Population response to contextual influences in the primary visual cortex

Microstimulation in the primary visual cortex: activity patterns and their relation to visual responses and evoked saccades

Intracortical microstimulation (ICMS) in the primary visual cortex (V1) can generate the visual perception of a small point of light, termed phosphene, and evoke saccades directed to the receptive field of the stimulated neurons. Although ICMS is widely used, a direct measurement of the spatio-temporal patterns of neural activity evoked by ICMS and their relation to the neural responses evoked by visual stimuli or how they relate to ICMS-evoked saccades are still missing. To investigate this, we combined ICMS with voltage-sensitive dye imaging in V1 of behaving monkeys and measured neural activity at a high spatial (meso-scale) and temporal resolution. We then compared the population response evoked by small visual stimuli to those evoked by microstimulation. Both stimulation types evoked population activity that spread over few millimeters in V1 and propagated to extrastriate areas. However, the population responses evoked by ICMS have shown faster dynamics for the activation transients and the horizontal propagation of activity revealed a wave-like propagation. Finally, neural activity in the ICMS condition was higher for trials with evoked saccades as compared with trials without saccades. Our results uncover the spatio-temporal patterns evoked by ICMS and their relation to visual processing and saccade generation.

Collinear search impairment is luminance contrast invariant

Collinear search impairment (CSI) is a phenomenon where a task-irrelevant collinear structure impairs a target search in a visual display. It has been suggested that CSI is monocular, occurs without the participants' access to consciousness and is possibly processed at an early visual site (e.g. V1). This effect has frequently been compared with a well-documented opposite effect called attentional capture (AC), in which salient and task-irrelevant basic features (e.g. color, orientation) enhance target detection. However, whether this phenomenon can be attributed to non-attentional factors such as collinear facilitation (CF) has not yet been formally tested. Here we used one well-established property of CF, i.e. that target contrast modulates its effect direction (facilitation vs suppression), to examine whether CSI shared similar signature profiles along different contrast levels. In other words, we tested whether CSI previously observed at the supra-threshold level was reduced or reversed at near-threshold contrast levels. Our results showed that, regardless of the luminance contrast levels, participants spent a longer time searching for targets displayed on the salient singleton collinear structure than those displayed off the structure. Contrast invariance suggests that it is unlikely that CSI is exclusively sub-served by an early vision mechanism (e.g. CF).

Functional ultrasound imaging of deep visual cortex in awake nonhuman primates

Nowadays, several techniques exist to study and better understand how the brain works (fMRI, EEG, electrophysiology, etc.). Each has its own advantages and disadvantages (spatiotemporal resolution, maximal recording depth, signal-to-noise ratio, etc.). In this article, we show that the new functional ultrasound (fUS) imaging technique is appropriate to record and map brain activity in awake primates on a scale previously unreachable. It allows distinguishing patterns similar to ocular dominance bands in the visual cortex through all layers of the cortex, which was impossible before with common techniques. This paper demonstrates the utility of fUS imaging for studying brain activity in awake primates and its interest to all neuroscientists.

Deep regions of the brain are not easily accessible to investigation at the mesoscale level in awake animals or humans. We have recently developed a functional ultrasound (fUS) technique that enables imaging hemodynamic responses to visual tasks. Using fUS imaging on two awake nonhuman primates performing a passive fixation task, we constructed retinotopic maps at depth in the visual cortex (V1, V2, and V3) in the calcarine and lunate sulci. The maps could be acquired in a single-hour session with relatively few presentations of the stimuli. The spatial resolution of the technology is illustrated by mapping patterns similar to ocular dominance (OD) columns within superficial and deep layers of the primary visual cortex. These acquisitions using fUS suggested that OD selectivity is mostly present in layer IV but with extensions into layers II/III and V. This imaging technology provides a new mesoscale approach to the mapping of brain activity at high spatiotemporal resolution in awake subjects within the whole depth of the cortex.

Cortical Interactions between Prosthetic and Natural Vision

Outer retinal degenerative diseases, such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD), are among the leading causes of incurable blindness in the Western world [1]. Retinal prostheses have been shown to restore some useful vision by electrically stimulating the remaining retinal neurons [2]. In contrast to inherited retinal degenerative diseases (e.g., RP), typically leading to a complete loss of the visual field, in AMD patients the disease is localized to the macula, leaving the peripheral vision intact. Implanting a retinal prosthesis in the central macula in AMD patients [3, 4] leads to an intriguing situation where the patient's central retina is stimulated electrically, whereas the peripheral healthy retina responds to natural light stimulation. An important question is whether the visual cortex responds to these two concurrent stimuli similarly to the interaction between two adjacent natural light stimuli projected onto healthy retina. Here, we investigated the cortical interactions between prosthetic and natural vision based on visually evoked potentials (VEPs) recorded in rats implanted with photovoltaic subretinal implants. Using this model, where prosthetic and natural vision information are combined in the visual cortex, we observed striking similarities in the interactions of natural and prosthetic vision, including similar effect of background illumination, linear summation of non-patterned stimuli, and lateral inhibition with spatial patterns [5], which increased with target contrast. These results support the idea of combined prosthetic and natural vision in restoration of sight for AMD patients.

High-resolution VSDI retinotopic mapping via a DLP-based projection system

High-resolution recording of visual cortex activity is an important tool for vision research. Using a customized digital mirror device (DMD) - based system equipped with retinal imaging, we projected visual stimuli directly on the rat retina and recorded cortical responses by voltage-sensitive dye imaging. We obtained robust cortical responses and generated high-resolution retinotopic maps at an unprecedented retinal resolution of 4.6 degrees in the field of view, while further distinguishing between normal and pathological retinal areas. This system is a useful tool for studying the cortical response to localized retinal stimulation and may shed light on various cortical plasticity processes.

Nonlinear Lateral Interactions in V1 Population Responses Explained by a Contrast Gain Control Model

How do cortical responses to local image elements combine to form a spatial pattern of population activity in primate V1? Here, we used voltage-sensitive dye imaging, which measures summed membrane potential activity, to examine the rules that govern lateral interactions between the representations of two small local-oriented elements in macaque (Macaca mulatta) V1. We find strong subadditive and mostly orientation-independent interactions for nearby elements [2-4 mm interelement cortical distance (IED)] that gradually become linear at larger separations (>6 mm IED). These results are consistent with a population gain control model describing nonlinear V1 population responses to single oriented elements. However, because of the membrane potential-to-spiking accelerating nonlinearity, the model predicts supra-additive lateral interactions of spiking responses for intermediate separations at a range of locations between the two elements, consistent with some prior facilitatory effects observed in electrophysiology and psychophysics. Overall, our results suggest that population-level lateral interactions in V1 are primarily explained by a simple orientation-independent contrast gain control mechanism.SIGNIFICANCE STATEMENT Interactions between representations of simple visual elements such as oriented edges in primary visual cortex (V1) are thought to contribute to our ability to easily integrate contours and segment surfaces, but the mechanisms that govern these interactions are primarily unknown. Our study provides novel evidence that lateral interactions at the population level are governed by a simple contrast gain-control mechanism, and we show how this divisive gain-control mechanism can give rise to apparently facilitatory spiking responses.

Increased gamma band activity for lateral interactions in humans

Collinear facilitation of contrast sensitivity supported by lateral interactions within primary visual cortex is implicated in contour and object perception, with neural correlates in several frequency bands. Although higher component of the ERP power spectrum, the gamma-band, is postulated to reflect object representation, attention and memory, its neuronal source has been questioned, suggesting it is an artifact reflecting saccadic eye movements. Here we explored the gamma-band activity during collinear facilitation with no saccade-related confounds. We used single-trial spectral analysis of ERP in occipital channels in a time-window of nearly complete saccadic suppression and discarded sporadic trials containing saccades, in order to avoid saccadic artifacts. Although converging evidence suggests that gamma-band oscillations emerge from local excitatory–inhibitory balance involving GABAergic inhibition, here we show activity amplification during facilitatory collinear interactions, presumably dominated by excitations, in the gamma-band 150–350 milliseconds following onset of low near-threshold contrast stimulus. This result highlights the potential role of gamma-band oscillations in neuronal encoding of basic processes in visual perception. Thus, our findings suggest that gamma-band ERP spectrum analysis may serve as a useful and reliable tool for exploring basic perception, both in normal adults and in special populations.

Space-Time Dynamics of Membrane Currents Evolve to Shape Excitation, Spiking, and Inhibition in the Cortex at Small and Large Scales

In the cerebral cortex, membrane currents, i.e., action potentials and other membrane currents, express many forms of space-time dynamics. In the spontaneous asynchronous irregular state, their space-time dynamics are local non-propagating fluctuations and sparse spiking appearing at unpredictable positions. After transition to active spiking states, larger structured zones with active spiking neurons appear, propagating through the cortical network, driving it into various forms of widespread excitation, and engaging the network from microscopic scales to whole cortical areas. At each engaged cortical site, the amount of excitation in the network, after a delay, becomes matched by an equal amount of space-time fine-tuned inhibition that might be instrumental in driving the dynamics toward perception and action.

Interocular contrast difference drives illusory 3D percept

Any processing delay between the two eyes can result in illusory 3D percepts for moving objects because of either changes in the pure disparities over time for disparity sensors or by changes to sensors that encode motion/disparity conjointly. This is demonstrated by viewing a fronto-parallel pendulum through a neutral density (ND) filter placed over one eye, resulting in the illusory 3D percept of the pendulum following an elliptical orbit in depth, the so-called Pulfrich phenomenon. Here we use a paradigm where a cylinder rotating in depth, defined by moving Gabor patches is presented at different interocular phases, generating strong to ambiguous depth percepts. This paradigm allows one to manipulate independently the contrast and the luminance of the patches to determine their influence on perceived motion-in-depth. Thus we show psychophysically that an interocular contrast difference can itself result in a similar illusory 3D percept of motion-in-depth. We argue that contrast, like luminance (ND filter) can modify the dynamics of visual neurons resulting in an interocular processing delay or an interocular velocity difference.

Temporal asynchrony and spatial perception

Collinear facilitation is an enhancement in the visibility of a target by laterally placed iso-oriented flankers in a collinear (COL) configuration. Iso-oriented flankers placed in a non-collinear configuration (side-by-side, SBS) produce less facilitation. Surprisingly, presentation of both configurations simultaneously (ISO-CROSS) abolishes the facilitation rather than increases it - a phenomenon that can’t be fully explained by the spatial properties of the target and flankers. Based on our preliminary data and recent studies, we hypothesized that there might be a novel explanation based on the temporal properties of the excitation and inhibition, resulting in asynchrony between the lateral inputs received from COL and SBS, leading to cancelation of the facilitatory component in ISO-CROSS. We explored this effect using a detection task in humans. The results replicated the previous results showing that the preferred facilitation for COL and SBS was abolished for the ISO-CROSS configuration. However, presenting the SBS flankers, but not the COL flankers 20 msec before ISO-CROSS restored the facilitatory effect. We propose a novel explanation that the perceptual advantage of collinear facilitation may be cancelled by the delayed input from the sides; thus, the final perception is determined by the overall spatial-temporal integration of the lateral interactions.

Noise and the Perceptual Filling-in effect

Nearby collinear flankers increase the false alarm rate (reports of the target being present when it is not) in a Yes-No experiment. This effect has been attributed to "filling-in" of the target location due to increased activity induced by the flankers. According to signal detection theory, false alarms are attributed to noise in the visual nervous system. Here we investigated the effect of external noise on the filling-in effect by adding white noise to a low contrast Gabor target presented between two collinear Gabor flankers at a range of target-flanker separations. External noise modulates the filling-in effect, reducing visual sensitivity (d') and increasing the filling-in effect (False Alarm rate). We estimated the amount of external noise at which the false alarm rate increases by the √2 (which we refer to as NFA). Across flank distances, both the false alarm rate and d' (with no external noise) are correlated with NFA. These results are consistent with the notion that nearby collinear flankers add both signal and noise to the target location. The increased signal results in higher d' values; the increased noise to higher false alarm rates (the filling effect).

Effects of GABAA kinetics on cortical population activity: computational studies and physiological confirmations

Voltage-sensitive dye (VSD) imaging produces an unprecedented real-time and high-resolution mesoscopic signal to measure the cortical population activity. We have previously shown that the neuronal compartments contributions to the signal are dynamic and stimulus-dependent (Chemla S, Chavane F. Neuroimage 53: 420-438, 2010). Moreover, the VSD signal can also be strongly affected by the network state, such as in anesthetized vs. awake preparations. Here, we investigated the impact of the network state, through GABAA receptors modulation, on the VSD signal using a computational approach. We therefore systematically measured the effect of the GABAA-mediated inhibitory postsynaptic potentials (IPSPs) decay time constant (τG) on our modeled VSD response to an input stimulus of increasing strength. Our simulations suggest that τG strongly modulates the dynamics of the VSD signal, affecting the amplitude, input response function, and the transient balance of excitation and inhibition. We confirmed these predictions experimentally on awake and anesthetized monkeys, comparing VSD responses to drifting gratings stimuli of various contrasts. Lastly, one in vitro study has suggested that GABAA receptors may also be directly affected by the VSDs themselves (Mennerick S, Chisari M, Shu H, Taylor A, Vasek M, Eisenman L, Zorumski C. J Neurosci 30: 2871-2879, 2010). Our modeling approach suggests that the type of modulation described in this study would actually have a negligible influence on the population response. This study highlights that functional results acquired with different techniques and network states must be compared with caution. Biophysical models are proposed here as an adequate tool to delineate the domain of VSD data interpretation.

Single additive mechanism predicts lateral interactions effects-computational model

The mechanism underlying the lateral interactions (LI) phenomenon is still an enigma. Over the years, several groups have tried to explain the phenomenon and suggested models to predict its psychophysical results. Most of these models comprise both inhibitory and facilitatory mechanisms for describing the LI phenomenon. Their studies' assumption that a significant inhibition mechanism exists is based on the classical interpretation of the threshold elevation perceived in psychophysical experiments. In this work, we suggest a different interpretation of the threshold elevation obtained experimentally. Our model proposes and demonstrates how a facilitatory additive mechanism can solely predict both the facilitation and "inhibition" aspects of the phenomenon, without the need for an additional inhibitory mechanism, at least for the two flankers' configurations. Though the model is simple it succeeds to predict the LI effect under a large variety of stimuli configurations and parameters. The model is in agreement with both classical and recent psychophysical and neurophysiological results. We suggest that the LI mechanism plays a role in creating an educated guess to form a continuation of gratings and textures based on the surrounding visual stimuli.

Representation of Color Surfaces in V1: Edge Enhancement and Unfilled Holes

The neuronal mechanism underlying the representation of color surfaces in primary visual cortex (V1) is not well understood. We tested on color surfaces the previously proposed hypothesis that visual perception of uniform surfaces is mediated by an isomorphic, filled-in representation in V1. We used voltage-sensitive-dye imaging in fixating macaque monkeys to measure V1 population responses to spatially uniform chromatic (red, green, or blue) and achromatic (black or white) squares of different sizes (0.5°-8°) presented for 300 ms. Responses to both color and luminance squares early after stimulus onset were similarly edge-enhanced: for squares 1° and larger, regions corresponding to edges were activated much more than those corresponding to the center. At later times after stimulus onset, responses to achromatic squares' centers increased, partially "filling-in" the V1 representation of the center. The rising phase of the center response was slower for larger squares. Surprisingly, the responses to color squares behaved differently. For color squares of all sizes, responses remained edge-enhanced throughout the stimulus. There was no filling-in of the center. Our results imply that uniform filled-in representations of surfaces in V1 are not required for the perception of uniform surfaces and that chromatic and achromatic squares are represented differently in V1.

SIGNIFICANCE STATEMENT

We used voltage-sensitive dye imaging from V1 of behaving monkeys to test the hypothesis that visual perception of uniform surfaces is mediated by an isomorphic, filled-in representation. We found that the early population responses to chromatic and achromatic surfaces are edge enhanced, emphasizing the importance of edges in surface processing. Next, we show for color surfaces that responses remained edge-enhanced throughout the stimulus presentation whereas response to luminance surfaces showed a slow neuronal 'filling-in' of the center. Our results suggest that isomorphic representation is not a general code for uniform surfaces in V1.

The effect of spatial frequency on peripheral collinear facilitation

The detection of a Gabor patch (target) can be decreased or improved by the presence of co-oriented Gabor patches (flankers) having the same spatial frequency as the target. These phenomena are thought to be mediated by lateral interactions. Depending on the distance between target and flankers, commonly defined as a multiple of the wavelength (λ) of the carrier, flankers can increase or decrease a target's detectability. Studies with foveal presentation showed that for target-to-flankers distances<2λ contrast thresholds for the central target increase, while for target-to-flankers distances>3λ contrast thresholds decrease. Earlier studies on collinear facilitation at the near-periphery of the visual field (4° of eccentricity) showed inconsistent facilitation (Shani & Sagi, 2005, Vision Research, 45, 2009-2024) whereas more recent studies showed consistent facilitation for larger separations (7-8λ) (Maniglia et al., 2011, PLoS ONE, 6, e25568; Lev & Polat, 2011, Vision Research, 51, 2488-2498). However, all of these studies used medium-to-high spatial frequencies (3-8 cpd). In this study we tested lower spatial frequencies (1, 2, and 3 cpd) with different target-to-flankers distances. The rationale was that near-peripheral vision is tuned for lower spatial frequencies and this could be reflected in collinear facilitation. Results show consistent collinear facilitation at 8λ for all the spatial frequencies tested, but also show collinear facilitation at shorter target-to-flanker distance (6λ) for the lowest spatial frequencies tested (1 cpd). Additionally, collinear facilitation decreases as spatial frequency increases; opposite to the findings of Polat (2009, Spatial Vision, 22, 179-193) in the fovea, indicating a different spatial frequency tuning between foveal and peripheral lateral interactions.

A contrast and surface code explains complex responses to black and white stimuli in V1

We investigated the cortical mechanisms underlying the visual perception of luminance-defined surfaces and the preference for black over white stimuli in the macaque primary visual cortex, V1. We measured V1 population responses with voltage-sensitive dye imaging in fixating monkeys that were presented with white or black squares of equal contrast around a mid-gray. Regions corresponding to the squares' edges exhibited higher activity than those corresponding to the center. Responses to black were higher than to white, surprisingly to a much greater extent in the representation of the square's center. Additionally, the square-evoked activation patterns exhibited spatial modulations along the edges and corners. A model comprised of neural mechanisms that compute local contrast, local luminance temporal modulations in the black and white directions, and cortical center-surround interactions, could explain the observed population activity patterns in detail. The model captured the weaker contribution of V1 neurons that respond to positive (white) and negative (black) luminance surfaces, and the stronger contribution of V1 neurons that respond to edge contrast. Also, the model demonstrated how the response preference for black could be explained in terms of stronger surface-related activation to negative luminance modulation. The spatial modulations along the edges were accounted for by surround suppression. Overall the results reveal the relative strength of edge contrast and surface signals in the V1 response to visual objects.

Cortico-cortical communication dynamics

In principle, cortico-cortical communication dynamics is simple: neurons in one cortical area communicate by sending action potentials that release glutamate and excite their target neurons in other cortical areas. In practice, knowledge about cortico-cortical communication dynamics is minute. One reason is that no current technique can capture the fast spatio-temporal cortico-cortical evolution of action potential transmission and membrane conductances with sufficient spatial resolution. A combination of optogenetics and monosynaptic tracing with virus can reveal the spatio-temporal cortico-cortical dynamics of specific neurons and their targets, but does not reveal how the dynamics evolves under natural conditions. Spontaneous ongoing action potentials also spread across cortical areas and are difficult to separate from structured evoked and intrinsic brain activity such as thinking. At a certain state of evolution, the dynamics may engage larger populations of neurons to drive the brain to decisions, percepts and behaviors. For example, successfully evolving dynamics to sensory transients can appear at the mesoscopic scale revealing how the transient is perceived. As a consequence of these methodological and conceptual difficulties, studies in this field comprise a wide range of computational models, large-scale measurements (e.g., by MEG, EEG), and a combination of invasive measurements in animal experiments. Further obstacles and challenges of studying cortico-cortical communication dynamics are outlined in this critical review.

Vobi One: a data processing software package for functional optical imaging

Optical imaging is the only technique that allows to record the activity of a neuronal population at the mesoscopic scale. A large region of the cortex (10-20 mm diameter) is directly imaged with a CCD camera while the animal performs a behavioral task, producing spatio-temporal data with an unprecedented combination of spatial and temporal resolutions (respectively, tens of micrometers and milliseconds). However, researchers who have developed and used this technique have relied on heterogeneous software and methods to analyze their data. In this paper, we introduce Vobi One, a software package entirely dedicated to the processing of functional optical imaging data. It has been designed to facilitate the processing of data and the comparison of different analysis methods. Moreover, it should help bring good analysis practices to the community because it relies on a database and a standard format for data handling and it provides tools that allow producing reproducible research. Vobi One is an extension of the BrainVISA software platform, entirely written with the Python programming language, open source and freely available for download at https://trac.int.univ-amu.fr/vobi_one.

Population responses to contour integration: early encoding of discrete elements and late perceptual grouping

The neuronal mechanisms underlying perceptual grouping of discrete, similarly oriented elements are not well understood. To investigate this, we measured neural population responses using voltage-sensitive dye imaging in V1 of monkeys trained on a contour-detection task. By mapping the contour and background elements onto V1, we could study their neural processing. Population response early in time showed activation patches corresponding to the contour/background individual elements. However, late increased activity in the contour elements, along with suppressed activity in the background elements, enabled us to visualize in single trials a salient continuous contour "popping out" from a suppressed background. This modulated activity in the contour and in background extended beyond the cortical representation of individual contour or background elements. Finally, the late modulation was correlated with behavioral performance of contour saliency and the monkeys' perceptual report. Thus, opposing responses in the contour and background may underlie perceptual grouping in V1.

Fovea-periphery axis symmetry of surround modulation in the human visual system

A visual stimulus activates different sized cortical area depending on eccentricity of the stimulus. Here, our aim is to understand whether the visual field size of a stimulus or cortical size of the corresponding representation determines how strongly it interacts with other stimuli. We measured surround modulation of blood-oxygenation-level-dependent signal and perceived contrast with surrounds that extended either towards the periphery or the fovea from a center stimulus, centered at 6° eccentricity. This design compares the effects of two surrounds which are identical in visual field size, but differ in the sizes of their cortical representations. The surrounds produced equally strong suppression, which suggests that visual field size of the surround determines suppression strength. A modeled population of neuronal responses, in which all the parameters were experimentally fixed, captured the pattern of results both in psychophysics and functional magnetic resonance imaging. Although the fovea-periphery anisotropy affects nearly all aspects of spatial vision, our results suggest that in surround modulation the visual system compensates for it.

Population response to natural images in the primary visual cortex encodes local stimulus attributes and perceptual processing

The primary visual cortex (V1) is extensively studied with a large repertoire of stimuli, yet little is known about its encoding of natural images. Using voltage-sensitive dye imaging in behaving monkeys, we measured neural population response evoked in V1 by natural images presented during a face/scramble discrimination task. The population response showed two distinct phases of activity: an early phase that was spread over most of the imaged area, and a late phase that was spatially confined. To study the detailed relation between the stimulus and the population response, we used a simple encoding model to compute a continuous map of the expected neural response based on local attributes of the stimulus (luminance and contrast), followed by an analytical retinotopic transformation. Then, we computed the spatial correlation between the maps of the expected and observed response. We found that the early response was highly correlated with the local luminance of the stimulus and was sufficient to effectively discriminate between stimuli at the single trial level. The late response, on the other hand, showed a much lower correlation to the local luminance, was confined to central parts of the face images, and was highly correlated with the animal's perceptual report. Our study reveals a continuous spatial encoding of low- and high-level features of natural images in V1. The low level is directly linked to the stimulus basic local attributes and the high level is correlated with the perceptual outcome of the stimulus processing.

Speeding up the brain: when spatial facilitation translates into latency shortening

Waves of activity following a focal stimulation are reliably observed to spread across the cortical tissue. The origin of these waves remains unclear and the underlying mechanisms and function are still debated. In this study, we ask whether waves of activity modulate the magnetoencephalography (MEG) signals recorded in humans during visual stimulation with Gabor patches sequentially flashed along a vertical path, eliciting a perception of vertical apparent motion. Building upon the functional properties of long-rang horizontal connections, proposed to contribute to spreading activity, we specifically probe the amplitude and latency of MEG responses as a function of Gabor contrast and orientation. The results indicate that in the left hemisphere the response amplitude is enhanced and the half height response latency is shortened for co-aligned Gabor as compared to misaligned Gabor patches at a low but not at a high contrast. Building upon these findings, we develop a biologically plausible computational model that performs a “spike time alignment” of the responses to elongated contours with varying contrast, endowing them with a phase advance relative to misaligned contours.

Collinear stimuli induce local and cross-areal coherence in the visual cortex of behaving monkeys

Background

Collinear patterns of local visual stimuli are used to study contextual effects in the visual system. Previous studies have shown that proximal collinear flankers, unlike orthogonal, can enhance the detection of a low contrast central element. However, the direct neural interactions between cortical populations processing the individual flanker elements and the central element are largely unknown.

Methodology/Principal Findings

Using voltage-sensitive dye imaging (VSDI) we imaged neural population responses in V1 and V2 areas in fixating monkeys while they were presented with collinear or orthogonal arrays of Gabor patches. We then studied the spatio-temporal interactions between neuronal populations processing individual Gabor patches in the two conditions. Time-frequency analysis of the stimulus-evoked VSDI signal showed power increase mainly in low frequencies, i.e., the alpha band (α; 7–14 Hz). Power in the α-band was more discriminative at a single trial level than other neuronal population measures. Importantly, the collinear condition showed an increased intra-areal (V1-V1 and V2-V2) and inter-areal (V1-V2) α-coherence with shorter latencies than the orthogonal condition, both before and after the removal of the stimulus contribution. α-coherence appeared between discrete neural populations processing the individual Gabor patches: the central element and the flankers.

Conclusions/Significance

Our findings suggest that collinear effects are mediated by synchronization in a distributed network of proximal and distant neuronal populations within and across V1 and V2.

Dynamics of local input normalization result from balanced short- and long-range intracortical interactions in area V1

To efficiently drive many behaviors, sensory systems have to integrate the activity of large neuronal populations within a limited time window. These populations need to rapidly achieve a robust representation of the input image, probably through canonical computations such as divisive normalization. However, little is known about the dynamics of the corticocortical interactions implementing these rapid and robust computations. Here, we measured the real-time activity of a large neuronal population in V1 using voltage-sensitive dye imaging in behaving monkeys. We found that contrast gain of the population increases over time with a time constant of ~30 ms and propagates laterally over the cortical surface. This dynamic is well accounted for by a divisive normalization achieved through a recurrent network that transiently increases in size after response onset with a slow swelling speed of 0.007-0.014 m/s, suggesting a polysynaptic intracortical origin. In the presence of a surround, this normalization pool is gradually balanced by lateral inputs propagating from distant cortical locations. This results in a centripetal propagation of surround suppression at a speed of 0.1-0.3 m/s, congruent with horizontal intracortical axons speed. We propose that a simple generalized normalization scheme can account for both the dynamical contrast response function through recurrent polysynaptic intracortical loops and for the surround suppression through long-range monosynaptic horizontal spread. Our results demonstrate that V1 achieves a rapid and robust context-dependent input normalization through a timely push-pull between local and lateral networks. We suggest that divisive normalization, a fundamental canonical computation, should be considered as a dynamic process.

Propagating waves in thalamus, cortex and the thalamocortical system: Experiments and models

Propagating waves of activity have been recorded in many species, in various brain states, brain areas, and under various stimulation conditions. Here, we review the experimental literature on propagating activity in thalamus and neocortex across various levels of anesthesia and stimulation conditions. We also review computational models of propagating waves in networks of thalamic cells, cortical cells and of the thalamocortical system. Some discrepancies between experiments can be explained by the "network state", which differs vastly between anesthetized and awake conditions. We introduce a network model displaying different states and investigate their effect on the spatial structure of self-sustained and externally driven activity. This approach is a step towards understanding how the intrinsically-generated ongoing activity of the network affects its ability to process and propagate extrinsic input.

Learning to be fast: gain accuracy with speed

Our recent neurophysiological findings provided evidence for collinear facilitation in detecting low-contrast Gabor patches (GPs) and for the abolishment of these collinear interactions by backward masking (BM) (Sterkin et al., 2008; Sterkin, Yehezkel, Bonneh, et al., 2009). We suggested that the suppression induced by the BM eliminates the collinear facilitation. Moreover, our recent study showed that training on a BM task overcomes the BM effect, hence, improves the processing speed (Polat, 2009). Here we applied training on detecting a target that is followed by BM in order to study whether reinforced facilitatory interactions can overcome the suppressive effects induced by BM. Event-Related Potentials (ERPs) were recorded before and after ten training sessions performed on different days. Low-contrast, foveal target GP was simultaneously flanked by two collinear high-contrast GPs. In the BM task, another identical mask was presented at different time-intervals (ISIs). Before training, BM induced suppression of target detection, at the ISI of 50 ms, in agreement with earlier behavioral findings. This ISI coincides with the active time-window of lateral interactions. After training, our results show a remarkable improvement in all behavioral measurements, including percent of correct responses, sensitivity (d'), reaction time (RT) and the decision criterion for this ISI. The ERP results show that before training,BM attenuated the physiological markers of facilitation at the same ISI of 50 ms, measured as the amplitude of the negative N1 peak (latency of 260 ms). After the training, the sensory representation, reflected by P1 peak, has not changed, consistent with the unchanged physical parameters of the stimulus. Instead, the shorter latency (by 20 ms, latency of 240 ms) and the increased amplitude of N1 represent the development of faster and stronger facilitatory lateral interactions between the target and the collinear flankers. Thus, previously effective backward masking became ineffective in disrupting the collinear facilitation. Moreover, a high-amplitude late peak (P4, latency of 610-630 ms) was not affected by training, however its high correlation with RT (95%) before training was significantly decreased (to 76%), consistent with a lower-level representation of a trained skill. We suggest that perceptual learning that strengthens collinear facilitation results in a faster processing speed.

Spatiotemporal effects of microsaccades on population activity in the visual cortex of monkeys during fixation

During visual fixation, the eyes make fast involuntary miniature movements known as microsaccades (MSs). When MSs are executed they displace the visual image over the retina and can generate neural modulation along the visual pathway. However, the effects of MSs on neural activity have substantial variability and are not fully understood. By utilizing voltage-sensitive dye imaging, we imaged the spatiotemporal patterns induced by MSs in V1 and V2 areas of behaving monkeys while they were fixating and presented with visual stimuli. We then investigated the neuronal modulation dynamics, induced by MSs, under different visual stimulation. MSs induced monophasic or biphasic neural responses depending on stimulus size. These neural responses were accompanied by different spatiotemporal patterns of synchronization. Finally, we show that a local patch of population response evoked by a small stimulus was clearly shifted over the V1 retinotopic map after each MS. Our results demonstrate the lack of visual stability in V1 following MSs and help clarify the substantial variability reported for MSs effects on neuronal responses. The observed neural effects suggest that MSs are associated with a continuum of neuronal responses in V1 area reflecting diverse spatiotemporal dynamics.

Interpreting local visual features as a global shape requires awareness

How the brain constructs a coherent representation of the environment from noisy visual input remains poorly understood. Here, we explored whether awareness of the stimulus plays a role in the integration of local features into a representation of global shape. Participants were primed with a shape defined either by position or orientation cues, and performed a shape-discrimination task on a subsequently presented probe shape. Crucially, the probe could either be defined by the same or different cues as the prime, which allowed us to distinguish the effect of priming by local features and global shape. We found a robust priming benefit for visible primes, with response times being faster when the probe and prime were the same shape, regardless of the defining cue. However, rendering the prime invisible uncovered a dissociation: position-defined primes produced behavioural benefit only for probes of the same cue type. Surprisingly, orientation-defined primes afforded an enhancement only for probes of the opposite cue. In further experiments, we showed that the effect of priming was confined to retinotopic coordinates and that there was no priming effect by invisible orientation cues in an orientation-discrimination task. This explains the absence of priming by the same cue in our shape-discrimination task. In summary, our findings show that while in the absence of awareness orientation signals can recruit retinotopic circuits (e.g. intrinsic lateral connections), conscious processing is necessary to interpret local features as global shape.

Suppressive lateral interactions at parafoveal representations in primary visual cortex

The perceptual salience and visibility of image elements is influenced by other elements in their vicinity. The perceptual effect of image elements on an adjacent target element depends on their relative orientation. Collinear flanking elements usually improve sensitivity for the target element, whereas orthogonal elements have a weaker effect. It is believed that the collinear flankers exert these effects through lateral interactions between neurons in the primary visual cortex (area V1), but the precise mechanisms underlying these contextual interactions remain unknown. Here, we directly examined this question by recording the effects of flankers on the responses of V1 neurons at parafoveal representations while monkeys performed a fixation task or a contrast detection task. We found, unexpectedly, that collinear flankers reduce the monkeys' perceptual sensitivity for a central target element. This behavioral effect was explained by a flanker-induced increase in the activity of V1 neurons in the absence of the central target stimulus, which reduced the amplitude of the target response. Our results indicate that the dominant effect of collinear flankers in parafoveal vision is suppression and suggest that these suppressive effects are caused by a decrease in the dynamic range of neurons coding the central target.