Perceptual consequences of centre-surround antagonism in visual motion processing

Linking cortical surface area to computational properties in human visual perception

Cortical structure and function are closely linked, shaping the neural basis of human behavior. This study explores how cortical surface area (SA), a structural feature, influences computational properties in human visual perception. Using a combination of psychophysical, neuroimaging, and computational modeling approaches, we find that variations in SA across the parietal and frontal cortices are linked to distinct behavioral patterns in a motion perception task. These differences in behavior correspond to specific parameters within a divisive normalization model, indicating a unique contribution of SA to the spatial organization of cortical circuitry. This work highlights the importance of cortical architecture in modifying computational processes that underlie perception, enhancing our understanding of how structural differences can influence neural function and behavior.

Dissociation of focal and large-scale inhibitory functions in the older adults: A multimodal MRI study

BACKGROUND

The decline of inhibitory in cognitive aging is linked to reduced cognitive and mental capacities in older adults. However, this decline often shows inconsistent clinical presentations, suggesting varied impacts on different inhibition-related tasks. Inhibitory control, a multifaceted construct, involves various types of inhibition. Understanding these components is crucial for comprehending how aging affects inhibitory functions. Our research investigates the influences of aging on large-scale and focal-scale inhibitory and examines the relationship with brain markers.

METHODS

We examined the impact of aging on inhibitory in 18 younger (20-35 years) and 17 older adults (65-85 years) using focal and large-scale inhibition tasks. The Gabor task assessed focal-scale inhibition, while the Stop Signal Task (SST) evaluated large-scale inhibition. Participants underwent neuropsychological assessments and MRI scans, including magnetic resonance spectroscopy (MRS) and structural and resting fMRI.

RESULTS

Older adults exhibited a marked decline in inhibitory function, with slower SST responses indicating compromised large-scale inhibition. Conversely, the Gabor task showed no significant age-related changes. MRS findings revealed decreased levels of GABA, glutamate, glutamine, and NAA in the pre-SMA, correlating with observed large-scale inhibition in older adults. Additionally, pre-SMA seed-based functional connectivity analysis showed reduced brain network connections in older adults, potentially contributing to inhibitory control deficits.

CONCLUSIONS

Our study elucidates the differential effects of aging on inhibitory functions. While large-scale inhibition is more vulnerable to aging, focal-scale inhibition is relatively preserved. These findings highlight the importance of targeted cognitive interventions and underscore the necessity of a multifaceted approach in aging research.

Visual surround suppression at the neural and perceptual levels

Surround suppression was initially identified as a phenomenon at the neural level in which stimuli outside the neuron's receptive field alone cannot activate responses but can modulate neural responses to stimuli covered inside the receptive field. Subsequent studies showed that surround suppression is not only a critical property of neurons across species and brain areas but also has been found in visual perceptions. More importantly, surround suppression varies across individuals and shows significant differences between normal controls and patients with certain mental disorders. Here, we combined results from related literature and summarized the findings derived from physiological and psychophysical evidence. We first outline the basic properties of surround suppression in the visual system and perceptions. Then, we mainly summarize the differences in perceptual surround suppression among different human subjects. Our review suggests that there is no consensus regarding whether the strength of perceptual surround suppression could be used as an effective index to distinguish particular populations. Then, we summarized the similar mechanisms for surround suppression and cognitive impairments to further explore the potential clinical applications of surround suppression. A clearer understanding of the mechanisms of surround suppression in neural responses and perceptions is necessary for facilitating its clinical applications.

No Evidence of Cross-Orientation Suppression Differences in Migraine with Aura Compared to Healthy Controls

It has been suggested that there may be an imbalance of excitation and inhibitory processes in the visual areas of the brain in people with migraine aura (MA). One idea is thalamocortical dysrhythmia, characterized by disordered oscillations, and thus disordered communication between the lateral geniculate nucleus and the cortex. Cross-orientation suppression is a visual task thought to rely on inhibitory processing, possibly originating in the lateral geniculate nucleus. We measured both resting-state oscillations and cross-orientation suppression using EEG over occipital areas in people with MA and healthy volunteers. We found evidence of cross-orientation suppression in the SSVEP responses, but no evidence of any group difference. Therefore, inhibitory processes related to cross-orientation suppression do not appear to be impaired in MA.

Visual orientation discrimination skills are tightly linked with specific aspects of human intelligence

We investigate the notion that basic visual information is acting as a building block for more complex cognitive processes in humans. Specifically, we measured individual visual orientation discrimination thresholds to report significant correlations against the total standardised intelligence quotient (IQ), verbal-IQ and non-verbal IQ scores evaluated using the Wechsler Abbreviated Scale of Intelligence Second Edition (WASI-II) test battery comprising Verbal Reasoning, Block Design, Similarities and Matrix Reasoning subtests (N = 92). A multiple linear regression analysis showed that participants' performance in our visual discrimination task, could be explained only by individual scores in Verbal Reasoning which quantifies the ability to comprehend and describe words and Matrix Reasoning, which evaluates general visual processing skills including abstract and spatial perception. Our results demonstrate that low-level visual abilities and high-level cognitive processes are more tightly interwoven together than previously thought and this result could pave the way for further research on how cognition can be defined by basic sensory processes.

Reduced visual context effects in global motion processing in depression

Research supports abnormal inhibitory visual motion processing in adults with remitted and current depression, but all studies to date have used paradigms with simple grating stimuli. Global motion processing, where multiple motion signals must be integrated, has not been explored in depression, nor have inhibitory processes within that domain. Depressed participants (n = 46) and healthy controls (n = 28) completed a direction discrimination task featuring a random dot pattern stimulus. Various signal (rightward or leftward dots) to noise (dots with randomly assigned directions) ratios modulated task difficulty. Metrics of global center surround suppression and facilitation were calculated. Accuracy in the baseline condition (i.e., no surrounding annulus) was not significantly different between depressed and healthy participants. Global center surround suppression and facilitation were not significantly different between healthy and depressed participants overall. When limiting the sample to unmedicated individuals, depressed participants (n = 27) showed a reduced global center surround suppression effect compared to controls, and there was no difference in global center surround facilitation. While global motion processing is intact in depression, abnormal center surround suppression effects in depression do extend to global motion stimuli. These alterations may be mitigated by the psychotropic medications taken by some subjects in our depressed sample. Future studies should explore the mechanisms underlying these effects.

Enhanced human contrast sensitivity with increased stimulation of melanopsin in intrinsically photosensitive retinal ganglion cells

The intrinsically photosensitive retinal ganglion cells (ipRGCs) are known to serve non-image-forming functions, such as photoentrainment of the circadian rhythm and pupillary light reflex. However, how they affect human spatial vision is largely unknown. The spatial contrast sensitivity function (CSF), which measures contrast sensitivity as a function of spatial frequency, was used in the current study to investigate the function of ipRGCs in pattern vision. To compare the effects of different background lights on the CSF, we utilized the silent substitution technique. We manipulated the stimulation level of melanopsin (i.e., the visual pigment of ipRGCs) from the background light while keeping the cone stimulations constant, or vice versa. We conducted four experiments to measure the CSFs at various spatial frequencies, eccentricities, and levels of background luminance. Results showed that melanopsin stimulation from the background light enhances spatial contrast sensitivity across different eccentricities and luminance levels. Our finding that melanopsin contributes to CSF, combined with the receptive field analysis, suggests a role for the magnocellular pathway and challenges the conventional view that ipRGCs are primarily responsible for non-visual functions.

Visual motion discrimination experiments reveal small differences between males and females

Recent results have shown that males have lower duration thresholds for motion direction discrimination than females. Measuring contrast thresholds, a previous study has shown that males have a greater sensitivity to fine details and fast flickering stimuli than females, and that females have a higher sensitivity to low spatial frequencies modulated at low temporal frequencies. Here, we present the data of a contrast-detection motion discrimination experiment and a reanalysis of four different motion discrimination experiments where we compare duration thresholds for males and females using different spatial frequencies, stimulus sizes, contrasts, and temporal frequencies (in two experiments, motion surround suppression was measured). Results from the main experiment and the reanalysis show that, in general, the association between sex and contrast and duration thresholds for motion discrimination is not significant, with males and females showing similar data patterns. Only the reanalysis of one out of four studies revealed different duration thresholds between males and females paired with a strong effect size supporting previous results in the literature, although motion surround suppression was identical between groups. Importantly, most of our results do not show significant differences between males and females in contrast and duration thresholds, suggesting that the sex variable may not be as relevant as previously claimed when testing visual motion discrimination.

Cortical Surface Area Relates to Distinct Computational Properties in Human Visual Perception

Understanding the relationship between cortical structure and function is essential for elucidating the neural basis of human behavior. However, the impact of cortical structural features on the computational properties of neural circuits remains poorly understood. In this study, we demonstrate that a simple structural feature - cortical surface area (SA) - relates to specific computational properties underlying human visual perception. By combining psychophysical, neuroimaging, and computational modeling approaches, we show that differences in SA in the parietal and frontal cortices are associated with distinct patterns of behavior in a motion perception task. These behavioral differences can be accounted for by specific parameters of a divisive normalization model, suggesting that SA in these regions contributes uniquely to the spatial organization of cortical circuitry. Our findings provide novel evidence linking cortical structure to distinct computational properties and offer a framework for understanding how cortical architecture can impact human behavior.

Detection of vertical interocular phase disparities using luster as cue

Interocular disparities in contrast generate an impression of binocular luster, providing a cue for their detection. Disparities in the carrier spatial phase of horizontally oriented Gabor patches also generate an impression of luster, so the question arises as to whether it is the disparities in local contrast that accompany the phase disparities that give rise to the luster. We examined this idea by comparing the detection of interocular spatial phase disparities with that of interocular contrast disparities in Gabor patches, in the latter case that differed in overall contrast rather than phase between the eyes. When bandwidth was held constant and Gabor spatial frequency was varied, the detection of phase and contrast disparities followed a similar pattern. However, when spatial frequency was fixed and Gabor envelope standard deviation (and hence number of modulation cycles) was varied, thresholds for detecting phase disparities followed a U-shaped function of Gabor standard deviation, whereas thresholds for contrast disparities, following an initial decline, were more-or-less constant as a function of Gabor standard deviation. After reviewing a number of possible explanations for the U-shape found with phase disparities, we suggest that the likely cause is binocular sensory fusion, the strength of which increases with the number of modulation cycles. Binocular sensory fusion would operate to reduce phase but not contrast disparities, thus selectively elevating phase disparity thresholds.

Gamma oscillations point to the role of primary visual cortex in atypical motion processing in autism

Neurophysiological studies suggest that abnormal neural inhibition may explain a range of sensory processing differences in autism spectrum disorders (ASD). In particular, the impaired ability of people with ASD to visually discriminate the motion direction of small-size objects and their reduced perceptual suppression of background-like visual motion may stem from deficient surround inhibition within the primary visual cortex (V1) and/or its atypical top-down modulation by higher-tier cortical areas. In this study, we estimate the contribution of abnormal surround inhibition to the motion-processing deficit in ASD. For this purpose, we used a putative correlate of surround inhibition-suppression of the magnetoencephalographic (MEG) gamma response (GR) caused by an increase in the drift rate of a large annular high-contrast grating. The motion direction discrimination thresholds for the gratings of different angular sizes (1° and 12°) were assessed in a separate psychophysical paradigm. The MEG data were collected in 42 boys with ASD and 37 typically developing (TD) boys aged 7-15 years. Psychophysical data were available in 33 and 34 of these participants, respectively. The results showed that the GR suppression in V1 was reduced in boys with ASD, while their ability to detect the direction of motion was compromised only in the case of small stimuli. In TD boys, the GR suppression directly correlated with perceptual suppression caused by increasing stimulus size, thus suggesting the role of the top-down modulations of V1 in surround inhibition. In ASD, weaker GR suppression was associated with the poor directional sensitivity to small stimuli, but not with perceptual suppression. These results strongly suggest that a local inhibitory deficit in V1 plays an important role in the reduction of directional sensitivity in ASD and that this perceptual deficit cannot be explained exclusively by atypical top-down modulation of V1 by higher-tier cortical areas.

Altered visual cortex excitability in premenstrual dysphoric disorder: Evidence from magnetoencephalographic gamma oscillations and perceptual suppression

Premenstrual dysphoric disorder (PMDD) is a psychiatric condition characterized by extreme mood shifts during the luteal phase of the menstrual cycle (MC) due to abnormal sensitivity to neurosteroids and unbalanced neural excitation/inhibition (E/I) ratio. We hypothesized that in women with PMDD in the luteal phase, these factors would alter the frequency of magnetoencephalographic visual gamma oscillations, affect modulation of their power by excitatory drive, and decrease perceptual spatial suppression. Women with PMDD and control women were examined twice-during the follicular and luteal phases of their MC. We recorded visual gamma response (GR) while modulating the excitatory drive by increasing the drift rate of the high-contrast grating (static, 'slow', 'medium', and 'fast'). Contrary to our expectations, GR frequency was not affected in women with PMDD in either phase of the MC. GR power suppression, which is normally associated with a switch from the 'optimal' for GR slow drift rate to the medium drift rate, was reduced in women with PMDD and was the only GR parameter that distinguished them from control participants specifically in the luteal phase and predicted severity of their premenstrual symptoms. Over and above the atypical luteal GR suppression, in both phases of the MC women with PMDD had abnormally strong GR facilitation caused by a switch from the 'suboptimal' static to the 'optimal' slow drift rate. Perceptual spatial suppression did not differ between the groups but decreased from the follicular to the luteal phase only in PMDD women. The atypical modulation of GR power suggests that neuronal excitability in the visual cortex is constitutively elevated in PMDD and that this E/I imbalance is further exacerbated during the luteal phase. However, the unaltered GR frequency does not support the hypothesis of inhibitory neuron dysfunction in PMDD.

Brain and spinal cord paired stimulation coupled with locomotor training facilitates motor output in human spinal cord injury

Combined interventions for neuromodulation leading to neurorecovery have gained great attention by researchers to resemble clinical rehabilitation approaches. In this randomized clinical trial, we established changes in the net output of motoneurons innervating multiple leg muscles during stepping when transcranial magnetic stimulation (TMS) of the primary motor cortex was paired with transcutaneous spinal (transspinal) stimulation over the thoracolumbar region during locomotor training. TMS was delivered before (TMS-transspinal) or after (transspinal-TMS) transspinal stimulation during the stance phase of the less impaired leg. Ten individuals with chronic incomplete or complete SCI received at least 20 sessions of training. Each session consisted of 240 paired stimuli delivered over 10-min blocks for 1 h during robotic assisted step training on a motorized treadmill. Body weight support, leg guidance force and treadmill speed were adjusted based on each subject's ability to step without knee buckling or toe dragging. Most transspinal evoked potentials (TEPs) recorded before and after each intervention from ankle and knee muscles during assisted stepping were modulated in a phase-dependent pattern. Transspinal-TMS and locomotor training affected motor neuron output of knee and ankle muscles with ankle TEPs to be modulated in a phase-dependent manner. TMS-transspinal and locomotor training increased motor neuron output for knee but not for ankle muscles. Our results support that targeted brain and spinal cord stimulation alters responsiveness of neurons over multiple spinal segments in people with chronic SCI. Noninvasive stimulation of the brain and spinal cord along with locomotor training is a novel neuromodulation method that can become a promising modality for rehabilitation in humans after SCI.

On the interplay of temporal resolution power and spatial suppression in their prediction of psychometric intelligence

As a measure of the brain’s temporal fine-tuning capacity, temporal resolution power (TRP) explained repeatedly a substantial amount of variance in psychometric intelligence. Recently, spatial suppression, referred to as the increasing difficulty in quickly perceiving motion direction as the size of the moving stimulus increases, has attracted particular attention, when it was found to be positively related to psychometric intelligence. Due to the conceptual similarities of TRP and spatial suppression, the present study investigated their mutual interplay in the relation to psychometric intelligence in 273 young adults to better understand the reasons for these relationships. As in previous studies, psychometric intelligence was positively related to a latent variable representing TRP but, in contrast to previous reports, negatively to latent and manifest measures of spatial suppression. In a combined structural equation model, TRP still explained a substantial amount of variance in psychometric intelligence while the negative relation between spatial suppression and intelligence was completely explained by TRP. Thus, our findings confirmed TRP to be a robust predictor of psychometric intelligence but challenged the assumption of spatial suppression as a representation of general information processing efficiency as reflected in psychometric intelligence. Possible reasons for the contradictory findings on the relation between spatial suppression and psychometric intelligence are discussed.

The Effects of Glare on the Perception of Visual Motion as a Function of Age

Purpose

The purpose of this study was to determine the impact of glare, that simulated the effects of oncoming vehicle headlights, and age on different aspects of motion perception in central and peripheral vision.

Methods

Twenty younger (mean age = 25 years, range = 20-32 years) and 20 older (mean age = 70 years, range = 60-79 years) visually healthy adults completed four visual motion tasks. Stimuli were presented centrally and at 15 degrees horizontal eccentricity for 2 viewing conditions: glare (continuous, off-axis) versus no glare. Motion tasks included minimum Gabor contrast required to discriminate direction of motion, translational global motion coherence, minimum duration of a Gabor to determine direction of motion (2 different size Gabors to determine spatial surround suppression), and biological motion detection in noise. Intraocular straylight was also measured (C-Quant).

Results

Older adults had increased intraocular straylight compared with younger adults (P < 0.001). There was no significant effect of glare on motion thresholds in either group for motion contrast (P = 0.47), translational global motion (P = 0.13), biological motion (P = 0.18), or spatial surround suppression of motion (P = 0.29). Older adults had elevated thresholds for motion contrast (P < 0.001), biological motion (P < 0.001), and differences in surround suppression of motion (P = 0.04), relative to the younger group, for both the glare and no-glare conditions.

Conclusions

Although older adults had elevated thresholds for some motion perception tasks, glare from a continuous off-axis light source did not further elevate these thresholds either in central or peripheral vision.

Translational Relevance

A glare source that simulated the effect of oncoming headlights, did not impact motion perception measures relevant to driving.

Motion processing impaired by transient spatial attention: Potential implications for the magnocellular pathway

Spatial cues presented prior to the presentation of a static stimulus usually improve its perception. However, previous research has also shown that transient exogenous cues to direct spatial attention to the location of a forthcoming stimulus can lead to reduced performance. In the present study, we investigated the effects of transient exogenous cues on the perception of briefly presented drifting Gabor patches. The spatial and temporal frequencies of the drifting Gabors were chosen to mainly engage the magnocellular pathway. We found better performance in the motion direction discrimination task when neutral cues were presented before the drifting target compared to a valid spatial cue. The behavioral results support the hypothesis that transient attention prolongs the internal response to the attended stimulus, thus reducing the temporal segregation of visual events. These results were complemented by applying a recently developed model for perceptual decisions to rule out a speed-accuracy trade-off and to further assess cueing effects on visual performance. In a model-based assessment, we found that valid cues initially enhanced processing but overall resulted in less efficient processing compared to neutral cues, possibly caused by reduced temporal segregation of visual events.

Increasing the spatial extent of attention strengthens surround suppression

Here we investigate how the extent of spatial attention affects center-surround interaction in visual motion processing. To do so, we measured motion direction discrimination thresholds in humans using drifting gratings and two attention conditions. Participants were instructed to limit their attention to the central part of the stimulus under the narrow attention condition, and to both central and surround parts under the wide attention condition. We found stronger surround suppression under the wide attention condition. The magnitude of the attention effect increased with the size of the surround when the stimulus had low contrast, but did not change when it had high contrast. Results also showed that attention had a weaker effect when the center and surround gratings drifted in opposite directions. Next, to establish a link between the behavioral results and the neuronal response characteristics, we performed computer simulations using the divisive normalization model. Our simulations showed that using smaller versus larger multiplicative attentional gain and parameters derived from the medial temporal (MT) area of the cortex, the model can successfully predict the observed behavioral results. These findings reveal the critical role of spatial attention on surround suppression and establish a link between neuronal activity and behavior. Further, these results also suggest that the reduced surround suppression found in certain clinical disorders (e.g., schizophrenia and autism spectrum disorder) may be caused by abnormal attention mechanisms.

Spatially distributed computation in cortical circuits

The traditional view of neural computation in the cerebral cortex holds that sensory neurons are specialized, i.e., selective for certain dimensions of sensory stimuli. This view was challenged by evidence of contextual interactions between stimulus dimensions in which a neuron's response to one dimension strongly depends on other dimensions. Here, we use methods of mathematical modeling, psychophysics, and electrophysiology to address shortcomings of the traditional view. Using a model of a generic cortical circuit, we begin with the simple demonstration that cortical responses are always distributed among neurons, forming characteristic waveforms, which we call neural waves. When stimulated by patterned stimuli, circuit responses arise by interference of neural waves. Results of this process depend on interaction between stimulus dimensions. Comparison of modeled responses with responses of biological vision makes it clear that the framework of neural wave interference provides a useful alternative to the standard concept of neural computation.

Common and independent processing of visual motion perception and oculomotor response

Visual motion signals are used not only to drive motion perception but also to elicit oculomotor responses. A fundamental question is whether perceptual and oculomotor processing of motion signals shares a common mechanism. This study aimed to address this question using visual motion priming, in which the perceived direction of a directionally ambiguous stimulus is biased in the same (positive priming) or opposite (negative priming) direction as that of a priming stimulus. The priming effect depends on the duration of the priming stimulus. It is assumed that positive and negative priming are mediated by high- and low-level motion systems, respectively. Participants were asked to judge the perceived direction of a π-phase-shifted test grating after a smoothly drifting priming grating during varied durations. Their eye movements were measured while the test grating was presented. The perception and eye movements were discrepant under positive priming and correlated under negative priming on a trial-by-trial basis when an interstimulus interval was inserted between the priming and test stimuli, indicating that the eye movements were evoked by the test stimulus per se. These findings suggest that perceptual and oculomotor responses are induced by a common mechanism at a low level of motion processing but by independent mechanisms at a high level of motion processing.

Neural correlates associated with impaired global motion perception in cerebral visual impairment (CVI)

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Cerebral visual impairment (CVI) is associated with impaired global motion processing.

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Mean motion coherence thresholds was higher in individuals with CVI.

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fMRI responses in area hMT+ showed an aberrant response profile in CVI.

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White matter tract reconstruction revealed cortico-cortical dysmyelination in CVI.

Cerebral visual impairment (CVI) is associated with a wide range of visual perceptual deficits including global motion processing. However, the underlying neurophysiological basis for these impairments remain poorly understood. We investigated global motion processing abilities in individuals with CVI compared to neurotypical controls using a combined behavioral and multi-modal neuroimaging approach. We found that CVI participants had a significantly higher mean motion coherence threshold (determined using a random dot kinematogram pattern simulating optic flow motion) compared to controls. Using functional magnetic resonance imaging (fMRI), we investigated activation response profiles in functionally defined early (i.e. primary visual cortex; area V1) and higher order (i.e. middle temporal cortex; area hMT+) stages of motion processing. In area V1, responses to increasing motion coherence were similar in both groups. However, in the CVI group, activation in area hMT+ was significantly reduced compared to controls, and consistent with a surround facilitation (rather than suppression) response profile. White matter tract reconstruction obtained from high angular resolution diffusion imaging (HARDI) revealed evidence of increased mean, axial, and radial diffusivities within cortico-cortical (i.e. V1-hMT+), but not thalamo-hMT+ connections. Overall, our results suggest that global motion processing deficits in CVI may be associated with impaired signal integration and segregation mechanisms, as well as white matter integrity at the level of area hMT+.

Neural Mechanisms of Visual Motion Anomalies in Autism: A Two-Decade Update and Novel Aetiology

The 21st century has seen dramatic changes in our understanding of the visual physio-perceptual anomalies of autism and also in the structure and development of the primate visual system. This review covers the past 20 years of research into motion perceptual/dorsal stream anomalies in autism, as well as new understanding of the development of primate vision. The convergence of this literature allows a novel developmental hypothesis to explain the physiological and perceptual differences of the broad autistic spectrum. Central to these observations is the development of motion areas MT+, the seat of the dorsal cortical stream, central area of pre-attentional processing as well as being an anchor of binocular vision for 3D action. Such development normally occurs via a transfer of thalamic drive from the inferior pulvinar → MT to the anatomically stronger but later-developing LGN → V1 → MT connection. We propose that autistic variation arises from a slowing in the normal developmental attenuation of the pulvinar → MT pathway. We suggest that this is caused by a hyperactive amygdala → thalamic reticular nucleus circuit increasing activity in the PIm → MT via response gain modulation of the pulvinar and hence altering synaptic competition in area MT. We explore the probable timing of transfer in dominance of human MT from pulvinar to LGN/V1 driving circuitry and discuss the implications of the main hypothesis.

From Molecular to Behavior: Higher Order Occipital Cortex in Major Depressive Disorder

Medial prefrontal cortex (MPFC) and other regions like the occipital cortex (OC) exhibit abnormal neural activity in major depressive disorder (MDD). Their relationship to specific biochemical, psychophysical, and psychopathological changes remains unclear, though. For that purpose, we focus on a particular subregion in OC, namely middle temporal (MT) visual area that is known to mediate the perception of visual motion. Using high-field 7 T magnetic resonance imaging (MRI), including resting state functional MRI and proton magnetic resonance spectroscopy, the amplitude of low-frequency fluctuations (ALFF) of the blood oxygen level-dependent signal in MT, MT-seeded functional connectivity (FC), and gamma-aminobutyric acid (GABA) in MT were investigated. Applying the vision motion psychophysical task, the motion suppression index of subjects was also examined. We demonstrate significantly elevated neural variability (as measured by ALFF) in MT together with decreases in both MT GABA and motion suppression in our MDD sample. Unlike in healthy subjects, MT neural variability no longer modulates the relationship of MT GABA and motion suppression in MDD. MT also exhibits reduction in global inter-regional FC to MPFC in MDD. Finally, elevated MT ALFF relates to specifically retardation in behavior as measured by the Hamilton subscore. Together, MT provides a strong candidate for biomarker in MDD.

The Certainty of Ambiguity in Visual Neural Representations

Some images evoke bistable percepts: two different visual experiences seen in alternation while continuously viewing an unchanged stimulus. The Necker Cube and Rubin's Vase are classic examples, each of which gives alternating percepts of different shapes. Other bistable percepts are alternating colors or directions of motion. Although stimuli that result in salient bistability are rare and sometimes cleverly constructed to emphasize ambiguity, they have been influential for over 150 years, since the work of von Helmholtz, who considered them to be evidence for perceptual visual processes that interpret retinal stimuli. While bistability in natural viewing is uncommon, the main point of this review is that implicit ambiguity in visual neural representations is pervasive. Resolving ambiguity, therefore, is a fundamental and ubiquitous process of vision that routinely affects what we see, not an oddity arising from cleverly crafted images. This review focuses on the causes of widespread ambiguity, historical perspectives on it, and modern knowledge and theory about resolving it.

Spatial suppression of chromatic motion

Center-surround antagonism, as a ubiquitous feature in visual processing, usually leads to inferior perception for a large stimulus compared to a small one. For example, it is more difficult to judge the motion direction of a large high-contrast pattern than that of a small one. However, this spatial suppression in the motion dimension was only reported for luminance motion, and was not found for chromatic motion. Given that center-surround suppression only occurs for strong visual inputs, we hypothesized that previous failure in finding spatial suppression of chromatic motion might be due to weak chromatic motion being induced with stimuli of limited parameters. In this study, we used phase-shift discrimination and motion-direction discrimination tasks to measure motion spatial suppression induced by stimuli of two spatial frequencies (0.5 and 2 cpd) and two contrasts (low and high). We found that spatial suppression of the chromatic motion was stably observed for stimuli of high spatial frequency (2 cpd) and high contrast and spatial summation occurred for stimuli of low spatial frequency (0.5 cpd). Intriguingly, there was no correlations between the motion spatial suppressions of luminance motion and chromatic motion, implying that the two types of spatial suppression are not originated from the same neural processing. Our findings indicate that spatial suppression also exists for chromatic motion, and the mechanisms underlying the spatial suppression of chromatic motion is different from that of luminance motion.

Visual surround suppression in people with epilepsy correlates with attentional-executive functioning, but not with epilepsy or seizure types

PURPOSE

Following reports that an index of visual surround suppression (SI) may serve as a biomarker for an imbalance of cortical excitation and inhibition in different psychiatric and neurological disorders including epilepsy, we evaluated whether SI is associated with seizure susceptibility, seizure spread, and inhibitory effects of antiseizure medication (ASM).

METHODS

In this prospective controlled study, we examined SI with a motion discrimination task in people with genetic generalized epilepsy (GGE) and focal epilepsy with and without focal to bilateral tonic-clonic seizures. Cofactors such as GABAergic ASM, attentional-executive functioning, and depression were taken into account.

RESULTS

Data of 45 patients were included in the final analysis. Suppression index was not related to epilepsy or seizure type, GABAergic ASM treatment or mood. However, SI correlated with attentional-executive functioning (r = 0.32), which in turn was associated with ASM load (r = -0.38). Repeated task administration (N = 7) proved a high stability over a one-week interval (r_tt = 0.89).

CONCLUSIONS

Our results do not support the hypothesis that SI is a reliable biomarker for mechanisms related to inhibition of seizure spread or seizure frequency, i.e., it does not seem to reflect inhibitory capacities in epilepsy. Likewise, SI did not differentiate GGE from focal epilepsy, nor was it influenced by ASM load or mode of action. Thus, in epilepsy, no added value of including SI to routine diagnostics can be concluded.

Opposing effects of selectivity and invariance in peripheral vision

Sensory processing necessitates discarding some information in service of preserving and reformatting more behaviorally relevant information. Sensory neurons seem to achieve this by responding selectively to particular combinations of features in their inputs, while averaging over or ignoring irrelevant combinations. Here, we expose the perceptual implications of this tradeoff between selectivity and invariance, using stimuli and tasks that explicitly reveal their opposing effects on discrimination performance. We generate texture stimuli with statistics derived from natural photographs, and ask observers to perform two different tasks: Discrimination between images drawn from families with different statistics, and discrimination between image samples with identical statistics. For both tasks, the performance of an ideal observer improves with stimulus size. In contrast, humans become better at family discrimination but worse at sample discrimination. We demonstrate through simulations that these behaviors arise naturally in an observer model that relies on a common set of physiologically plausible local statistical measurements for both tasks.

Linking Neuronal Direction Selectivity to Perceptual Decisions About Visual Motion

Psychophysical and neurophysiological studies of responses to visual motion have converged on a consistent set of general principles that characterize visual processing of motion information. Both types of approaches have shown that the direction and speed of target motion are among the most important encoded stimulus properties, revealing many parallels between psychophysical and physiological responses to motion. Motivated by these parallels, this review focuses largely on more direct links between the key feature of the neuronal response to motion, direction selectivity, and its utilization in memory-guided perceptual decisions. These links were established during neuronal recordings in monkeys performing direction discriminations, but also by examining perceptual effects of widespread elimination of cortical direction selectivity produced by motion deprivation during development. Other approaches, such as microstimulation and lesions, have documented the importance of direction-selective activity in the areas that are active during memory-guided direction comparisons, area MT and the prefrontal cortex, revealing their likely interactions during behavioral tasks.

Adaptive mechanisms of visual motion discrimination, integration, and segregation

Under ecological conditions, the luminance impinging on the retina varies within a dynamic range of 220 dB. Stimulus contrast can also vary drastically within a scene and eye movements leave little time for sampling luminance. Given these fundamental problems, the human brain allocates a significant amount of resources and deploys both structural and functional solutions that work in tandem to compress this range. Here we propose a new dynamic neural model built upon well-established canonical neural mechanisms. The model consists of two feed-forward stages. The first stage encodes the stimulus spatially and normalizes its activity by extracting contrast and discounting the background luminance. These normalized activities allow a second stage to implement a contrast-dependent spatial-integration strategy. We show how the properties of this model can account for adaptive properties of motion discrimination, integration, and segregation.

Selective postsaccadic enhancement of motion perception

Saccadic eye movements can drastically affect motion perception: during saccades, the stationary surround is swept rapidly across the retina and contrast sensitivity is suppressed. However, after saccades, contrast sensitivity is enhanced for color and high-spatial frequency stimuli and reflexive tracking movements known as ocular following responses (OFR) are enhanced in response to large field motion. Additionally, OFR and postsaccadic enhancement of neural activity in primate motion processing areas are well correlated. It is not yet known how this postsaccadic enhancement arises. Therefore, we tested if the enhancement can be explained by changes in the balance of centre-surround antagonism in motion processing, where spatial summation is favoured at low contrasts and surround suppression is favoured at high contrasts. We found motion perception was selectively enhanced immediately after saccades for high spatial frequency stimuli, consistent with previously reported selective postsaccadic enhancement of contrast sensitivity for flashed high spatial frequency stimuli. The observed enhancement was also associated with changes in spatial summation and suppression, as well as contrast facilitation and inhibition, suggesting that motion processing is augmented to maximise visual perception immediately after saccades. The results highlight that spatial and contrast properties of underlying neural mechanisms for motion processing can be affected by an antecedent saccade for highly detailed stimuli and are in line with studies that show behavioural and neuronal enhancement of motion processing in non-human primates.

White matter deficits correlate with visual motion perception impairments in dyslexic carriers of the DCDC2 genetic risk variant

Motion perception deficits in dyslexia show a large intersubjective variability, partly reflecting genetic factors influencing brain architecture development. In previous work, we have demonstrated that dyslexic carriers of a mutation of the DCDC2 gene have a very strong impairment in motion perception. In the present study, we investigated structural white matter alterations associated with the poor motion perception in a cohort of twenty dyslexics with a subgroup carrying the DCDC2 gene deletion (DCDC2d+) and a subgroup without the risk variant (DCDC2d–). We observed significant deficits in motion contrast sensitivity and in motion direction discrimination accuracy at high contrast, stronger in the DCDC2d+ group. Both motion perception impairments correlated significantly with the fractional anisotropy in posterior ventral and dorsal tracts, including early visual pathways both along the optic radiation and in proximity of occipital cortex, MT and VWFA. However, the DCDC2d+ group showed stronger correlations between FA and motion perception impairments than the DCDC2d– group in early visual white matter bundles, including the optic radiations, and in ventral pathways located in the left inferior temporal cortex. Our results suggest that the DCDC2d+ group experiences higher vulnerability in visual motion processing even at early stages of visual analysis, which might represent a specific feature associated with the genotype and provide further neurobiological support to the visual-motion deficit account of dyslexia in a specific subpopulation.

Comparing the influence of stimulus size and contrast on the perception of moving gratings and random dot patterns-A registered report protocol

Modern accounts of visual motion processing in the primate brain emphasize a hierarchy of different regions within the dorsal visual pathway, especially primary visual cortex (V1) and the middle temporal area (MT). However, recent studies have called the idea of a processing pipeline with fixed contributions to motion perception from each area into doubt. Instead, the role that each area plays appears to depend on properties of the stimulus as well as perceptual history. We propose to test this hypothesis in human subjects by comparing motion perception of two commonly used stimulus types: drifting sinusoidal gratings (DSGs) and random dot patterns (RDPs). To avoid potential biases in our approach we are pre-registering our study. We will compare the effects of size and contrast levels on the perception of the direction of motion for DSGs and RDPs. In addition, based on intriguing results in a pilot study, we will also explore the effects of a post-stimulus mask. Our approach will offer valuable insights into how motion is processed by the visual system and guide further behavioral and neurophysiological research.

Age effects on saccadic suppression of luminance and color

Saccadic eye movements modulate visual perception: they initiate and terminate high acuity vision at a certain location in space, but before and during their execution visual contrast sensitivity is strongly attenuated for 100 to 200 ms. Transient perisaccadic perceptual distortions are assumed to be an important mechanism to maintain visual stability. Little is known about age effects on saccadic suppression, even though for healthy adults other major age-related changes are well documented, like a decrease of visual contrast sensitivity for intermediate and high spatial frequencies or an increase of saccade latencies. Here, we tested saccadic suppression of luminance and isoluminant chromatic flashes in 100 participants from eight to 78 years. To estimate the effect of saccadic suppression on contrast sensitivity, we used a two-alternative forced choice (2AFC) design and an adaptive staircase procedure to modulate the luminance or chromatic contrast of a flashed detection target during fixation and 15 ms after saccade onset. The target was a single horizontal luminance or chromatic line flashed 2° above or below the fixation or saccade target. Compared to fixation, average perisaccadic contrast sensitivity decreased significantly by 66% for luminance and by 36% for color. A significant correlation was found for the strength of saccadic suppression of luminance and color. However, a small age effect was found only for the strength of saccadic suppression of luminance, which increased from 64% to 70% from young to old age. We conclude that saccadic suppression for luminance and color is present in most participants independent of their age and that mechanisms of suppression stay relatively stable during healthy aging.

Investigating the neurochemistry of the human visual system using magnetic resonance spectroscopy

Biochemical processes underpin the structure and function of the visual cortex, yet our understanding of the fundamental neurochemistry of the visual brain is incomplete. Proton magnetic resonance spectroscopy (¹H-MRS) is a non-invasive brain imaging tool that allows chemical quantification of living tissue by detecting minute differences in the resonant frequency of molecules. Application of MRS in the human brain in vivo has advanced our understanding of how the visual brain consumes energy to support neural function, how its neural substrates change as a result of disease or dysfunction, and how neural populations signal during perception and plasticity. The aim of this review is to provide an entry point to researchers interested in investigating the neurochemistry of the visual system using in vivo measurements. We provide a basic overview of MRS principles, and then discuss recent findings in four topics of vision science: (i) visual perception, plasticity in the (ii) healthy and (iii) dysfunctional visual system, and (iv) during visual stimulation. Taken together, evidence suggests that the neurochemistry of the visual system provides important novel insights into how we perceive the world.

Contrast Sensitivity on 1/f Noise Is More Greatly Impacted by Older Age for the Fovea Than Parafovea

SIGNIFICANCE

Contrast sensitivity changes across the visual field with age and is often measured clinically with various forms of perimetry on plain backgrounds. In daily life, the visual scene is more complicated, and therefore, the standard clinical measures of contrast sensitivity may not predict a patient's visual experience in more natural environments.

PURPOSE

This study aims to determine whether contrast thresholds in older adults are different from younger adults when measured on a 1/f noise background (a nonuniform background whose spatial frequency content is similar to those present in the natural vision environments).

METHODS

Twenty younger (age range, 20 to 35 years) and 20 older adults (age range, 61 to 79 years) with normal ocular health were recruited. Contrast thresholds were measured for a Gabor patch of 6 cycles per degree (sine wave grating masked by a Gaussian envelope of standard deviation 0.17°) presented on 1/f noise background (root-mean-square contrast, 0.05 and 0.20) that subtended 15° diameter of the central visual field. The stimulus was presented at four eccentricities (0°, 2°, 4°, and 6°) along the 45° meridian in the noise background, and nine contrast levels were tested at each eccentricity. The proportion of correct responses for detecting the target at each eccentricity was obtained, and psychometric functions were fit to estimate the contrast threshold.

RESULTS

Older adults demonstrate increased contrast thresholds compared with younger adults. There was an eccentricity-dependent interaction with age, with the difference between groups being highest in the fovea compared with other eccentricities. Performance was similar for the two noise backgrounds tested.

CONCLUSIONS

Our results revealed a strong eccentricity dependence in performance between older and younger adults, highlighting age-related differences in the contrast detection mechanisms between fovea and parafovea for stimuli presented on nonuniform backgrounds.

Critical aspects of neurodevelopment

Organisms have the unique ability to adapt to their environment by making use of external inputs. In the process, the brain is shaped by experiences that go hand-in-hand with optimisation of neural circuits. As such, there exists a time window for the development of different brain regions, each unique for a particular sensory modality, wherein the propensity of forming strong, irreversible connections are high, referred to as a critical period of development. Over the years, this domain of neurodevelopmental research has garnered considerable attention from many scientists, primarily because of the intensive activity-dependent nature of development. This review discusses the cellular, molecular, and neurophysiological bases of critical periods of different sensory modalities, and the disorders associated in cases the regulators of development are dysfunctional. Eventually, the neurobiological bases of the behavioural abnormalities related to developmental pathologies are discussed. A more in-depth insight into the development of the brain during the critical period of plasticity will eventually aid in developing potential therapeutics for several neurodevelopmental disorders that are categorised under critical period disorders.

Contribution of the slow motion mechanism to global motion revealed by an MAE technique

Two different motion mechanisms have been identified with motion aftereffect (MAE). (1) A slow motion mechanism, accessed by a static MAE, is sensitive to high-spatial and low-temporal frequency; (2) a fast motion mechanism, accessed by a flicker MAE, is sensitive to low-spatial and high-temporal frequency. We examined their respective responses to global motion after adapting to a global motion pattern constructed of multiple compound Gabor patches arranged circularly. Each compound Gabor patch contained two gratings at different spatial frequencies (0.53 and 2.13 cpd) drifting in opposite directions. The participants reported the direction and duration of the MAE for a variety of global motion patterns. We discovered that static MAE durations depended on the global motion patterns, e.g., longer MAE duration to patches arranged to see rotation than to random motion (Exp 1), and increase with global motion strength (patch number in Exp 2). In contrast, flicker MAEs durations are similar across different patterns and adaptation strength. Further, the global integration occurred at the adaptation stage, rather than at the test stage (Exp 3). These results suggest that slow motion mechanism, assessed by static MAE, integrate motion signals over space while fast motion mechanisms do not, at least under the conditions used.

Motion perception at mesopic light levels: effects of physiological ageing and eccentricity

PURPOSE

To explore the differential effects of age and eccentricity on the perception of motion at photopic and mesopic light levels.

METHODS

Thirty-six visually normal participants (18 younger; mean age 25 years, range: 20-31) and (18 older; mean age 70 years, range: 60-79) underwent two testing sessions, one at photopic and one at mesopic light levels. In each session, motion perception was tested binocularly at two eccentricities (centrally, and peripherally at 15° rightwards and 5° superior to the horizontal) for four motion tasks: minimum contrast of a drifting Gabor to identify motion direction (motion contrast); translational global motion coherence; biological motion embedded in noise and the minimum duration of a high-contrast Gabor to determine the direction of motion, using two Gabor sizes to measure spatial surround suppression of motion.

RESULTS

There was a significant main effect of light condition (higher thresholds in mesopic) for motion contrast (p < 0.001), translational global motion (p = 0.001) and biological motion (p < 0.001); a significant main effect of age (higher thresholds in older adults) for motion contrast (p < 0.001) and biological motion (p = 0.04) and a significant main effect of eccentricity (higher thresholds peripherally) for motion contrast (p < 0.001) and biological motion (p < 0.001). Additionally, we found a significant three-way interaction between light levels, age and eccentricity for translational global motion (similar increase in mesopic thresholds centrally for both groups, but a much larger deterioration in older adult's peripheral mesopic thresholds, p = 0.02). Finally, we found a two-way interaction between light condition and eccentricity for translational global motion (higher values in central mesopic relative to peripheral photopic, p = 0.001) and for biological motion (higher values in peripheral mesopic relative to central photopic, p < 0.001).

CONCLUSIONS

For the majority of tasks assessed, motion perception was reduced in mesopic relative to photopic conditions, to a similar extent in both age groups. However, because some older adults exhibited elevated thresholds even under photopic conditions, particularly in the periphery, the ability to detect mesopic moving stimuli even at high contrast was markedly impaired in some individuals. Our results imply age-related differences in the detection of peripheral moving stimuli at night that might impact hazard avoidance and night driving ability.

Reduced surround suppression in monocular motion perception

Motion discrimination of large stimuli is impaired at high contrast and short durations. This psychophysical result has been linked with the center-surround suppression found in neurons of area MT. Recent physiology results have shown that most frontoparallel MT cells respond more strongly to binocular than to monocular stimulation. Here we measured the surround suppression strength under binocular and monocular viewing. Thirty-nine participants took part in two experiments: (a) where the nonstimulated eye viewed a blank field of the same luminance (n = 8) and (b) where it was occluded with a patch (n = 31). In both experiments, we measured duration thresholds for small (1 deg diameter) and large (7 deg) drifting gratings of 1 cpd with 85% contrast. For each subject, a Motion Suppression Index (MSI) was computed by subtracting the duration thresholds in logarithmic units of the large minus the small stimulus. Results were similar in both experiments. Combining the MSI of both experiments, we found that the strength of suppression for binocular condition (MSIbinocular = 0.249 ± 0.126 log10 (ms)) is 1.79 times higher than under monocular viewing (MSImonocular = 0.139 ± 0.137 log10 (ms)). This increase is too high to be explained by the higher perceived contrast of binocular stimuli and offers a new way of testing whether MT neurons account for surround suppression. Potentially, differences in surround suppression reported in clinical populations may reflect altered binocular processing.

Differential Circuit Mechanisms of Young and Aged Visual Cortex in the Mammalian Brain

The main goal of this review is to summarize and discuss (1) age-dependent structural reorganization of mammalian visual cortical circuits underlying complex visual behavior functions in primary visual cortex (V1) and multiple extrastriate visual areas, and (2) current evidence supporting the notion of compensatory mechanisms in aged visual circuits as well as the use of rehabilitative therapy for the recovery of neural plasticity in normal and diseased aging visual circuit mechanisms in different species. It is well known that aging significantly modulates both the structural and physiological properties of visual cortical neurons in V1 and other visual cortical areas in various species. Compensatory aged neural mechanisms correlate with the complexity of visual functions; however, they do not always result in major circuit alterations resulting in age-dependent decline in performance of a visual task or neurodegenerative disorders. Computational load and neural processing gradually increase with age, and the complexity of compensatory mechanisms correlates with the intricacy of higher form visual perceptions that are more evident in higher-order visual areas. It is particularly interesting to note that the visual perceptual processing of certain visual behavior functions does not change with age. This review aims to comprehensively discuss the effect of normal aging on neuroanatomical alterations that underlie critical visual functions and more importantly to highlight differences between compensatory mechanisms in aged neural circuits and neural processes related to visual disorders. This type of approach will further enhance our understanding of inter-areal and cortico-cortical connectivity of visual circuits in normal aging and identify major circuit alterations that occur in different visual deficits, thus facilitating the design and evaluation of potential rehabilitation therapies as well as the assessment of the extent of their rejuvenation.

Reduction of higher-order occipital GABA and impaired visual perception in acute major depressive disorder

Major depressive disorder (MDD) is a complex state-dependent psychiatric illness for which biomarkers linking psychophysical, biochemical, and psychopathological changes remain yet elusive, though. Earlier studies demonstrate reduced GABA in lower-order occipital cortex in acute MDD leaving open its validity and significance for higher-order visual perception, though. The goal of our study is to fill that gap by combining psychophysical investigation of visual perception with measurement of GABA concentration in middle temporal visual area (hMT+) in acute depressed MDD. Psychophysically, we observe a highly specific deficit in visual surround motion suppression in a large sample of acute MDD subjects which, importantly, correlates with symptom severity. Both visual deficit and its relation to symptom severity are replicated in the smaller MDD sample that received MRS. Using high-field 7T proton Magnetic resonance spectroscopy (¹H-MRS), acute MDD subjects exhibit decreased GABA concentration in visual MT+ which, unlike in healthy subjects, no longer correlates with their visual motion performance, i.e., impaired SI. In sum, our combined psychophysical-biochemical study demonstrates an important role of reduced occipital GABA for altered visual perception and psychopathological symptoms in acute MDD. Bridging the gap from the biochemical level of occipital GABA over visual-perceptual changes to psychopathological symptoms, our findings point to the importance of the occipital cortex in acute depressed MDD including its role as candidate biomarker.

Dyslexia and the magnocellular-parvocellular coactivaton hypothesis

Previous studies showed that the lateral masking of a fast-moving low spatial frequency (SF) target was strong when exerted by static flankers of lower or equal to the target SF and absent when flankers' SF was higher than the target's one. These masking and unmasking effects have been interpreted as due to Magnocellular-Magnocellular (M-M) inhibition and Parvocellular-on-Magnocellular (P-M) disinhibitory coactivation, respectively. Based on the hypothesis that the balance between the two systems is perturbed in Developmental Dyslexia (DD), we asked whether dyslexic children (DDs) behaved differently than Typically Developing children (TDs) in conditions of lateral masking. DDs and TDs performed a motion discrimination task, of a .5c/deg Gabor target moving at 16 deg/sec, either isolated or flanked by static Gabors with a SF of .125, .5 or 2 c/deg (Experiment 1). As a control, they also performed a contrast detection task of a static target, either isolated or flanked (Experiment 2). DDs did not perform any different from TDs with either a static target or an isolated moving target of low spatial frequency, thus suggesting efficient feedforward Magnocellular (M) and Parvocellular (P) processing. Also, DDs showed similar contrast thresholds to TDs in the M-M inhibition condition. Conversely, DDs did not recover from lateral masking in the M-P coactivation condition. In addition, their performance in this condition negatively correlated with non-words accuracy, supporting the suggestion that an inefficient Magno-Parvo coactivation may possibly be associated to both higher visual suppression and reduced perceptual stability during reading.

Adaptive Surround Modulation of MT Neurons: A Computational Model

The classical receptive field (CRF) of a spiking visual neuron is defined as the region in the visual field that can generate spikes when stimulated by a visual stimulus. Many visual neurons also have an extra-classical receptive field (ECRF) that surrounds the CRF. The presence of a stimulus in the ECRF does not generate spikes but rather modulates the response to a stimulus in the neuron's CRF. Neurons in the primate Middle Temporal (MT) area, which is a motion specialist region, can have directionally antagonistic or facilitatory surrounds. The surround's effect switches between directionally antagonistic or facilitatory based on the characteristics of the stimulus, with antagonistic effects when there are directional discontinuities but facilitatory effects when there is directional coherence. Here, we present a computational model of neurons in area MT that replicates this observation and uses computational building blocks that correlate with observed cell types in the visual pathways to explain the mechanism of this modulatory effect. The model shows that the categorization of MT neurons based on the effect of their surround depends on the input stimulus rather than being a property of the neurons. Also, in agreement with neurophysiological findings, the ECRFs of the modeled MT neurons alter their center-surround interactions depending on image contrast.

Evidence for different spatiotemporal mechanisms using duration thresholds: An individual differences approach

The study of motion perception through classical psychophysical methods has suggested that independent spatiotemporal filters acting over specific locations in retinal images carry out early motion processing. On the other hand, individual differences approaches have been able to identify a structure of spatiotemporal filters too. In this same fashion-through an individual differences approach-the present study aims to uncover a structure of spatiotemporal frequency selective motion mechanisms. This is done, for the first time, using supra-threshold contrast stimuli in a motion direction discrimination task. Two experiments were performed measuring duration thresholds for drifting 2D Gabor gratings of 0.25, 0.5, 0.75, 1, 1.5, 2, 3 and 6 c/deg. They moved with a speed of 2 deg/sec, with Michelson contrasts of 0.1 or 0.9 (Experiment 1) or had a contrast of 0.9 drifting with a temporal frequency of 2 Hz or 8 Hz (Experiment 2). Principal component analyses uncover three factors in each of four conditions. When Varimax-rotated, these are seen to be selective to spatial frequencies lower than 0.5 c/deg, intermediate ones from 0.5 to 1-1.5 c/deg, and frequencies greater than 1-1.5 c/deg. Direct Oblimin rotations indicate that factors are moderately correlated. Further analyses show very slight differences in the correlational structures between contrast conditions (0.1 vs. 0.9), and no differences between temporal frequency conditions (2 Hz vs. 8 Hz). To conclude, the idea of a three-factor structure in motion processing for low, intermediate, and high spatial frequencies is supported.

Distinct patterns of surround modulation in V1 and hMT

Modulation of a neuron's responses by the stimuli presented outside of its classical receptive field is ubiquitous in the visual system. This "surround modulation" mechanism is believed to be critical for efficient processing and leads to many well-known perceptual effects. The details of surround modulation, however, are still not fully understood. One of the open questions is related to the differences in surround modulation mechanisms in different cortical areas, and their interactions. Here we study patterns of surround modulation in primary visual cortex (V1) and middle temporal complex (hMT+) utilizing a well-studied effect in motion perception, where human observers' ability to discriminate the drift direction of a grating improves as its size gets bigger if the grating has a low contrast, and deteriorates if it has a high contrast. We first replicated the findings in the literature with a behavioral experiment using small and large (1.67 and 8.05 degrees of visual angle) drifting gratings with either low (2%) or high (99%) contrast presented at the periphery. Next, using functional MRI, we found that in V1 with increasing size cortical responses increased at both contrast levels. Whereas in hMT+ with increasing size cortical responses remained unchanged or decreased at high contrast, and increased at low contrast, reflecting the perceptual effect. We also show that the divisive normalization model successfully predicts these activity patterns, and establishes a link between the behavioral results and hMT+ ​activity. We conclude that surround modulation patterns in V1 and hMT+ ​are different, and that the size-contrast interaction in motion perception is likely to originate in hMT+.

Invisible Adaptation: The Effect of Awareness on the Strength of the Motion Aftereffect

The ability to process information despite the lack of perceptual awareness is one of the most fascinating aspects of the visual system. Such unconscious processing is often investigated using adaptation, where any presence of the former can be traced by its footprint on aftereffects following the latter. We have investigated the mechanisms of the motion aftereffect (MAE) using random dot displays of varying motion coherence as well as crowding to modulate both the physical as well as the perceptual strength of the adaptation stimulus. Perceptual strength was quantitatively measured as the performance in a forced-choice direction-discrimination task. A motion-nulling technique was used to quantitatively measure the strength of the MAE. We show that the strength of the dynamic MAE is independently influenced by both the physical stimulus strength as well as the subjective perceptual strength, with the effect of the former being more prominent than that of the latter. We further show that the MAE still persists under conditions of subthreshold perception. Our results suggest that perceptual awareness can influence the strength of visual processing, but the latter is not fully dependent on the former and can still take place at its partial or even total absence.

Natural scene statistics predict how humans pool information across space in surface tilt estimation

Visual systems estimate the three-dimensional (3D) structure of scenes from information in two-dimensional (2D) retinal images. Visual systems use multiple sources of information to improve the accuracy of these estimates, including statistical knowledge of the probable spatial arrangements of natural scenes. Here, we examine how 3D surface tilts are spatially related in real-world scenes, and show that humans pool information across space when estimating surface tilt in accordance with these spatial relationships. We develop a hierarchical model of surface tilt estimation that is grounded in the statistics of tilt in natural scenes and images. The model computes a global tilt estimate by pooling local tilt estimates within an adaptive spatial neighborhood. The spatial neighborhood in which local estimates are pooled changes according to the value of the local estimate at a target location. The hierarchical model provides more accurate estimates of groundtruth tilt in natural scenes and provides a better account of human performance than the local estimates. Taken together, the results imply that the human visual system pools information about surface tilt across space in accordance with natural scene statistics.

Visual systems estimate three-dimensional (3D) properties of scenes from two-dimensional images on the retinas. To solve this difficult problem as accurately as possible, visual systems use many available sources of information, including information about how the 3D properties of the world are spatially arranged. This manuscript reports a systematic analysis of 3D surface tilt in natural scenes, a model of surface tilt estimation that makes use of these scene statistics, and human psychophysical data on the estimation of surface tilt from natural images. The results show that the regularities present in the natural environment predict both how to maximize the accuracy of tilt estimation and how to maximize the prediction of human performance. This work contributes to a growing line of work that establishes links between rigorous measurements of natural scenes and the function of sensory and perceptual systems.

Functional preservation and enhanced capacity for visual restoration in subacute occipital stroke

Stroke damage to the primary visual cortex (V1) causes a loss of vision known as hemianopia or cortically-induced blindness. While perimetric visual field improvements can occur spontaneously in the first few months post-stroke, by 6 months post-stroke, the deficit is considered chronic and permanent. Despite evidence from sensorimotor stroke showing that early injury responses heighten neuroplastic potential, to date, visual rehabilitation research has focused on patients with chronic cortically-induced blindness. Consequently, little is known about the functional properties of the post-stroke visual system in the subacute period, nor do we know if these properties can be harnessed to enhance visual recovery. Here, for the first time, we show that 'conscious' visual discrimination abilities are often preserved inside subacute, perimetrically-defined blind fields, but they disappear by ∼6 months post-stroke. Complementing this discovery, we now show that training initiated subacutely can recover global motion discrimination and integration, as well as luminance detection perimetry, just as it does in chronic cortically-induced blindness. However, subacute recovery was attained six times faster; it also generalized to deeper, untrained regions of the blind field, and to other (untrained) aspects of motion perception, preventing their degradation upon reaching the chronic period. In contrast, untrained subacutes exhibited spontaneous improvements in luminance detection perimetry, but spontaneous recovery of motion discriminations was never observed. Thus, in cortically-induced blindness, the early post-stroke period appears characterized by gradual-rather than sudden-loss of visual processing. Subacute training stops this degradation, and is far more efficient at eliciting recovery than identical training in the chronic period. Finally, spontaneous visual improvements in subacutes were restricted to luminance detection; discrimination abilities only recovered following deliberate training. Our findings suggest that after V1 damage, rather than waiting for vision to stabilize, early training interventions may be key to maximize the system's potential for recovery.

Aging and the perception of global structure

A single experiment required 40 younger and older adults to discriminate global shape as depicted by Glass patterns (concentric and radial organizations). Such patterns have been widely used for decades, because in order to successfully perceive the depicted shape, the visual system has to detect both locally oriented features (dipoles) and their alignments across extended regions of space. In the current study, we manipulated the number of constituent dipoles in the stimulus patterns (40 or 200), the noise-to-signal ratio (zero, 1.0, & 5.0), and the pattern size (6.0 & 25.0 degrees visual angle). The observers’ shape discrimination accuracies (d’ values) decreased markedly as the amount of noise increased, and there were smaller (but significant) effects of both overall pattern size and the number of stimulus dipoles. Interestingly, while there was a significant effect of age, it was relatively small: the overall d’ values for older and younger adults were 2.07 and 2.34, respectively. Older adults therefore retain an effective ability to visually perceive global shape, even for sparsely-defined patterns embedded in noise.