation has an activity-dependent suppressive effect.
The Journal of Neuroscience.
The effects of transcranial magnetic stimulation (TMS) vary depending on the brain state at the stimulation moment. Four mechanisms have been proposed to underlie these effects: (1) virtual lesion—TMS suppresses neural signals; (2) preferential activation of less active neurons— TMS drives up activity in the stimulated area, but active neurons are saturating; (3) noise generation—TMS adds random neuronal activity, and its effect interacts with stimulus intensity; and (4) noise generation—TMS adds random neuronal activity, and its effect depends on TMS intensity. Here we explore these hypotheses by investigating the effects of TMS on early visual cortex by assessing the contrast response function while varying the adaptation state of the observers. We tested human participants in an orientation discrimination task, in which performance is contingent upon contrast sensitivity. Before each trial, neuronal activation of visual cortex was altered through contrast adaptation to two flickering gratings. In a factorial design, with or without adaptation, a single TMS pulse was delivered simultaneously with targets of varying contrast. Adaptation decreased contrast sensitivity. The effect of TMS on performance was state dependent: TMS decreased contrast sensitivity in the absence of adaptation but increased it after adaptation. None of the proposed mechanisms can account for the results in their entirety, in particular, for the facilitatory effect at intermediate to high contrasts after adaptation. We propose an alternative hypothesis: TMS effects are activity dependent, so that TMS suppresses the most active neurons and thereby changes the balance between excitation and inhibition.
[TMS; adaptation, orientation discrimination, contrast sensitivity]
Barbot A, Landy MS & Carrasco M (2012).
Differential effects of exogenous and endogenous attention on second-order texture contrast sensitivity.
Journal of Vision.
The visual system can use a rich variety of contours to segment visual scenes into distinct perceptually coherent regions. However, successfully segmenting an image is a computationally expensive process. Previously we have shown that exogenous attention—the more automatic, stimulus-driven component of spatial attention—helps extract contours by enhancing contrast sensitivity for second-order, texture-defined patterns at the attended location, while reducing sensitivity at unattended locations, relative to a neutral condition. Interestingly, the effects of exogenous attention depended on the second-order spatial frequency of the stimulus. At parafoveal locations, attention enhanced second-order contrast sensitivity to relatively high, but not to low second-order spatial frequencies. In the present study we investigated whether endogenous attention—the more voluntary, conceptually-driven component of spatial attention—affects second-order contrast sensitivity, and if so, whether its effects are similar to those of exogenous attention. To that end, we compared the effects of exogenous and endogenous attention on the sensitivity to second-order, orientation-defined, texture patterns of either high or low second-order spatial frequencies. The results show that, like exogenous attention, endogenous attention enhances second-order contrast sensitivity at the attended location and reduces it at unattended locations. However, whereas the effects of exogenous attention are a function of the second-order spatial frequency content, endogenous attention affected second-order contrast sensitivity independent of the second-order spatial frequency content. This finding supports the notion that both exogenous and endogenous attention can affect second-order contrast sensitivity, but that endogenous attention is more flexible, benefitting performance under different conditions.
[spatial attention, contrast sensitivity, texture segmentation, second-order processing]
Raio CM, Carmel D, Carrasco M & Phelps EA (2012).
Nonconscious fear is quickly acquired but swiftly forgotten.
[emotion, fear learning]
Herrmann K, Heeger DJ & Carrasco M (2012).
Feature-based attention enhances performance by increasing response gain.
Covert spatial attention can increase contrast sensitivity either by changes in contrast gain or by changes in response gain, depending on the size of the attention field and the size of the stimulus (Herrmann et al., 2010), as predicted by the normalization model of attention (Reynolds & Heeger, 2009). For feature-based attention, unlike spatial attention, the model predicts only changes in response gain, regardless of whether the featural extent of the attention field is small or large. To test this prediction, we measured the contrast dependence of feature-based attention. Observers performed an orientation-discrimination task on a spatial array of grating patches. The spatial locations of the gratings were varied randomly so that observers could not attend to specific locations. Feature-based attention was manipulated with a 75% valid and 25% invalid pre-cue, and the featural extent of the attention field was manipulated by introducing uncertainty about the upcoming grating orientation. Performance accuracy was better for valid than for invalid pre-cues, consistent with a change in response gain, when the featural extent of the attention field was small (low uncertainty) or when it was large (high uncertainty) relative to the featural extent of the stimulus. These results for feature-based attention clearly differ from results of analogous experiments with spatial attention, yet both support key predictions of the normalization model of attention.
[modeling; feature based attention, contrast sensitivity]
Spering M & Carrasco M (2012).
Similar effects of feature-based attention on motion perception and pursuit eye movements at different levels of awareness.
The Journal of Neuroscience.
Feature-based attention enhances visual processing and improves perception, even for visual features that we are not aware of. Does feature-based attention also modulate motor behavior in response to visual information that does or does not reach awareness? Here we compare the effect of feature-based attention on motion perception and smooth-pursuit eye movements in response to moving dichoptic plaids—stimuli composed of two orthogonally drifting gratings, presented separately to each eye—in human observers. Monocular adaptation to one grating before the presentation of both gratings renders the adapted grating perceptually weaker than the unadapted grating and decreases the level of awareness. Feature-based attention was directed to either the adapted or the unadapted grating’s motion direction or to both (neutral condition). We show that observers were better at detecting a speed change in the attended than the unattended motion direction, indicating that they had successfully attended to one grating. Speed change detection was also better when the change occurred in the unadapted than the adapted grating, indicating that the adapted grating was perceptually weaker. In neutral conditions, perception and pursuit in response to plaid motion were dissociated: While perception followed one grating’s motion direction almost exclusively (component motion), the eyes tracked the average of both gratings (pattern motion). In attention conditions, perception and pursuit were shifted toward the attended component. These results suggest that attention affects perception and pursuit similarly even though only the former reflects awareness. The eyes can track an attended feature even if observers do not perceive it.
[feature based attention, eye movements, motion perception, adaptation, awareness]
Abrams J, Nizam A & Carrasco M (2012).
Isoeccentric locations are not equivalent: The extent of the vertical meridian asymmetry.
Performance in visual tasks is limited by the low-level mechanisms that sample the visual field. It is well documented that contrast sensitivity and spatial resolution decrease as a function of eccentricity and that those factors impair performance in ‘‘higher level’’ tasks, such as visual search. Performance also varies consistently at isoeccentric locations in the visual field. Specifically, at a fixed eccentricity, performance is better along the horizontal meridian than the vertical meridian, and along the lower than the upper vertical meridian. Whether these asymmetries in visual performance fields are confined to the vertical meridian or extend across the whole upper versus lower visual hemifield has been a matter of debate. Here, we measure the extent of the upper versus lower asymmetry. Results reveal that this asymmetry is most pronounced at the vertical meridian and that it decreases gradually as the angular distance (polar angle) from the vertical meridian increases, with eccentricity held constant. Beyond 30° of polar angle from the vertical meridian, the upper to lower asymmetry is no longer reliable. Thus, the vertical meridian is uniquely asymmetric and uniquely insensitive. This pattern of results is consistent with early anatomical properties of the visual system and reflects constraints that are critical to our understanding of visual information processing.
[performance fields, orientation discrimination]
Anton-Erxleben K, Abrams J & Carrasco M (2011).
Equality judgments cannot distinguish between attention effects on appearance and criterion: A reply to Schneider (2011).
Journal of Vision.
Whether attention modulates the appearance of stimulus features is debated. Whereas many previous studies using a comparative judgment have found evidence for such an effect, two recent studies using an equality judgment have not. Critically, these studies have relied on the assumption that the equality paradigm yields bias-free PSE estimates and is as sensitive as the comparative judgment, without testing these assumptions. Anton-Erxleben, Abrams, and Carrasco (2010) compared comparative judgments and equality judgments with and without the manipulation of attention. They demonstrated that the equality paradigm is less sensitive than the comparative judgment and also bias-prone. Furthermore, they reported an effect of attention on the PSE using both paradigms. Schneider (2011) questions the validity of the latter finding, stating that the data in the equality experiment are corrupted because of skew in the response distributions. Notably, this argument supports the original conclusion by Anton-Erxleben et al.: that the equality paradigm is bias-prone. Additionally, the necessary analyses to show that the attention effect observed in Anton-Erxleben et al. was due to skew in the data were not conducted. Here, we provide these analyses and show that although the equality judgment is bias-prone, the effects we observe are consistent with an increase of apparent contrast by attention.
[modeling; spatial attention, appearance, contrast]
Pestilli F, Carrasco M, Heeger DJ & Gardner JL (2011).
Attentional enhancement via selection and poling of early sensory responses in human visual cortex.
The computational processes by which attention improves behavioral performance were characterized by measuring visual cortical activity with functional magnetic resonance imaging as humans performed a contrast-discrimination task with focal and distributed attention. Focal attention yielded robust improvements in behavioral performance accompanied by increases in cortical responses. Quantitative analysis revealed that if performance were limited only by the sensitivity of the measured sensory signals, the improvements in behavioral performance would have corresponded to an unrealistically large reduction in response variability. Instead, behavioral performance was well characterized by a pooling and selection process for which the largest sensory responses, those most strongly modulated by attention, dominated the perceptual decision. This characterization predicts that high-contrast distracters that evoke large responses should negatively impact behavioral performance. We tested and confirmed this prediction. We conclude that attention enhanced behavioral performance predominantly by enabling efficient selection of the behaviorally relevant sensory signals.
[fMRI, eye-tracking; spatial attention, contrast]
Corbett JE & Carrasco M (2011).
Visual performance fields: Frames of reference.
Performance in most visual discrimination tasks is better along the horizontal than the vertical meridian (Horizontal-Vertical Anisotropy, HVA), and along the lower than the upper vertical meridian (Vertical Meridian Asymmetry, VMA), with intermediate performance at intercardinal locations. As these inhomogeneities are prevalent throughout visual tasks, it is important to understand the perceptual consequences of dissociating spatial reference frames. In all studies of performance fields so far, allocentric environmental references and egocentric observer reference frames were aligned. Here we quantified the effects of manipulating head-centric and retinotopic coordinates on the shape of visual performance fields. When observers viewed briefly presented radial arrays of Gabors and discriminated the tilt of a target relative to homogeneously oriented distractors, performance fields shifted with head tilt (Experiment 1), and fixation (Experiment 2). These results show that performance fields shift in-line with egocentric referents, corresponding to the retinal location of the stimulus.
[eye-tracking; performance fields, spatial reference frames, orientation discrimination]
Carrasco M (2011).
Visual attention: The past 25 years.
This review focuses on covert attention and how it alters early vision. I explain why attention is considered a selective process, the constructs of covert attention, spatial endogenous and exogenous attention, and feature-based attention. I explain how in the last 25 years research on attention has characterized the effects of covert attention on spatial filters and how attention influences the selection of stimuli of interest. This review includes the effects of spatial attention on discriminability and appearance in tasks mediated by contrast sensitivity and spatial resolution; the effects of feature-based attention on basic visual processes, and a comparison of the effects of spatial and feature-based attention. The emphasis of this review is on psychophysical studies, but relevant electrophysiological and neuroimaging studies and models regarding how and where neuronal responses are modulated are also discussed.
White AL & Carrasco M (2011).
Feature-based attention involuntarily and simultaneously improves visual performance across locations.
Journal of Vision.
Selective attention can selectively increase sensitivity to particular visual features in order to prioritize behaviorally relevant stimuli. Moreover, neural responses to attended feature values are boosted even at ignored locations. We provide behavioral evidence for involuntary and simultaneous effects of this “global” feature-based attention on visual performance. Observers were cued to attend to dots moving in a particular direction at one location (the primary task), while discriminating which of two groups of moving dots on the other side of the screen contained coherent motion (the secondary task). An analogous experiment tested selective attention to orientation. The secondary tasks did not require observers to discriminate or selectively attend to the particular feature values present. Nonetheless, sensitivity was highest when the direction or orientation happened to match the one cued in the primary task. By comparing performance to a neutral condition, we revealed more enhancement of attended feature values than suppression of others.
[FBA, orientation, motion]
Barbot A, Landy MS & Carrasco M (2011).
Exogenous attention enhances 2nd-order contrast sensitivity.
Natural scenes contain a rich variety of contours that the visual system extracts to segregate the retinal image into perceptually coherent regions. Covert spatial attention helps extract contours by enhancing contrast sensitivity for 1st-order, luminance-defined patterns at attended locations, while reducing sensitivity at unattended locations, relative to neutral attention allocation. However, humans are also sensitive to 2nd-order patterns such as spatial variations of texture, which are predominant in natural scenes and cannot be detected by linear mechanisms. We assess whether and how exogenous attention—the involuntary and transient capture of spatial attention—affects the contrast sensitivity of channels sensitive to 2nd-order, texture-defined patterns. Using 2nd-order, texture-defined stimuli, we demonstrate that exogenous attention increases 2nd-order contrast sensitivity at the attended location, while decreasing it at unattended locations, relative to a neutral condition. By manipulating both 1st- and 2nd-order spatial frequency, we find that the effects of attention depend both on 2nd-order spatial frequency of the stimulus and the observer’s 2nd-order spatial resolution at the target location. At parafoveal locations, attention enhances 2nd-order contrast sensitivity to high, but not to low 2nd-order spatial frequencies; at peripheral locations attention also enhances sensitivity to low 2nd-order spatial frequencies. Control experiments rule out the possibility that these effects might be due to an increase in contrast sen- sitivity at the 1st-order stage of visual processing. Thus, exogenous attention affects 2nd-order contrast sensitivity at both attended and unattended locations.
[eye-tracking; spatial attention, contrast sensitivity, texture segmentation, second-order processing]
Spering M, Pomplun M & Carrasco M (2011).
Tracking without perceiving: A dissociation between eye movements and motion perception.
Can people react to objects in their visual field that they do not consciously perceive? We investigated how visual perception and motor action respond to moving objects whose visibility is reduced, and we found a dissociation between motion processing for perception and for action. We compared motion perception and eye movements evoked by two orthogonally drifting gratings, each presented separately to a different eye. The strength of each monocular grating was manipulated by inducing adaptation to one grating prior to the presentation of both gratings. Reflexive eye movements tracked the vector average of both gratings (pattern motion) even though perceptual responses followed one motion direction exclusively (component motion). Observers almost never perceived pattern motion.This dissociation implies the existence of visual-motion signals that guide eye movements in the absence of a corresponding conscious percept.
[eye-tracking; eye movements, motion]
Herrmann K, Montaser-Kouhsari L, Carrasco M & Heeger DJ (2010).
When size matters: attention affects performance by contrast or response gain.
Covert attention, the selective processing of visual information in the absence of eye movements, improves behavioral performance. We found that attention, both exogenous (involuntary) and endogenous (voluntary), can affect performance by contrast or response gain changes, depending on the stimulus size and the relative size of the attention field. These two variables were manipulated in a cueing task while stimulus contrast was varied. We observed a change in behavioral performance consonant with a change in contrast gain for small stimuli paired with spatial uncertainty and a change in response gain for large stimuli presented at one location (no uncertainty) and surrounded by irrelevant flanking distracters. A complementary neuroimaging experiment revealed that observers’ attention fields were wider with than without spatial uncertainty. Our results support important predictions of the normalization model of attention and reconcile previous, seemingly contradictory findings on the effects of visual attention.
[fMRI, modeling; spatial attention, size, contrast]
Anton-Erxleben K, Abrams J & Carrasco M (2010).
Evaluating comparative and equality judgments in contrast perception: Attention alters appearance.
Journal of Vision.
Covert attention not only improves performance in many visual tasks but also modulates the appearance of several visual features. Studies on attention and appearance have assessed subjective appearance using a task contingent upon a comparative judgment (e.g., M. Carrasco, S. Ling, & S. Read, 2004). Recently, K. A. Schneider and M. Komlos (2008) questioned the validity of those results because they did not !nd a signi!cant effect of attention on contrast appearance using an equality task. They claim that such equality judgments are bias-free whereas comparative judgments are bias- prone and propose an alternative interpretation of the previous !ndings based on a decision bias. However, to date there is no empirical support for the superiority of the equality procedure. Here, we compare biases and sensitivity to shifts in perceived contrast of both paradigms. We measured contrast appearance using both a comparative and an equality judgment. Observers judged the contrasts of two simultaneously presented stimuli, while either the contrast of one stimulus was physically incremented (Experiments 1 and 2) or exogenous attention was drawn to it (Experiments 3 and 4). We demonstrate several methodological limitations of the equality paradigm. Nevertheless, both paradigms capture shifts in PSE due to physical and perceived changes in contrast and show that attention enhances apparent contrast.
[modeling; spatial attention, appearance, contrast]
Abrams J, Barbot A & Carrasco M (2010).
Voluntary attention increases perceived spatial frequency.
Attention, Perception & Psychophysics.
Voluntary covert attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenology has received little empirical investigation. Involuntary attention increases perceived spatial frequency (Gobell & Carrasco, 2005), but involuntary attention can differ from voluntary attention in its effects on performance in tasks mediated by spatial resolution (Yeshurun, Montagna, & Carrasco, 2008). Therefore, we ask whether voluntary attention affects the subjective appearance of spatial frequency—a fundamental dimension of visual perception underlying spatial resolution. We used a demanding rapid serial visual presentation task to direct voluntary attention and measured perceived spatial frequency at the attended and unattended locations. Attention increased the perceived spatial frequency of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task. In the control experiment, we ruled out response bias as an alternative account by using a lengthened interstimulus interval, which allows observers to disengage attention from the cued location. In contrast to the main experiment, the observers showed neither increased perceived spatial frequency nor improved orientation discrimination at the attended location. Thus, this study establishes that voluntary attention increases perceived spatial frequency. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.
[spatial attention, appearance, spatial frequency]
Carrasco M (2010).
Attention: Effects on perception.
Encyclopedia of Perception.
[book chapter; attention]
Ferneyhough E, Stanley DA, Phelps EA & Carrasco M (2010).
Cuing effects of faces are dependent on handedness and visual field.
Psychonomic Bulletin & Review.
Faces are unlike other visual objects we encounter, in that they alert us to potentially relevant social information. Both face processing and spatial attention are dominant in the right hemisphere of the human brain, with a stronger lateralization in right- than in left-handers. Here, we demonstrate behavioral evidence for an effect of handedness on performance in tasks using faces to direct attention. Nonpredictive, peripheral cues (faces or dots) directed exogenous attention to contrast-varying stimuli (Gabor patches)—a tilted target, a vertical distractor, or both; observers made orientation discriminations on the target stimuli. Whereas cuing with dots increased contrast sensitivity in both groups, cuing with faces increased contrast sensitivity in right- but not in left-handers, for whom opposite hemifield effects resulted in no net increase. Our results reveal that attention modulation by face cues critically depends on handedness and visual hemifield. These previously unreported interactions suggest that such lateralized systems may be functionally connected.
Carrasco M (2009).
Cross-modal attention enhances perceived contrast.
In this issue of PNAS, Störmer, McDonald, and Hillyard (1) investigate the neurophysiological basis of phenomenological experience, probing a central question in perception: Does attention alter our subjective experience of the world? Their study is the first to show converging evidence from human electrophysiology and behavior indicating that attention alters appearance.
[commentary; attention, appearance, contrast]
Carrasco M & Yeshurun Y (2009).
Covert attention effects on spatial resolution.
Progress in Brain Research.
First, we review the characteristics of endogenous (sustained) and exogenous (transient) spatial covert attention. Then we examine the effects of these two types of attention on spatial resolution in a variety of tasks, such as acuity, visual search, and texture segmentation. Both types of covert attention enhance resolution; directing attention to a given location allows us to better resolve the fine details of the visual scene at that location. With exogenous attention, but not with endogenous attention, this is the case even when enhanced spatial resolution hampers performance. The enhanced resolution at the attended location comes about at the expense of lower resolution at the unattended locations.
[book chapter; spatial attention, spatial resolution]
Carrasco M, Eckstein M, Verghese P, Boynton G & Treue S (2009).
Visual attention: Neurophysiology, psychophysics and cognitive neuroscience.
Fuller S, Park Y & Carrasco M (2009).
Cue contrast modulates the effects of exogenous attention on appearance.
Exogenous spatial attention can be automatically engaged by a cue presented in the visual periphery. To investigate the effects of exogenous attention, previous studies have generally used highly salient cues that reliably trigger attention. However, the cueing threshold of exogenous attention has been unexamined. We investigated whether the attentional effect varies with cue salience. We examined the magni- tude of the attentional effect on apparent contrast [Carrasco, M., Ling, S., & Read, S. (2004). Attention alters appearance. Nature Neuroscience, 7(3), 308–313.] elicited by cues with negative Weber contrast between 6% and 100%. Cue contrast modulated the attentional effect, even at cue contrasts above the level at which observers can perfectly localize the cue; hence, the result is not due to an increase in cue visibility. No attentional effect is observed when the 100% contrast cue is presented after the stimuli, ruling out cue bias or sensory interaction between cues and stimuli as alternative explanations. A second experiment, using the same paradigm with high contrast motion stimuli gave similar results, providing further evidence against a sensory interaction explanation, as the stimuli and task were defined on a visual dimension independent from cue contrast. Although exogenous attention is triggered automatically and involuntarily, the attentional effect is gradual.
[spatial attention; appearance]
Giordano AM, McElree B & Carrasco M (2009).
On the automaticity and flexibility of covert attention: A speed-accuracy trade-off analysis.
Journal of Vision.
Exogenous covert attention improves discriminability and accelerates the rate of visual information processing (M. Carrasco & B. McElree, 2001). Here we investigated and compared the effects of both endogenous (sustained) and exogenous (transient) covert attention. Specifically, we directed attention via spatial cues and evaluated the automaticity and flexibility of exogenous and endogenous attention by manipulating cue validity in conjunction with a response-signal speed-accuracy trade-off (SAT) procedure, which provides conjoint measures of discriminability and information accrual. To investigate whether discriminability and rate of information processing differ as a function of cue validity (chance to 100%), we compared how both types of attention affect performance while keeping experimental conditions constant. With endogenous attention, both the observed benefits (valid-cue) and the costs (invalid-cue) increased with cue validity. However, with exogenous attention, the benefits and costs in both discriminability and processing speed were similar across cue validity conditions. These results provide compelling time-course evidence that whereas endogenous attention can be flexibly allocated according to cue validity, exogenous attention is automatic and unaffected by cue validity.
Montagna B, Pestilli F & Carrasco M (2009).
Attention trades off spatial acuity.
Covertly attending to a stimulus location increases spatial acuity. Is such increased spatial acuity coupled with a decreased acuity at unattended locations? We measured the effects of exogenous (transient and involuntary) and endogenous (sustained and voluntary) attention on observers’ acuity thresholds for a Landolt gap resolution task at both attended and unattended locations. Both types of attention increased acuity at the attended and decreased it at unattended locations relative to a neutral baseline condition. These trade-off findings support the idea that limited processing resources affect early vision, even when the display is impoverished and there is no location uncertainty. There was no benefit without a cost.
[spatial attention; spatial acuity]
Montaser-Kouhsari L & Carrasco M (2009).
Perceptual asymmetries are preserved in short-term memory tasks.
Attention, Perception, & Psychophysics.
Visual performance is heterogeneous at isoeccentric locations; it is better on the horizontal than on the vertical meridian and worse at the upper than at the lower region of the vertical meridian (Carrasco, Talgar, & Cameron, 2001; Talgar & Carrasco, 2002). It is unknown whether these performance inhomogeneities are also present in spatial frequency tasks and whether asymmetries present during encoding of visual information also emerge in visual short-term memory (VSTM) tasks. Here, we investigated the similarity of the perceptual and VSTM tasks in spatial frequency discrimination (Experiments 1 and 2) and perceived spatial frequency (Experiments 3 and 4). We found that (1) performance in both simultaneous (perceptual) and delayed (VSTM) spatial frequency discrimi- nation tasks varies as a function of location; it is better along the horizontal than along the vertical meridian; and (2) perceived spatial frequency in both tasks is higher along the horizontal than along the vertical meridian. These results suggest that perceived spatial frequency may mediate performance differences in VSTM tasks across the visual field, implying that the quality with which we encode information affects VSTM.
[performance fields, appearance, visual short-term memory, spatial frequency]
Fuller S & Carrasco M (2009).
Perceptual consequences of visual performance fields: The case of the line motion illusion.
Journal of Vision.
Illusory line motion (ILM) is the illusion that a line, preceded by a small dot (cue) near one end, is perceived to shoot out from the dot even though the line is physically presented at once. Does this illusion result from a low-level motion effect, a gradient of exogenous spatial attention, or both? Given that exogenous attention speeds visual processing unequally at isoeccentric cardinal locations (M. Carrasco, A. M. Giordano, & B. McElree, 2004), we hypothesized that the contribution of attention to ILM would follow the same pattern. We characterized psychometric functions of perceived line motion direction, for 1.5° stimuli with varying amounts of physical line motion (8 levels) at four cardinal locations. We used three cue conditions to separate the effects of attention from low-level motionVa single cue to draw focal attention to the stimulus location, a distributed cue with elements near all four possible stimulus locations, and no visual cue. Distributed and single cues generate identical effects along the horizontal meridian, but the effect of the single cue is progressively greater along the vertical meridian, more so at the top location (“North”). We conclude that the low-level motion explanation accounts for the majority of the canonical example of the ILM (line preceded by a single dot) effect used in our study.
[spatial attention, performance fields, illusions, motion, appearance]
Pestilli F, Ling S & Carrasco M (2009).
A population-coding model of attention's influence on contrast response: Estimating neural effects from psychophysical data.
Human psychophysics and monkey physiology studies have shown that attention modulates early vision – contrast sensitivity and processing. But how can we bridge the effects of attention on perceptual per- formance to their neural underpinnings? Here we implement a population-coding model that estimates attentional effects on population contrast response given psychophysical data. Model results show that whereas endogenous (sustained, voluntary) attention changes population contrast-response via contrast gain, exogenous (transient, involuntary) attention changes population contrast-response via response gain.
[modeling; spatial attention, contrast]
Liu T, Abrams J & Carrasco M (2009).
Voluntary attention enhances contrast appearance.
Voluntary (endogenous, sustained) covert spatial attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenology has received little empirical investigation. We asked whether voluntary attention affects the subjective appearance of contrast—a fundamental dimension of visual perception. We used a demanding rapid serial visual presentation (RSVP) task to direct endogenous attention to a given location and mea- sured perceived contrast at the attended and unattended locations. Attention increased perceived contrast of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task at the cued location. We ruled out response bias as an alternative ac- count of the pattern of results. Thus, this study establishes that voluntary attention enhances perceived contrast. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.
[spatial attention, appearance, contrast]
Ling S, Liu T & Carrasco M (2009).
How spatial and feature-based attention affect the gain and tuning of population responses.
How does attention optimize our visual system for the task at hand? Two mechanisms have been pro- posed for how attention improves signal processing: gain and tuning. To distinguish between these two mechanisms we use the equivalent-noise paradigm, which measures performance as a function of external noise. In the present study we explored how spatial and feature-based attention affect perfor- mance by assessing their threshold-vs-noise (TvN) curves with regard to the signature behavioral effects of gain and tuning. Furthermore, we link our psychophysical results to neurophysiology by implementing a simple, biologically-plausible model to show that attention affects the gain and tuning of population responses differentially, depending on the type of attention being deployed: Whereas spatial attention operates by boosting the gain of the population response, feature-based attention operates by both boost- ing the gain and sharpening the tuning of the population response.
[modeling; spatial attention, feature based attention, motion]
Carrasco M (2009).
Attention: Psychophysical approaches.
The Oxford Companion to Consciousness.
Attention is the mechanism that allows us to selectively process the vast amount of information that we receive and to guide our behaviour. Visual spatial attention can be deployed overtly, accompanied by eye movements to the relevant location, or covertly, without eye movements (Helmholtz 1910/1925, Posner 1980). There are two types of covert attention: sustained attention refers to the voluntary, endogenous directing of attention to a location in the visual field, and transient attention refers to the automatic, exogenous capture of attention to a location, brought about by a sudden change in the environment (Nakayama and Mackeben 1989, Posner 1980). Following a procedure devised by Posner (1980), whereby observers are cued to attend to specific locations while keeping their gaze at a central fixation point, many studies have characterized the effects of covert attention on perception. Attention improves performance (higher accuracy and shorter reaction times) on many tasks, involving several dimensions of early vision (contrast sensitivity and spatial resolution), such as detection, discrimination, localization, and visual search. Moreover, attention increases the haemodynamic response, an index of neural activity, in visual areas (for reviews see Reynolds and Chelazzi 2004, Carrasco 2006).
[book chapter; spatial attention]
Carrasco M, Fuller S & Ling S (2008).
Transient attention does increase perceived contrast of supra threshold stimuli: A reply to Prinzmetal, Long & Leonhardt.
Perception & Psychophysics.
Carrasco, Ling, and Read (2004) showed that transient attention increases perceived contrast. However, Prinz metal, Long, and Leonhardt (2008) suggest that for targets of low visibility, observers may bias their response to ward the cued location, and they propose a cuebias explanation for our previous results. Our response is threefold. First, we outline several key methodological differences between the studies that could account for the different results. We conclude that the cuebias hypothesis is a plausible explanation for Prinzmetal et al.’s (2008) results, given the characteristics of their stimuli, but not for the studies by Carrasco and colleagues, in which the stimuli were suprathreshold (Carrasco, Ling, & Read, 2004; Fuller, Rodriguez, & Carrasco, 2008; Ling & Carrasco, 2007). Second, we conduct a study to show that the stimuli used in our previous studies are not nearthreshold, but suprathreshold (Experiment 1, Phase 1). Furthermore, we found an increase in apparent contrast for a high contrast stimulus when it was precued, but not when it was postcued, providing more evidence against a cuebias hypothesis (Experiment 1, Phase 2). We also show that the visibility of the stimuli in Prinzmetal et al. (2008) was much lower than that of Carrasco, Ling, and Read, rendering their stimuli susceptible to their cuebias explanation (Experiment 2). Third, we present a comprehensive summary of all the control conditions used in different labs that have ruled out a cue bias explanation of the appearance studies. We conclude that a cuebias explanation may operate with nearthreshold and lowvisibility stimuli, as was the case in Prinzmetal et al. (2008), but that such an explanation has no bearing on studies with suprathreshold stimuli. Consistent with our previous studies, the present data support the claim that attention does alter the contrast appearance of suprathreshold stimuli.
[spatial attention, appearance, contrast]
Yeshurun Y, Carrasco M & Maloney LT (2008).
Bias and sensitivity in two-interval forced choice procedures: Tests of the difference model.
We assess four common claims concerning the two-interval forced choice (2-IFC) procedure and the standard Difference Model of 2-IFC performance. The first two are (1) that it is unbiased and (2) that the structure of the 2-IFC task does not in itself alter sensitivity. The remaining two concern a claimed √2 enhancement in sensitivity in 2-IFC relative to that measured in a Yes–No task. We review relevant past research and re-analyze seventeen experiments from previous studies across three laboratories. We then report an experiment comparing 2-IFC performance with performance in a second task designed to elucidate observers’ decision processes. This second task is simply two successive Yes–No signal detection tasks with the same timing as in the 2-IFC experiment. We find little evidence supporting the claims that 2-IFC is unbiased and that it does not alter sensitivity and we also reject the two claims associated with the Difference Model as a model of performance in our own experiment.
[review; modeling; decision making, methodology]
Carmel D & Carrasco M (2008).
Perceptual learning and dynamic changes in primary visual cortex.
Perceptual learning is the improved performance that follows practice in a perceptual task. In this issue of Neuron, Yotsumoto et al. use fMRI to show that stimuli presented at the location used in training initially evoke greater activation in primary visual cortex than stimuli presented elsewhere, but this difference disappears once learning asymptotes.
[commentary, fMRI; perceptual learning]
Fuller S, Rodriguez RZ & Carrasco M (2008).
Apparent contrast differs across the vertical meridian: Visual and attentional factors.
Journal of Vision.
It is known that visual performance is better on the horizontal than the vertical meridian, and in the lower than the upper region of the vertical meridian (Vertical Meridian Asymmetry, “VMA”), and that exogenous spatial attention increases the apparent contrast of a stimulus. Here we investigate whether the VMA also leads to differences in the subjective appearance of contrast between the upper and lower vertical meridian, and how the effects of exogenous spatial attention on appearance interact with the VMA. Two Gabor stimuli were presented North and South of fixation at 4- eccentricity along the vertical meridian. Observers were asked to report the orientation of the Gabor that was higher in contrast. By assessing which stimulus observers perceived to be higher in contrast, we obtained psychometric functions and their concomitant points of subjective equality (PSE). These functions were measured both when a neutral cue was presented in the middle of the display and transient attention was deployed via a peripheral cue to the location of one of the stimuli. Observers were told that the cues were uninformative as to the stimulus contrast or its orientation. We report two novel findings. First, apparent contrast is higher on the lower vertical meridian than on the upper. Second, the attentional enhancement of apparent contrast is asymmetrical with both low and high contrast stimuli; the effect of exogenous spatial attention is greater on the lower than the upper vertical meridian. As in prior studies, we find no corresponding asymmetry in orientation discrimination. Signal detection-based models explain the asymmetrical appearance effects as a function of differential multiplicative gain factors for the North and South locations, and predict a similar but much smaller asymmetry for orientation discrimination.
[modeling; spatial attention, performance fields, contrast, appearance]
Yeshurun Y, Montagna B & Carrasco M (2008).
On the flexibility of sustained attention and its effects on a texture segmentation task.
Previously we have shown that transient attention—the more automatic, stimulus-driven component of spatial attention—enhances spatial resolution. Specifically, transient attention improves texture segmentation at the periphery, where spatial resolution is too low, but impairs performance at central locations, where spatial resolution is already too high for the task. In the present study we investigated whether sustained attention—the more controlled component of spatial attention—can also affect texture segmentation, and if so whether its effect will be similar to that of transient attention. To that end we combined central, symbolic cues with texture displays in which the target appears at several eccentricities. We found that sustained attention can also affect texture segmentation, but unlike transient attention, sustained attention improved performance at all eccentricities. Comparing the effect of pre-cues and post-cues indicated that the benefit brought about by sustained attention is significantly greater than the effect of location uncertainty reduction. These findings indicate that sustained attention is a more flexible mechanism that can optimize performance at all eccentricities in a task where performance is constrained by spatial resolution.
[spatial attention, texture segmentation, spatial resolution]
Yeshurun Y & Carrasco M (2008).
The effects of transient attention on spatial resolution and the size of the attentional cue.
Perception & Psychophysics.
It has been shown that transient attention enhances spatial resolution, but is the effect of transient attention on spatial resolution modulated by the size of the attentional cue? Would a gradual increase in the size of the cue lead to a gradual decrement in spatial resolution? To test these hypotheses, we used a texture segmentation task in which performance depends on spatial resolution, and systematically manipulated the size of the attentional cue: A bar of different lengths (Experiment 1) or a frame of different sizes (Experiments 2–3) indicated the target region in a texture segmentation display. Observers indicated whether a target patch region (oriented line elements in a background of an orthogonal orientation), appearing at a range of eccentricities, was present in the first or the second interval. We replicated the attentional enhancement of spatial resolution found with small cues; attention improved performance at peripheral locations but impaired performance at central locations. However, there was no evidence of gradual resolution decrement with large cues. Transient attention enhanced spatial resolution at the attended location when it was attracted to that location by a small cue but did not affect resolution when it was attracted by a large cue. These results indicate that transient attention cannot adapt its operation on spatial resolution on the basis of the size of the attentional cue.
[spatial attention, texture segmentation, spatial resolution]
Ling S & Carrasco M (2007).
Transient covert attention does alter appearance: A reply to Schneider.
Perception & Psychophysics.
We recently demonstrated that transient covert attention increases the apparent contrast of a stimulus (Carrasco, Ling, & Read, 2004). Schneider (2006) proposes that the observed increase in apparent contrast is largely due to sensory interactions occurring between the precue and stimulus, rather than to attention. Specifically, he reports that cuing effects only occur at contrasts near detection threshold, and that there are confounding sensory inter- actions between the cue and stimulus at suprathreshold detection contrasts. Our response is twofold. First, we outline the key methodological differences between our original study and Schneider’s that are likely to account for the different results, and explain how we had ruled out the sensory interaction explanation of the cue. Second, we directly test the prediction put forth by Schneider: If the effects were due to sensory interactions, reversing the luminance polarity of the precue in our paradigm should lead to differential cuing effects. We replicate one of the experiments of our original study and add a condition in which the cue luminance is either black or white. Our results replicated our previous findings—they showed an increase in apparent contrast to a high-contrast stimulus when it was precued. Moreover, we found that the black cue and the white cue had the same effect, thus ruling out the alternative explanation proposed by Schneider. Transient attention does alter contrast appearance.
[spatial attention, appearance, contrast]
Liu T, Larsson J & Carrasco M (2007).
Feature-based attention modulates orientation selective responses in human visual cortex.
How does feature-based attention modulate neural responses? We used adaptation to quantify the effect of feature-based attention on orientation-selective responses in human visual cortex. Observers were adapted to two superimposed oblique gratings while attending to one grating only. We measured the magnitude of attention-induced orientation-selective adaptation both psychophysically, by the behavioral tilt aftereffect, and physiologically, using fMRI response adaptation. We found evidence for orientation-selective attentional modulation of neuronal responses—a lower fMRI response for the attended than the unattended orientation—in multiple visual areas, and a significant correlation between the magnitude of the tilt aftereffect and that of fMRI response adaptation in V1, the earliest site of orientation coding. These results show that feature-based attention can selectively increase the response of neuronal subpopulations that prefer the attended feature, even when the attended and unattended features are coded in the same visual areas and share the same retinotopic location.
[fMRI; feature-based attention, adaptation, orientation]
Pestilli F, Viera G & Carrasco M (2007).
How do attention and adaptation affect contrast sensitivity.
Journal of Vision.
Attention and adaptation are both mechanisms that optimize visual performance. Attention optimizes performance by increasing contrast sensitivity for and neural response to attended stimuli while decreasing them for unattended stimuli; adaptation optimizes performance by increasing contrast sensitivity for and neural response to changing stimuli while decreasing them for unchanging stimuli. We investigated whether and how the adaptation state and the attentional effect on contrast sensitivity interact. We measured contrast sensitivity with an orientation-discrimination task, in two adaptation conditions - adapt to 0% or 100% contrast - in focused, distributed, and withdrawn attentional conditions. We used threshold and asymptotic performance to index the magnitude of the attentional effect - enhancement or impairment in contrast sensitivity - before and after adapting to high-contrast stimuli. The results show that attention and adaptation affect the contrast psychometric function in a similar but opposite way: Attention increases stimulus salience, whereas adaptation reduces stimulus salience. An interesting finding is that the adaptation state does not modulate the magnitude of the attentional effect. This suggests that attention affects the normalized signal once the effect of contrast adaptation has taken place and that these two mechanisms act separately to change contrast sensitivity. Attention can overcome adaptation to restore contrast sensitivity.
[spatial attention, adaptation, contrast sensitivity]
Liu T, Stevens ST & Carrasco M (2007).
Comparing the time course and efficacy of spatial and feature-based attention.
We investigated the time course of feature-based attention and compared it to the time course of spatial attention in an experiment with identical stimuli and task. Observers detected a speed increment in a compound motion stimulus preceded by cues that indicated either the target location or direction. The cue-target stimulus-onset-asynchrony (SOA) was varied to assess the time course of the attentional effect. We found that spatial attention was deployed earlier than feature-based attention and that both types of attention improved performance to a similar extent at a longer SOA. Results indicate that attention is a flexible mechanism allowing us to efficiently select task-relevant information based on either spatial or feature dimensions, but that spatial attention exert its effects faster.
[spatial attention, feature-based attention, motion]
Liu T, Heeger DJ & Carrasco M (2006).
Neural correlates of the visual vertical meridian asymmetry.
Journal of Vision.
Human visual performance is better below than above fixation along the vertical meridian - a phenomenon we refer to as vertical meridian asymmetry (VMA). Here, we used fMRI to investigate the neural correlates of the VMA. We presented stimuli of two possible sizes and spatial frequencies on the horizontal and vertical meridians and analyzed the fMRI data in subregions of early visual cortex (V1/V2) that corresponded retinotopically to the stimulus locations. Asymmetries in both the spatial extent and amplitude of the fMRI measurements correlated with the behavioral VMA. These results demonstrate that the VMA has a neural basis at the earliest stages of cortical visual processing and imply that visual performance is limited by the pooled sensory responses of large populations of neurons in the visual cortex.
[fMRI; performance fields, spatial frequency]
Carrasco M (2006).
Covert attention increases contrast sensitivity, psychophysical, neurophysiological, and neuroimaging studies.
Progress in Brain Research.
This chapter focuses on the effect of covert spatial attention on contrast sensitivity, a basic visual dimension where the best mechanistic understanding of attention has been achieved. I discuss how models of contrast sensitivity, as well as the confluence of psychophysical, single-unit recording, and neuroimaging studies, suggest that attention increases contrast sensitivity via contrast gain, an effect akin to a change in the physical contrast stimulus. I suggest possible research directions and ways to strengthen the interaction among different levels of analysis to further our understanding of visual attention.
[book chapter; spatial attention, contrast sensitivity]
Liu T, Fuller S & Carrasco M (2006).
Attention alters the appearance of motion coherence.
Psychonomic Bulletin & Review.
Selective attention enhances visual information processing, as measured by behavioral performance and neural activity. However, little is known about its effects on subjective experience. Here, we investigated the effect of transient (exogenous) attention on the appearance of visual motion, using a psychophysical procedure that directly measures appearance and controls for response bias. Observers viewed pairs of moving dot patterns and reported the motion direction of the more coherent pattern. Directing attention (via a peripheral precue) to a stimulus location increased its perceived coherence level and improved performance on a direction discrimination task. In a control experiment, we ruled out response bias by lengthening the time interval between the cue and the stimuli, so that the effect of transient attention could no longer be exerted. Our results are consistent with those of neurophysiological studies showing that attention modulates motion processing and provide evidence of a subjective perceptual correlate of attention, with a concomitant effect on performance.
[spatial attention, appearance, motion]
Carrasco M, Loula F & Ho Y-X (2006).
How attention enhances spatial resolution: Evidence from selective adaptation to spatial frequency.
Perception and Psychophysics.
In this study, we investigated how spatial resolution and covert attention affect performance in a texture segmentation task in which performance peaks at midperiphery and drops at peripheral and central retinal locations. The central impairment is called the central performance drop (CPD; Kehrer, 1989). it has been established that attending to the target location improves performance in the periphery where resolution is too low for the task, but impairs it at central locations where resolution is too high. This is called the central attention impairment (Cai; Yeshurun & Carrasco, 1998, 2000). We employed a cuing procedure in conjunction with selective adaptation to explore (1) whether the CPD is due to the inhibition of low spatial frequency responses by high spatial frequency responses in central locations, and (2) whether the Cai is due to attention’s shifting sensitivity to higher spatial frequencies. We found that adaptation to low spatial frequencies does not change performance in this texture segmentation task. However, adaptation to high spatial frequencies diminishes the CPD and eliminates the Cai. These results indicate that the CPD is primarily due to the dominance of high spatial frequency responses and that covert attention enhances spatial resolution by shifting sensitivity to higher spatial frequencies.
[adaptation; spatial attention, texture segmentation, spatial resolution, spatial frequency]
Fuller S & Carrasco M (2006).
Exogenous attention and color perception: Performance and appearance of saturation and hue.
Exogenous covert attention is an automatic, transient form of attention that can be triggered by sudden changes in the periphery. Here we test for the effects of attention on color perception. We used the methodology developed by Carrasco, Ling, and Read [Carrasco, M., Ling, S., & Read, S. (2004). Attention alters appearance. Nature Neuroscience, 7 (3) 308–313] to explore the effects of exogenous attention on appearance of saturation (Experiment 1) and of hue (Experiment 2). We also tested orientation discrimination performance for single stimuli defined by saturation or hue (Experiment 3). The results indicate that attention increases apparent saturation, but does not change apparent hue, notwithstanding the fact that it improves orientation discrimination for both saturation and hue stimuli.
[spatial attention, appearance, color, orientation]
Ling S & Carrasco M (2006).
When sustained attention impairs perception.
Virtually all behavioral and neurophysiological studies have shown that sustained (endogenous, conceptually driven) attention enhances perception. But can this enhancement be held indefinitely? We assessed the time course of attention’s effects on contrast sensitivity, reasoning that if attention does indeed boost stimulus strength, the strengthened representation could result in stronger adaptation over time. We found that attention initially enhances contrast sensitivity, but that over time sustained attention can actually impair sensitivity to an attended stimulus.
[spatial attention, appearance, contrast sensitivity]