Biology subjects

Carrasco, M.

Publications and source records attributed to Carrasco, M..

At least 37 records · Page 2Linked to original sources

bioRxiv2024

Spatial attention alters visual cortical representation during target anticipation

Attention enables us to efficiently and flexibly interact with the environment by prioritizing specific image locations and features in preparation for responding to stimuli. Using a concurrent psychophysics-fMRI experiment, we investigated how covert spatial attention modulates responses in human visual cortex before target onset and how it affects subsequent behavioral performance. Performance improved at cued locations and worsened at uncued locations compared to distributed attention, demonstrating a selective processing tradeoff. Pre-target BOLD responses in cortical visual field maps revealed two key changes: First, a stimulus-independent baseline shift, with increases near cued locations and decreases elsewhere, paralleling behavioral results. Second, a shift in population receptive field centers toward the attended location. Both effects increased in higher visual areas. Together, these findings reveal that spatial attention has large effects on visual cortex prior to target appearance, altering neural response properties across multiple visual field maps and enhancing performance through anticipatory mechanisms.

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bioRxiv2024

Do microsaccades vary with discriminability around the visual field?

Microsaccades-tiny fixational eye movements- improve discriminability in high acuity tasks in the foveola. To investigate whether they help compensate for low discriminability at perifovea, we examined MS characteristics relative to the adult visual performance field, which is characterized by two perceptual asymmetries: Horizontal-Vertical Anisotropy (better discrimination along the horizontal than vertical meridian), and Vertical Meridian Asymmetry (better discrimination along the lower-than upper-vertical meridian). We investigated whether and to what extent microsaccade directionality varies when stimuli are at isoeccentric locations along the cardinals under conditions of heterogeneous discriminability (Experiment 1) and homogeneous discriminability, equated by adjusting stimulus contrast (Experiment 2). Participants performed a two-alternative forced-choice orientation discrimination task. In both experiments, performance was better on trials without microsaccades between ready signal onset and stimulus offset than on trials with microsaccades. Across the trial sequence the microsaccade rate and directional pattern were similar across locations. Our results indicate that microsaccades were similar regardless of stimulus discriminability and target location, except during the response period-once the stimuli were no longer present and target location no longer uncertain-when microsaccades were biased toward the target location. Thus, this study reveals that microsaccades do not flexibly adapt as a function of varying discriminability in a basic visual task around the visual field.

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bioRxiv2023

When to attend? Temporal attention interacts with expectation

Temporal attention is voluntarily deployed at specific moments, whereas temporal expectation is deployed according to timing probabilities. When the target appears at an expected moment in a sequence, temporal attention improves performance at the attended moments, but the timing and the precision of the attentional window remain unknown. Here we independently and concurrently manipulated temporal attention-via behavioral relevance-and temporal expectation-via session-wise precision and trial-wise hazard rate-to investigate whether and how these mechanisms interact to improve perception. Our results reveal that temporal attention interacts with temporal expectation-the higher the precision, the stronger the attention benefit, but surprisingly this benefit decreased with delayed onset despite the increasing probability of stimulus appearance. When attention was suboptimally deployed to earlier than expected moments, it could not be reoriented to a later time point. These findings provide evidence that temporal attention and temporal expectation are different mechanisms, and highlight their interplay in optimizing visual performance. RelevanceOur ability to process visual information is limited both across space and time. Here we disentangle how two mechanisms-attention and expectation-help us overcome temporal limitations. We concurrently manipulated attention and expectation independently to investigate whether and how they interact. We found that temporal attention interacts with two distinct forms of expectation. Temporal expectation strengthens the benefits of temporal attention on performance for the attended time points, depending on how precise the expectations are. Surprisingly, the advantages of attention decrease when stimuli occur later than expected, suggesting a limitation of attention to reorient from earlier to later time points. This study provides further evidence that humans cannot sustain temporal attention even over short periods, reveals that although temporal attention and expectation interact to improve visual performance, expectation suboptimally guides attention, and highlights that attention and expectation are different temporal mechanisms.

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bioRxiv2023

Presaccadic attention enhances and reshapes the Contrast Sensitivity Function around the visual field

Contrast sensitivity, which constrains our vision, decreases from fovea to periphery, from the horizontal to the vertical meridian, and from the lower vertical to the upper vertical meridian. The Contrast Sensitivity Function (CSF) depicts how contrast sensitivity varies with spatial frequency (SF). To overcome these visual constraints, we constantly make saccadic eye movements to foveate on relevant objects in the scene. Already before saccade onset, presaccadic attention shifts to the saccade target and enhances perception. However, it is unknown whether and how it modulates the interplay between contrast sensitivity and SF, and if this effect varies around polar angle locations. Contrast sensitivity enhancement may result from a horizontal or vertical shift of the CSF, increase in bandwidth, or any combination. Here, we investigated these possibilities by extracting key attributes of the CSF using Hierarchical Bayesian Modeling, which enables precise estimation of the CSF parameters by decomposing the variability of the dataset into multiple levels. The results reveal that presaccadic attention (1) enhances contrast sensitivity across SF, (2) increases the most preferred and the highest discernable SF, and (3) narrows the bandwidth. Therefore, presaccadic attention bridges the gap between presaccadic and post-saccadic input by increasing visibility at the saccade target. Counterintuitively, the presaccadic enhancement in contrast sensitivity was more pronounced where perception is better -along the horizontal than the vertical meridian- exacerbating polar angle asymmetries. Our results call for an investigation of the differential neural modulations underlying presaccadic perceptual changes for different saccade directions. Significance statementThe contrast sensitivity function (CSF) describes how our ability to perceive contrast depends on spatial frequency. Contrast sensitivity is highest at the fovea and decreases in the periphery, especially at vertical locations. We thus make saccadic eye movements to view objects in detail. Already before moving our eyes, presaccadic attention enhances perception at the target location. But how does it influence the interplay between spatial frequency and contrast sensitivity, and does its effect vary around the visual field? Using Hierarchical Bayesian Modeling, we show that presaccadic attention enhances and reshapes the CSF to prepare the periphery for upcoming fixation. Counterintuitively, it does so more at horizontal locations where vision is stronger, suggesting smoother perception across horizontal than vertical eye movements.

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bioRxiv2023

Adaptation and exogenous attention interact in the early visual cortex: A TMS study

Transcranial magnetic stimulation (TMS) to early visual cortex modulates the effect of adaptation and eliminates the effect of exogenous (involuntary) attention on contrast sensitivity. Here we investigated whether adaptation modulates exogenous attention under TMS to V1/V2. Observers performed an orientation discrimination task while attending to one of two stimuli, with or without adaptation. Following an attentional cue, two stimuli were presented in the stimulated region and its contralateral symmetric region. A response cue indicated the stimulus whose orientation observers had to discriminate. Without adaptation, in the distractor-stimulated condition, contrast sensitivity increased at the attended location and decreased at the unattended location via response gain-but these effects were eliminated in the target-stimulated condition. Critically, after adaptation, exogenous attention altered performance similarly in both distractor-stimulated and target-stimulated conditions. These results reveal that (1) adaptation and attention interact in the early visual cortex, and (2) adaptation shields exogenous attention from TMS effects.

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bioRxiv2023

The Extrafoveal Preview Effect is More Pronounced Where Perception is Poor

The pre-saccadic preview of a peripheral target enhances the efficiency of its post-saccadic processing, termed the extrafoveal preview effect. Peripheral visual performance -and thus the quality of the preview- varies around the visual field, even at iso-eccentric locations: it is better along the horizontal than vertical meridian and along the lower than upper vertical meridian. To investigate whether these polar angle asymmetries influence the preview effect, we asked human participants (to preview four tilted gratings at the cardinals, until a central cue indicated to which one to saccade. During the saccade, the target orientation either remained or slightly changed (valid/invalid preview). After saccade landing, participants discriminated the orientation of the (briefly presented) second grating. Stimulus contrast was titrated with adaptive staircases to assess visual performance. Expectedly, valid previews increased participants post-saccadic contrast sensitivity. This preview benefit, however, was inversely related to polar angle perceptual asymmetries; largest at the upper, and smallest at the horizontal meridian. This finding reveals that the visual system compensates for peripheral asymmetries when integrating information across saccades, by selectively assigning higher weights to the less-well perceived preview information. Our study supports the recent line of evidence showing that perceptual dynamics around saccades vary with eye movement direction. Significance StatementWe constantly make saccadic eye movements to bring relevant visual information into the fovea, which has the highest acuity. Before each saccade, we use "previewed" peripheral information to support our post-saccadic vision. Our sensitivity varies around the visual field -at the same eccentricity it is best along the horizontal meridian and worst at the upper vertical meridian. An optimal visual system should rely more on previewed information with higher precision. Our study reveals the opposite: peripheral preview shapes subsequent post-saccadic foveal processing more at locations where peripheral vision is worse. This finding implies that the human visual system compensates for sensitivity differences around the visual field when integrating information across eye movements.

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bioRxiv2023

Featural representation and internal noise underlie the eccentricity effect in contrast sensitivity

Human visual performance for basic visual dimensions (e.g., contrast sensitivity and acuity) peaks at the fovea and decreases with eccentricity. The eccentricity effect is related to the larger surface area of the visual cortex corresponding to the fovea, but it is unknown if differential feature tuning contributes to this eccentricity effect. Here, we investigated two system-level computations underlying the eccentricity effect: featural representation (tuning) and internal noise. Observers (both sexes) detected a Gabor embedded in filtered white noise which appeared at the fovea or one of four perifoveal locations. We used psychophysical reverse correlation to estimate the weights assigned by the visual system to a range of orientations and spatial frequencies (SFs) in noisy stimuli, which are conventionally interpreted as perceptual sensitivity to the corresponding features. We found higher sensitivity to task-relevant orientations and SFs at the fovea than the perifovea, and no difference in selectivity for either orientation or SF. Concurrently, we measured response consistency using a double-pass method, which allowed us to infer the level of internal noise by implementing a noisy observer model. We found lower internal noise at the fovea than perifovea. Finally, individual variability in contrast sensitivity correlated with sensitivity to and selectivity for task-relevant features as well as with internal noise. Moreover, the behavioral eccentricity effect mainly reflects the foveal advantage in orientation sensitivity compared to other computations. These findings suggest that the eccentricity effect stems from a better representation of task-relevant features and lower internal noise at the fovea than at the perifovea. SignificancePerformance in many visual tasks worsens with eccentricity. Many studies attribute this eccentricity effect to retinal and cortical factors, like higher cone density and a larger cortical surface area representing the foveal than peripheral locations. We investigated whether system-level computations for task-relevant visual features also underlie this eccentricity effect. Measuring contrast sensitivity in visual noise, we showed that the fovea better represents task-relevant orientation and spatial frequency and has lower internal noise than the perifovea, and that individual variability in these two computations correlates with that in performance. These findings reveal that both representations of these basic visual features and internal noise underlie the difference in performance with eccentricity.

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bioRxiv2023

Visual field asymmetries vary between adolescents and adults

For human adults, visual perception varies with polar angle at isoeccentric locations from the center of gaze. The same visual information yields better performance along the horizontal than vertical meridian (horizontal vertical anisotropy, HVA) and along the lower than upper vertical meridian (vertical meridian asymmetry, VMA). For children, performance is better along the horizontal than vertical meridian (HVA) but does not differ between the lower and upper vertical meridian. Here, we investigated whether the extent of the HVA varies and whether the VMA emerges and develops during adolescence, or whether the VMA only emerges in adulthood. We found that both the HVA and VMA develop gradually throughout adolescence and become as pronounced as those of adults only in late adolescence.

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bioRxiv2023

Dissociable roles of human frontal eye fields and early visual cortex in presaccadic attention -- evidence from TMS

Shortly before each saccadic eye movement, presaccadic attention improves visual sensitivity at the saccade target1-5 at the expense of lowered sensitivity at non-target locations6-11. Some behavioral and neural correlates of presaccadic attention and covert attention -which likewise enhances sensitivity, but during fixation12-are similar13. This resemblance has led to the debatable13-18 notion that presaccadic and covert attention are functionally equivalent and rely on the same neural circuitry19-21. At a broad scale, oculomotor brain structures (e.g., FEF) are also modulated during covert attention22-24 - yet by distinct neuronal subpopulations25-28. Perceptual benefits of presaccadic attention rely on feedback from oculomotor structures to visual cortices29,30 (Fig. 1a); micro-stimulation of FEF in non-human primates affects activity in visual cortex31-34 and enhances visual sensitivity at the movement field of the stimulated neurons35-37. Similar feedback projections seem to exist in humans: FEF+ activation precedes occipital activation during saccade preparation38,39 and FEF TMS modulates activity in visual cortex40-42 and enhances perceived contrast in the contralateral hemifield40. We investigated presaccadic feedback in humans by applying TMS to frontal or visual areas during saccade preparation. By simultaneously measuring perceptual performance, we show the causal and differential roles of these brain regions in contralateral presaccadic benefits at the saccade target and costs at non-targets: Whereas rFEF+ stimulation reduced presaccadic costs throughout saccade preparation, V1/V2 stimulation reduced benefits only shortly before saccade onset. These effects provide causal evidence that presaccadic attention modulates perception through cortico-cortical feedback and further dissociate presaccadic and covert attention. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=127 SRC="FIGDIR/small/529691v2_fig1.gif" ALT="Figure 1"> View larger version (37K): org.highwire.dtl.DTLVardef@1567165org.highwire.dtl.DTLVardef@2ecec9org.highwire.dtl.DTLVardef@10a00adorg.highwire.dtl.DTLVardef@1b29dfd_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFigure 1.C_FLOATNO Background and experimental design. (a) Feedback connections originally established in non-human primates29,30,36,90,104,105 assumed to underly perceptual correlates of human presaccadic attention. FEF: Frontal eye field, SC: Superior Colliculus; (b) Response gain effect of presaccadic attention on the contrast response function: presaccadic attention scales the response by a multiplicative gain factor, resulting in an increase of the asymptotic response dmax (the maximal response achieved at high contrast)9,48. (c) Determining occipital (V1/V2) stimulation sites (Exp.1 & 2a) and stimulus placement for both experiments via phosphene mapping: Observers were stimulated laterally around the occipital pole until they perceived a phosphene (in the contralateral visual field), then drew its outline on the screen. The center of the phosphene drawings (stimulated region) and the symmetric region (not stimulated) were used for stimulus placement in the main experiment, where we applied sub-phosphene-threshold TMS using identical coil positioning. (d) Determining rFEF+ stimulation sites (Exp.2b). rFEF+ (yellow ROI) was localized on each individual observers anatomy (two exemplary observers shown here) via a probabilistic topography atlas76 and verified by anatomical landmarks (junction of the precentral and superior frontal sulcus; light gray areas)64,65,67. (e) Presaccadic orientation discrimination task. After a fixation period, a central direction cue (black line) appeared. Observers were instructed to make a saccade to the indicated target marked by placeholder dots. Note that the saccade was equally likely directed to the stimulated and to the symmetric region / hemifield. 100ms after cue onset, a tilted test Gabor patch was presented at either the saccade target (valid; 50%) or at the opposite location (invalid), randomly intermixed; a vertical Gabor was presented at the other location. Importantly, stimuli were presented during saccade preparation, i.e., while gaze was still at the screen center. After saccade offset, a response cue (white dots on placeholder) indicated the location at which the test Gabor had appeared, and observers reported its orientation. In the neutral condition (separately blocked) the cue pointed to both placeholders and observers kept fixating (supplemental Video S1 demonstrates the trials sequence of each condition). (f) Trial timeline. Observers received double-pulse TMS (50ms inter-pulse interval) locked to stimuli onset (Exp.1) or at various times during saccade preparation (0-200ms relative to cue onset; Exp.2). Whereas grating contrast was varied to measure contrast response functions in Exp.1, contrast was fixed to 46% in Exp.2. (g) Experimental conditions. The test was equally likely presented at the stimulated region (test stimulated) or in the opposite hemifield (not stimulated). Moreover, the test was equally likely presented at the saccade target (valid) or opposite of it (invalid). Note that the fixation condition (neutral) was only tested in Exp.1. C_FIG

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bioRxiv2022

Presaccadic attention depends on eye movement direction

With every saccadic eye movement, humans bring new information into their fovea to be processed with high visual acuity. Notably, perception is enhanced already before a relevant item is foveated: During saccade preparation, presaccadic attention shifts to the upcoming fixation location, which can be measured via behavioral correlates such as enhanced visual performance or modulations of sensory feature tuning. The coupling between saccadic eye movements and attention is assumed to be robust and mandatory, and considered a mechanism facilitating the integration of pre- and post-saccadic information. However, until recently it had not been investigated as a function of saccade direction. Here, we measured contrast response functions during fixation and saccade preparation in male and female observers and found that the pronounced response gain benefit typically elicited by presaccadic attention is selectively lacking before upward saccades at the group level - some observers even showed a cost. Individual observers sensitivity before upward saccades was negatively related to their amount of surface area in primary visual cortex representing the saccade target, suggesting a potential compensatory mechanism that optimizes the use of the limited neural resources processing the upper vertical meridian. Our results raise the question how perceptual continuity is achieved and upward saccades can be accurately targeted despite the lack of - theoretically required- presaccadic attention. Significance StatementWhen we make a saccadic eye movement to a target location in the visual field, perception improves at the saccade target, already before the eyes start moving. This benefit afforded by presaccadic attention is thought to be mandatory and independent of eye movement direction. We show that this is not the case; moving our eyes horizontally or downwards, but not upwards, enhances contrast sensitivity. At the neural level, however, humans with less V1 cortical tissue representing the target location for upwards saccades have some presaccadic enhancement. The finding that presaccadic attention is dependent upon eye movement direction challenges the view that the presaccadic benefit is automatic and mandatory in nature.

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bioRxiv2022

Dynamics of temporal attention and expectation revealed by continuous tracking of visual cortical responses

We can often anticipate the precise moment when a stimulus will be relevant for our behavioral goals. Voluntary temporal attention, the prioritization of sensory information at task-relevant time points, enhances visual perception. However, the neural mechanisms of voluntary temporal attention have not been isolated from those of temporal expectation, which reflects timing predictability rather than relevance. Here we used time-resolved steady-state visual evoked responses (SSVER) to investigate how temporal attention dynamically modulates visual activity when temporal expectation is controlled. We recorded MEG while observers directed temporal attention to one of two sequential grating targets with predictable timing. Meanwhile, we used a co-localized SSVER probe to continuously track visual cortical modulations both before and after the target stimuli. In the pre-target period, the SSVER gradually ramped up as the targets approached, reflecting temporal expectation. Furthermore, we found a low-frequency modulation of the SSVER, which shifted approximately half a cycle in phase according to which target was attended. In the post-target period, temporal attention to the first target transiently modulated the SSVER shortly after target onset. Thus, temporal attention dynamically modulates visual cortical responses via both periodic pre-target and transient post-target mechanisms to prioritize sensory information at precise moments.

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bioRxiv2022

Presaccadic attention sharpens visual acuity

Visual perception is limited by spatial resolution, which declines with eccentricity and differs around polar angle locations. To compensate for poor peripheral resolution, we make rapid eye movements -saccades- to bring peripheral objects into high-acuity foveal vision. Already before saccade onset, visual attention shifts to the saccade target location and prioritizes visual processing. This presaccadic shift of attention improves performance in many visual tasks, but whether it changes resolution is unknown. Here, we investigated whether presaccadic attention sharpens peripheral spatial resolution; and if so, whether such effect interacts with polar angle locations. We measured acuity thresholds in an orientation discrimination task during fixation and saccade preparation around the visual field. The results revealed that presaccadic attention sharpens acuity, which can facilitate a smooth transition from peripheral to foveal representation. This acuity enhancement is similar across the four cardinal locations; thus, presaccadic attention does not change polar angle differences in resolution.

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bioRxiv2022

Transcranial magnetic stimulation to frontal but not occipital cortex disrupts endogenous attention

Covert endogenous (voluntary) attention improves visual performance. Human neuroimaging studies suggest that the putative human homolog of macaque FEF (FEF+) is critical for this improvement, whereas early visual areas are not. Yet, MRI methods are correlational, as they do not manipulate brain function. Here we investigated whether rFEF+ or V1/V2 play a causal role in endogenous attention. We used transcranial magnetic stimulation (TMS) to alter activity in visual cortex (Exp.1) or rFEF+ (Exp.2) when observers performed an orientation discrimination task while attention was manipulated. On every trial, they received double-pulse TMS at a predetermined site (stimulated region) around the occipital pole or the rFEF+. Two cortically magnified gratings were presented, one in the stimulated region (contralateral to the stimulated cortical area) and another in the symmetric (ipsilateral) non-stimulated region. Grating contrast was varied to measure contrast response functions (CRFs) for all attention and stimulation combinations. In Exp.1, the CRFs were similar at the stimulated and non-stimulated regions, indicating that early visual areas do not modulate endogenous attention during stimulus presentation. In contrast, occipital TMS eliminates exogenous (involuntary) attention effects on performance (1). In Exp.2, rFEF+ stimulation decreased the overall attentional effect; neither benefits at the attended location nor cost at the unattended location were significant. This pattern is mimicked in the frequency and directionality of microsaccades: Whereas occipital stimulation did not affect microsaccades, rFEF+ stimulation caused a higher microsaccade rate selectively directed toward the stimulated hemifield. These results provide causal evidence of the role of this frontal region for endogenous attention. SIGNIFICANCE STATEMENTHuman neuroimaging studies have revealed activity in frontal regions (e.g., FEF+) as a neural correlate of endogenous (voluntary) attention, and early visual areas (V1/V2) as neural correlates of both endogenous and exogenous (involuntary) attention. Using a causal manipulation-transcranial magnetic stimulation-we show that briefly disrupting activity in rFEF+ weakens endogenous attentions benefits at attended and costs at unattended locations. In contrast, V1/V2 stimulation did not alter endogenous attention (although we have previously demonstrated that it eliminates effects of exogenous attention). Correspondingly, whereas stimulation to rFEF+ increased the rate of microsaccades directed toward the stimulated hemifield, occipital stimulation did not. Together, these results provide causal evidence for the role of rFEF+ but not V1/V2 in endogenous attention.

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bioRxiv2022

Perceptual learning improves discrimination while distorting appearance

Perceptual sensitivity often improves with training, a phenomenon known as perceptual learning. Another important perceptual dimension is appearance, the subjective sense of stimulus magnitude. Are training-induced improvements in sensitivity accompanied by more accurate appearance? Here, we examine this question by measuring both discrimination and estimation capabilities for near-horizontal motion perception, before and after training. Human observers who trained on either discrimination or estimation exhibited improvements in discrimination accuracy alongside increased biases in their estimates away from horizontal. To explain this counterintuitive finding, we developed a computational observer model in which perceptual learning arises from increases in the precision of underlying neural representations. For each observer, the fitted model accounted for both discrimination performance and the distribution of estimates, and their changes after training. Our empirical findings and modeling suggest that learning enhances distinctions between categories, a potentially important aspect of real-world perception and perceptual learning.

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bioRxiv2022

Comparing visual cortex of children and adults reveals a late-stage change in the architecture of V1

Adult visual performance differs with angular location -it is better for stimuli along the horizontal than vertical, and lower than upper vertical meridian of the visual field. These perceptual asymmetries are paralleled by asymmetries in cortical surface area in primary visual cortex (V1). Children, unlike adults, have similar visual performance at the lower and upper vertical meridian. Do children have similar V1 surface area representing the upper and lower vertical meridian? Using MRI, we measure the surface area of retinotopic maps (V1-V3) in children and adults. Many features of the maps are similar between groups, including greater V1 surface area for the horizontal than vertical meridian. However, unlike adults, children have a similar amount of V1 surface area representing the lower and upper vertical meridian. These data reveal a late-stage change in V1 organization that may relate to the emergence of the visual performance asymmetry along the vertical meridian by adulthood.

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bioRxiv2022

Cortical magnification underlies differences across but not around the visual field

Human visual performance changes dramatically both across (eccentricity) and around (polar angle) the visual field. Performance is better at the fovea, decreases with eccentricity, and is better along the horizontal than vertical meridian and along the lower than the upper vertical meridian. However, all neurophysiological and virtually all behavioral studies of cortical magnification have investigated eccentricity effects without considering polar angle. Most performance differences due to eccentricity are eliminated when stimulus size is cortically magnified (M-scaled) to equate the size of its cortical representation in primary visual cortex (V1). But does cortical magnification underlie performance differences around the visual field? Here, to assess contrast sensitivity, human adult observers performed an orientation discrimination task with constant stimulus size at different locations as well as when stimulus size was M-scaled according to stimulus eccentricity and polar angle location. We found that although M-scaling stimulus size eliminates differences across eccentricity, it does not eliminate differences around the polar angle. This finding indicates that limits in contrast sensitivity across eccentricity and around the visual field are mediated by different anatomical and computational constraints.

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bioRxiv2021

Linking contrast sensitivity to cortical magnification in human primary visual cortex

A central question in neuroscience is how the organization of cortical maps relates to perception, for which human primary visual cortex (V1) is an ideal model system. V1 nonuniformly samples the retinal image, with greater cortical magnification (surface area per degree of visual field) at the fovea than periphery and at the horizontal than vertical meridian. Moreover, the size and cortical magnification of V1 varies greatly across individuals. Here, we used fMRI and psychophysics in the same observers to quantify individual differences in V1 cortical magnification and contrast sensitivity at the four polar angle meridians. Across observers, the overall size of V1 and localized cortical magnification positively correlated with contrast sensitivity. Moreover, greater cortical magnification and higher contrast sensitivity at the horizontal than the vertical meridian were strongly correlated. These data reveal a link between cortical anatomy and visual perception at the level of individual observer and stimulus location.

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bioRxiv2021

Presaccadic attention enhances contrast sensitivity, but not at the upper vertical meridian

Human visual performance is not only better at the fovea and decreases with eccentricity, but also has striking radial asymmetries around the visual field: At a fixed eccentricity, it is better along (1) the horizontal than vertical meridian and (2) the lower than upper vertical meridian. These asymmetries are not alleviated by covert exogenous or endogenous attention, but have been studied exclusively during eye fixation. However, a major driver of everyday attentional orienting is saccade preparation, during which visual attention automatically shifts to the future eye fixation. This presaccadic shift of attention is considered strong and compulsory, and relies on fundamentally different neural computations and substrates than covert attention. Given these differences, we investigated whether presaccadic attention can compensate for the ubiquitous performance asymmetries observed during eye fixation. Our data replicate polar performance asymmetries during fixation and document the same asymmetries during saccade preparation. Crucially, however, presaccadic attention enhanced contrast sensitivity at the horizontal and lower vertical meridian, but not at the upper vertical meridian. Thus, instead of attenuating polar performance asymmetries, presaccadic attention exacerbates them.

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