Role Of Primary Visual Cortex Research Articles

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.

Gamma Oscillations Point to the Role of Primary Visual Cortex in Atypical Motion Processing in Autism

Gamma Oscillations Point to the Role of Primary Visual Cortex in Atypical Motion Processing in Autism

Spontaneous ocular positioning during visual imagery in patients with hemianopia and/or hemineglect

Spontaneous eye movements during imagery are not random and can be used to study and reveal mental visualization processes (Fourtassi et al., 2013; Johansson et al. 2006). For example, we previously showed that during memory recall of French towns via imagery healthy individuals looks straight ahead when recalling Paris and their subsequent gaze positions are significantly correlated with the real GPS coordinates of the recalled towns. This correlation suggests that memory retrieval is done via depictive representations as it is never found when the towns are recalled using verbal fluency. In the present paper we added to this finding by showing that the mental image is spontaneously centered on the head or body midline.In order to investigate the capacities of visual imagery in patients, and by extension, the role of primary visual cortex and fronto-parietal cortex in spatial visual imagery, we recorded gaze positions during memory recall of French towns in an imagery task, a non-imagery task (verbal fluency), and a visually-guided task in five patients with left or right hemianopia and in four patients with hemineglect (two with left hemianopia and two without).The correlation between gaze position and real GPS coordinates of the recalled towns was significant in all hemianopic patients, but in patients with hemineglect this was only the case for towns located on the right half of the map of France. This suggests hemianopic patients can perform spatially consistent mental imagery despite direct or indirect unilateral lesions of the primary visual cortex. In contrast, the left-sided towns recalled by hemineglect patients, revealed that they have some spatial inconsistency or representational difficulty.Hemianopic patients positioned and maintained their gaze in their contralesional hemispace, suggesting that their mental map was not centered on their head or body midline. This contralesional gaze positioning appeared to be a general compensation strategy and was not observed in patients with neglect (with or without hemianopia). Instead, neglect patients positioned their gaze in their ipsilesional hemispace and only when performing the visual imagery task.These findings are discussed in the context of the role of occipital and fronto-parietal cortices in the neuroanatomical model of visual imagery developed by Kosslyn et al. (2006).

Synaptic Correlates of Low-Level Perception in V1.

The computational role of primary visual cortex in low-level perception remains debated. The expression of this "pop-out" perception is often assumed to require attention-related processes, such as top-down feedback from higher cortical areas. Using intracellular techniques in the anesthetized cat and novel analysis methods, we reveal unexpected structural-functional biases in the synaptic integration and dynamic association properties of V1 neurons. These structural-functional biases provide a substrate, within V1, for contour detection and, more unexpectedly, global motion flow sensitivity at saccadic speed, even in the absence of attentional processes. We argue for the concept of a "dynamic association field" in V1 neurons, whose spatial extent and anisotropy changes with retinal flow statistics, and more generally for a renewed focus on intracortical computation.

106. Parietal cortex involvement in visual perceptual learning: A rTMS investigation

Practice-induced visual perceptual learning is thought to reflect the plasticity of sensory brain regions. The hypothesis that changes in early visual areas may depend on top-down modulation from higher level brain regions by attentional mechanisms is still debated. We used a causal approach by repetitive transcranial magnetic stimulation (rTMS) to evaluate the role of primary visual cortex (V1) and posterior parietal cortex (PPC) in visual perceptual learning. Twelve healthy volunteers performed a forced-choice orientation discrimination task of gabor patches (consisting of 500 trials in five blocks) without any intervention or immediately after a conditioning inhibitory 1-Hz rTMS protocol delivered over the V1, the right PPC, and the vertex (control area), in four different days. Conditioning inhibition by 1-Hz rTMS over the V1 produced an overall worsening of the performance accuracy but no effect in the learning rate, as measured by the slope of the learning curve. The learning rate was lower after right PPC inhibition compared to the other conditions, when evaluated during the first two blocks (200 trials) where occurred most of the learning. These preliminary findings suggest that higher level brain regions may be involved in the mechanisms underlying perceptual learning.

Reconstruction of shape contours from V1 activity at high resolution

The role of primary visual cortex (V1) in encoding physical stimulus features is well known, while stimulus categorization is mainly attributed to higher visual areas. However, visual experience is not stripped down to invariant, categorical-only “labels.” Rather, visual experiences are remarkably rich with details resulting in high-resolution perception of objects. If V1 is involved in this process, high-resolution readout of shape contours should be possible from V1 activity. To test this, we presented various shapes to awake, fixating monkeys while recording V1 population activity using voltage-sensitive dye imaging. A simplified bottom-up model was constructed based on known cortical properties and without an image prior. Contours were reconstructed from single trials, in sub-degree resolution by applying the inverse model to neuronal responses. These novel reconstruction results suggest V1 can be an important constituent in the detailed internal representation of visual experiences.

Continuous Flash Suppression Modulates Cortical Activity in Early Visual Cortex

A salient visual stimulus can be rendered invisible by presenting it to one eye while flashing a mask to the other eye. This procedure, called continuous flash suppression (CFS), has been proposed as an ideal way of studying awareness as it can make a stimulus imperceptible for extended periods of time. Previous studies have reported robust suppression of cortical activity in higher visual areas during CFS, but the role of primary visual cortex (V1) is still controversial. In this study, we resolve this controversy on the role of V1 in CFS and also begin characterizing the computational processes underlying CFS. Early visual cortical activity was measured with functional magnetic resonance imaging while human subjects viewed stimuli composed of target and mask, presented to the same or different eyes. Functional MRI responses in early visual cortex were smaller when target and mask were in different eyes compared with the same eye, not only for the lowest contrast target rendered invisible by CFS, but also for higher contrast targets, which were visible even when presented to the eye opposite the mask. We infer that CFS is based on modulating the gain of neural responses, akin to reducing target contrast.

Continuous flash suppression modulates cortical activity in early visual cortex.

A salient visual stimulus can be rendered invisible by presenting it to one eye while flashing a mask to the other eye. This procedure, called continuous flash suppression (CFS), has been proposed as an ideal way of studying awareness as it can make a stimulus imperceptible for extended periods of time. Previous studies have reported robust suppression of cortical activity in higher visual areas during CFS, but the role of primary visual cortex (V1) is still controversial. In this study, we resolve this controversy on the role of V1 in CFS and also begin characterizing the computational processes underlying CFS. Early visual cortical activity was measured with functional magnetic resonance imaging while human subjects viewed stimuli composed of target and mask, presented to the same or different eyes. Functional MRI responses in early visual cortex were smaller when target and mask were in different eyes compared with the same eye, not only for the lowest contrast target rendered invisible by CFS, but also for higher contrast targets, which were visible even when presented to the eye opposite the mask. We infer that CFS is based on modulating the gain of neural responses, akin to reducing target contrast.

Orientation pop-out processing in human visual cortex

Visual stimuli can “pop out” if they are different to their background. There has been considerable debate as to the role of primary visual cortex (V1) versus higher visual areas (esp. V4) in pop-out processing. Here we parametrically modulated the relative orientation of stimuli and their backgrounds to investigate the neural correlates of pop-out in visual cortex while subjects were performing a demanding fixation task in a scanner. Whole brain and region of interest analyses confirmed a representation of orientation contrast in extrastriate visual cortex (V4), but not in striate visual cortex (V1). Thus, although previous studies have shown that human V1 can be involved in orientation pop-out, our findings demonstrate that there are cases where V1 is “blind” and pop-out detection is restricted to higher visual areas. Pop-out processing is presumably a distributed process across multiple visual regions.

Neural correlates of stimulus spatial frequency-dependent contrast detection

Psychophysical studies on human and non-human vertebrate species have shown that visual contrast sensitivity function (CSF) peaks at a certain stimulus spatial frequency and declines in both lower and higher spatial frequencies. The underlying neural substrate and mechanisms remain in debate. Here, we investigated the role of primary visual cortex (V1: area 17) in spatial frequency-dependent contrast detection in cats. Perceptual CSFs of three cats were measured using a two-alternative forced-choice task. The responses of V1 neurons to their optimal visual stimuli in a range of luminance contrast levels (from 0 to 1.0) were recorded subsequently using in vivo extracellular single-unit recording techniques. The contrast sensitivity of each neuron was determined. The neuronal CSF for each cat was constructed from the mean contrast sensitivity of neurons with different preferred stimulus spatial frequencies. (1) The perceptual and neuronal CSFs of each of the three cats exhibited a similar shape with peak amplitude near 0.4 cpd. (2) The neuronal CSF of each cat was highly correlated with its perceptual CSF. (3) V1 neurons with different preferred stimulus spatial frequencies had different contrast gains. (1) Contrast detection of visual stimuli with different spatial frequencies may likely involve population coding of V1 neurons with different preferred stimulus spatial frequencies. (2) Difference in contrast gain may underlie the observed contrast sensitivity variation of V1 neurons with different preferred stimulus spatial frequencies, possibly from either evolution or postnatal visual experiences.

What does Neural Plasticity Tell us about Role of Primary Visual Cortex (V1) in Visual Awareness?

The complete loss of visual awareness resulting from a lesion to the primary visual cortex (V1) suggests that this region is indispensable for conscious visual perception. There are however a number cases of conscious perception in the absence of V1 which appear to challenge this conclusion. These include reports of patients with bilateral V1 lesions sustained at an early age whose conscious vision has spontaneously recovered, as well as stroke patients who have recovered some conscious vision with the help of rehabilitation programs. In addition, the phenomenon of hemianopic completion and percepts induced by brain stimulation suggest that V1 may not be necessary for conscious perception in all circumstances. Furthermore, that the visual abilities in the cat are associated with the recovery of normal extrastriate tuning properties rather than emulation of V1 functions suggests that there is nothing unique about the functional properties of this region in visual awareness. Rather, the dramatic effect of a V1 lesion on visual awareness may be due to its role in providing the majority of extrastriate visual input, the loss of which abolishes normal neural responsiveness throughout the visual cortex.

The role of the primary visual cortex in perceptual suppression of salient visual stimuli.

The role of primary visual cortex (area V1) in subjective perception has intrigued students of vision for decades. Specifically, the extent to which the activity of different types of cells (monocular versus binocular) and electrophysiological signals (i.e., local field potentials versus spiking activity) reflect perception is still debated. To address these questions we recorded from area V1 of the macaque using tetrodes during the paradigm of binocular flash suppression, where incongruent images presented dichoptically compete for perceptual dominance. We found that the activity of a minority (20%) of neurons reflect the perceived visual stimulus and these cells exhibited perceptual modulations substantially weaker compared with their sensory modulation induced by congruent stimuli. Importantly, perceptual modulations were found equally often for monocular and binocular cells, demonstrating that perceptual competition in V1 involves mechanisms across both types of neurons. The power of the local field potential (LFP) also showed moderate perceptual modulations with similar percentages of sites showing significant effects across frequency bands (18-22%). The possibility remains that perception may be strongly reflected in more elaborate aspects of activity in V1 circuits (e.g., specific neuronal subtypes) or perceptual states might have a modulatory role on more intricate aspects of V1 firing patterns (e.g., synchronization), not necessarily altering the firing rates of single cells or the LFP power dramatically.

Divergence of fMRI and neural signals in V1 during perceptual suppression in the awake monkey

The role of primary visual cortex (V1) in determining the contents of perception is controversial. Human functional magnetic resonance imaging (fMRI) studies of perceptual suppression have revealed a robust drop in V1 activity when a stimulus is subjectively invisible. In contrast, monkey single-unit recordings have failed to demonstrate such perception-locked changes in V1. To investigate the basis of this discrepancy, we measured both the blood oxygen level-dependent (BOLD) response and several electrophysiological signals in two behaving monkeys. We found that all signals were in good agreement during conventional stimulus presentation, showing strong visual modulation to presentation and removal of a stimulus. During perceptual suppression, however, only the BOLD response and the low-frequency local field potential (LFP) power showed decreases, whereas the spiking and high-frequency LFP power were unaffected. These results demonstrate that the coupling between the BOLD and electrophysiological signals in V1 is context dependent, with a marked dissociation occurring during perceptual suppression.

Physiological Mechanisms Underlying Motion-Induced Blindness

AbstractVisual disappearance illusions - such as motion-induced blindness (MIB) - are commonly used to study the neural underpinnings of visual perception. In such illusions a salient visual target becomes perceptually invisible. Previous studies are inconsistent regarding the role of primary visual cortex (V1) in these illusions. Here we provide physiological and psychophysical evidence supporting a role for V1 in generating MIB.

Role of primary visual cortex in the binocular integration of plaid motion perception

This study assessed the early mechanisms underlying perception of plaid motion. Thus, two superimposed gratings drifting in a rightward direction composed plaid stimuli whose global motion direction was perceived as the vector sum of the two components. The first experiment was aimed at comparing the perception of plaid motion when both components were presented to both eyes (dioptic) or separately to each eye (dichoptic). When components of the patterns had identical spatial frequencies, coherent motion was correctly perceived under dioptic and dichoptic viewing condition. However, the perceived direction deviated from the predicted direction when spatial frequency differences were introduced between components in both conditions. The results suggest that motion integration follows similar rules for dioptic and dichoptic plaids even though performance under dichoptic viewing did not reach dioptic levels. In the second experiment, the role of early cortical areas in the processing of both plaids was examined. As convergence of monocular inputs is needed for dichoptic perception, we tested the hypothesis that primary visual cortex (V1) is required for dichoptic plaid processing by delivering repetitive transcranial magnetic stimulation to this area. Ten minutes of magnetic stimulation disrupted subsequent dichoptic perception for approximately 15 min, whereas no significant changes were observed for dioptic plaid perception. Taken together, these findings suggest that V1 is not crucial for the processing of dioptic plaids but it is necessary for the binocular integration underlying dichoptic plaid motion perception.

Cone inputs in macaque primary visual cortex.

To understand the role of primary visual cortex (V1) in color vision, we measured directly the input from the 3 cone types in macaque V1 neurons. Cells were classified as luminance-preferring, color-luminance, or color-preferring from the ratio of the peak amplitudes of spatial frequency responses to red/green equiluminant and to black/white (luminance) grating patterns, respectively. In this study we used L-, M-, and S-cone-isolating gratings to measure spatial frequency response functions for each cone type separately. From peak responses to cone-isolating stimuli we estimated relative cone weights and whether cone inputs were the same or opposite sign. For most V1 cells the relative S-cone weight was <0.1. All color-preferring cells were cone opponent and their L/M cone weight ratio was clustered around a value of -1, which is roughly equal and opposite L and M cone signals. Almost all cells (88%) classified as luminance cells were cone nonopponent, with a broad distribution of cone weights. Most cells (73%) classified as color-luminance cells were cone opponent. This result supports our conclusion that V1 color-luminance cells are double-opponent. Such neurons are more sensitive to color boundaries than to areas of color and thereby could play an important role in color perception. The color-luminance population had a broad distribution of L/M cone weight ratios, implying a broad distribution of preferred colors for the double-opponent cells.

Layer 4C in monkey V1 may linearize the output of the LGN

In primates, most LGN fibers terminate in cortical layer 4C, an anatomically prominent structure of unexplained function. We hypothesize that the enormous number of cells in layer 4C of monkey primate visual cortex functions as a neural network “hidden layer” that inverts distortions introduced by transmitting visual signals through the LGN. This hypothesis helps explain how simple cells respond (quasi-) linearly to visual inputs in spite of nonlinearities present in LGN responses. Linearization averts prematurely discarding visual information, in keeping with the role of primary visual cortex as the source of raw visual information to the rest of the brain.

Spatial distribution of contextual interactions in primary visual cortex and in visual perception.

To examine the role of primary visual cortex in visuospatial integration, we studied the spatial arrangement of contextual interactions in the response properties of neurons in primary visual cortex of alert monkeys and in human perception. We found a spatial segregation of opposing contextual interactions. At the level of cortical neurons, excitatory interactions were located along the ends of receptive fields, while inhibitory interactions were strongest along the orthogonal axis. Parallel psychophysical studies in human observers showed opposing contextual interactions surrounding a target line with a similar spatial distribution. The results suggest that V1 neurons can participate in multiple perceptual processes via spatially segregated and functionally distinct components of their receptive fields.

The role of primary visual cortex (V1) in visual awareness

In the search for the neural correlate of visual awareness, much controversy exists about the role of primary visual cortex. Here, the neurophysiological data from V1 recordings in awake monkeys are examined in light of two general classes of models of visual awareness. In the first model type, visual awareness is seen as being mediated either by a particular set of areas or pathways, or alternatively by a specific set of neurons. In these models, the role of V1 seems rather limited, as the mere activity of V1 cells seems insufficient to mediate awareness. In the second model type, awareness is hypothesized to be mediated by a global mechanism, i.e. a specific kind of activity not linked to a particular area or cell type. Two separate versions of global models are discussed, synchronous oscillations and spike rate modulations. It is shown that V1 synchrony does not reflect perception but rather the horizontal connections between neurons, indicating that V1 synchrony cannot be a direct neural correlate of conscious percepts. However, the rate of spike discharges of V1 neurons is strongly modulated by perceptual context, and these modulations correlate very well with aspects of perceptual organization, visual awareness, and attention. If these modulations serve as a neural correlate of visual awareness, then V1 contributes to that neural correlate. Whether V1 plays a role in the neural correlate of visual awareness thus strongly depends on the way visual awareness is hypothesized to be implemented in the brain.

Improvement in visual sensitivity by changes in local context: Parallel studies in human observers and in V1 of alert monkeys

To explore the role of primary visual cortex in contour integration, we measured the contextual sensitivity of human contrast thresholds and of superficial layer complex cells in monkey V1. An observer's contrast detection was 40% improved by a second suprathreshold bar; the effect was decreased as the two bars were separated along their axis of orientation, were displaced from colinearity, and had their relative orientation changed. Recordings from V1 showed that 42% of complex cells demonstrated facilitation for a second bar outside their classical receptive fields with a similar dependency on relative location and orientation. Both effects were eliminated by an orthogonal line between the two iso-oriented lines. Multiple randomly placed and oriented lines in the receptive field surround often caused a reduction in a cell's response to an optimally oriented stimulus, but this inhibition could be eliminated by changing the orientation of a few of these elements to colinearity with the centrally located target.