Different Temporal Structure for Form versus Surface Cortical Color Systems – Evidence from Chromatic Non-Linear VEP

Physiological studies of color processing have typically measured responses to spatially varying chromatic stimuli such as gratings, while psychophysical studies of color include color naming, color and light, as well as spatial and temporal chromatic sensitivities. This raises the question of whether we have one or several cortical color processing systems. Here we show from non-linear analysis of human visual evoked potentials (VEP) the presence of distinct and independent temporal signatures for form and surface color processing. Surface color stimuli produced most power in the second order Wiener kernel, indicative of a slowly recovering neural system, while chromatic form stimulation produced most power in the first order kernel (showing rapid recovery). We find end-spectral saturation-dependent signals, easily separable from achromatic signals for surface color stimuli. However physiological responses to form color stimuli, though varying somewhat with saturation, showed similar waveform components. Lastly, the spectral dependence of surface and form color VEP was different, with the surface color responses almost vanishing with yellow-grey isoluminant stimulation whereas the form color VEP shows robust recordable signals across all hues. Thus, surface and form colored stimuli engage different neural systems within cortex, pointing to the need to establish their relative contributions under the diverse chromatic stimulus conditions used in the literature

The Biological Basis of a Universal Constraint on Color Naming: Cone Contrasts and the Two-Way Categorization of Colors

Many studies have provided evidence for the existence of universal constraints on color categorization or naming in various languages, but the biological basis of these constraints is unknown. A recent study of the pattern of color categorization across numerous languages has suggested that these patterns tend to avoid straddling a region in color space at or near the border between the English composite categories of “warm” and “cool”. This fault line in color space represents a fundamental constraint on color naming. Here we report that the two-way categorization along the fault line is correlated with the sign of the L- versus M-cone contrast of a stimulus color. Moreover, we found that the sign of the L-M cone contrast also accounted for the two-way clustering of the spatially distributed neural responses in small regions of the macaque primary visual cortex, visualized with optical imaging. These small regions correspond to the hue maps, where our previous study found a spatially organized representation of stimulus hue. Altogether, these results establish a direct link between a universal constraint on color naming and the cone-specific information that is represented in the primate early visual system

Enhancing spatio-chromatic representation with more-than-three color coding for image description

The extraction of spatio-chromatic features from color images is usually performed independently on each color channel. Usual 3D color spaces, such as RGB, present a high inter-channel correlation for natural images. This correlation can be reduced using color-opponent representations, but the spatial structure of regions with small color differences is not fully captured in two generic Red-Green and Blue-Yellow channels. To overcome these problems, we propose new color coding that is adapted to the specific content of each image. Our proposal is based on two steps: (a) setting the number of channels to the number of distinctive colors we find in each image (avoiding the problem of channel correlation), and (b) building a channel representation that maximizes contrast differences within each color channel (avoiding the problem of low local contrast). We call this approach more-than-three color coding (MTT) to emphasize the fact that the number of channels is adapted to the image content. The higher the color complexity of an image, the more channels can be used to represent it. Here we select distinctive colors as the most predominant in the image, which we call color pivots, and we build the new color coding strategy using these color pivots as a basis. To evaluate the proposed approach, we measure the efficiency in an image categorization task. We show how a generic descriptor improves performance at the description level when applied to the MTT coding

NICE : A Computational solution to close the gap from colour perception to colour categorization

The segmentation of visible electromagnetic radiation into chromatic categories by the human visual system has been extensively studied from a perceptual point of view, resulting in several colour appearance models. However, there is currently a void when it comes to relate these results to the physiological mechanisms that are known to shape the pre-cortical and cortical visual pathway. This work intends to begin to fill this void by proposing a new physiologically plausible model of colour categorization based on Neural Isoresponsive Colour Ellipsoids (NICE) in the cone-contrast space defined by the main directions of the visual signals entering the visual cortex. The model was adjusted to fit psychophysical measures that concentrate on the categorical boundaries and are consistent with the ellipsoidal isoresponse surfaces of visual cortical neurons. By revealing the shape of such categorical colour regions, our measures allow for a more precise and parsimonious description, connecting well-known early visual processing mechanisms to the less understood phenomenon of colour categorization. To test the feasibility of our method we applied it to exemplary images and a popular ground-truth chart obtaining labelling results that are better than those of current state-of-the-art algorithms

Spatial and color hallucinations in a mathematical model of primary visual cortex

We study a simplified model of the representation of colors in the primate primary cortical visual area V1. The model is described by an initial value problem related to a Hammerstein equation. The solutions to this problem represent the variation of the activity of populations of neurons in V1 as a function of space and color. The two space variables describe the spatial extent of the cortex while the two color variables describe the hue and the saturation represented at every location in the cortex. We prove the well-posedness of the initial value problem. We focus on its stationary, i.e. independent of time, and periodic in space solutions. We show that the model equation is equivariant with respect to the direct product G of the group of the Euclidean transformations of the planar lattice determined by the spatial periodicity and the group of color transformations, isomorphic to O(2), and study the equivariant bifurcations of its stationary solutions when some parameters in the model vary. Their variations may be caused by the consumption of drugs and the bifurcated solutions may represent visual hallucinations in space and color. Some of the bifurcated solutions can be determined by applying the Equivariant Branching Lemma (EBL) by determining the axial subgroups of G . These define bifurcated solutions which are invariant under the action of the corresponding axial subgroup. We compute analytically these solutions and illustrate them as color images. Using advanced methods of numerical bifurcation analysis we then explore the persistence and stability of these solutions when varying some parameters in the model. We conjecture that we can rely on the EBL to predict the existence of patterns that survive in large parameter domains but not to predict their stability. On our way we discover the existence of spatially localized stable patterns through the phenomenon of "snaking".Comment: 30 pages, 12 figure

Spatial and color hallucinations in a mathematical model of primary visual cortex

We study a simplified model of the representation of colors in the primate primary cortical visual area V1. The model is described by an initial value problem related to a Hammerstein equation. The solutions to this problem represent the variation of the activity of populations of neurons in V1 as a function of space and color. The two space variables describe the spatial extent of the cortex while the two color variables describe the hue and the saturation represented at every location in the cortex. We prove the well-posedness of the initial value problem. We focus on its stationary, i.e. independent of time, and periodic in space solutions. We show that the model equation is equivariant with respect to the direct product $\mathcal{G}$ of the group of the Euclidean transformations of the planar lattice determined by the spatial periodicity and the group of color transformations, isomorphic to $O(2)$, and study the equivariant bifurcations of its stationary solutions when some parameters in the model vary. Their variations may be caused by the consumption of drugs and the bifurcated solutions may represent visual hallucinations in space and color. Some of the bifurcated solutions can be determined by applying the Equivariant Branching Lemma (EBL) by determining the axial subgroups of $\mathcal{G}$. These define bifurcated solutions which are invariant under the action of the corresponding axial subgroup. We compute analytically these solutions and illustrate them as color images. Using advanced methods of numerical bifurcation analysis we then explore the persistence and stability of these solutions when varying some parameters in the model. We conjecture that we can rely on the EBL to predict the existence of patterns that survive in large parameter domains but not to predict their stability. On our way we discover the existence of spatially localized stable patterns through the phenomenon of “snaking”

Neural mechanisms of divided feature-selective attention to colour

The work was partly supported by an Experimental Psychology Society summer studentship to Karol Puch under the supervision of JM.Peer reviewedPostprin

A neural field model for color perception unifying assimilation and contrast

37 pages, 17 figures, 3 ancillary filesInternational audienceWe propose a neural field model of color perception in context, for the visual area V1 in the cortex. This model reconciles into a common framework two opposing perceptual phenomena, simultaneous contrast and chromatic assimilation. Previous works showed that they act simultaneously, and can produce larger shifts in color matching when acting in synergy with a spatial pattern. At some point in an image,the color perceptually seems more similar to that of the adjacent locations, while being more dissimilar from that of remote neighbors. The influence of neighbors hence reverses its nature above some characteristic scale. Our model fully exploits the balance between attraction and repulsion in color space, combined at small or large scales in physical space. For that purpose we rely on the opponent color theory introduced by Hering, and suppose a hypercolumnar structure coding for colors. At some neural mass, the pointwise influence of neighbors is spatially integrated to obtain the final effect that we call a color sensation. Alongside this neural field model, we describe the search for a color match in asymmetric matching experiments as a mathematical projector. We validate it by fitting the parameters of the model to data from (Monnier and Shevell, 2004) and (Monnier, 2008) and our own data. All the results show that we are able to explain the nonlinear behavior of the observed shifts along one or two dimensions in color space, which cannot be done using a simple linear model

Anatomy and Physiology of Macaque Visual Cortical Areas V1, V2, and V5/MT : Bases for Biologically Realistic Models

The cerebral cortex of primates encompasses multiple anatomically and physiologically distinct areas processing visual information. Areas V1, V2, and VS/MT are conserved across mammals and are central for visual behavior. To facilitate the generation of biologically accurate computational models of primate early visual processing, here we provide an overview of over 350 published studies of these three areas in the genus Macaca, whose visual system provides the closest model for human vision. The literature reports 14 anatomical connection types from the lateral geniculate nucleus of the thalamus to V1 having distinct layers of origin or termination, and 194 connection types between V1, V2, and VS, forming multiple parallel and interacting visual processing streams. Moreover, within V1, there are reports of 286 and 120 types of intrinsic excitatory and inhibitory connections, respectively. Physiologically, tuning of neuronal responses to 11 types of visual stimulus parameters has been consistently reported. Overall, the optimal spatial frequency (SF) of constituent neurons decreases with cortical hierarchy. Moreover, VS neurons are distinct from neurons in other areas for their higher direction selectivity, higher contrast sensitivity, higher temporal frequency tuning, and wider SF bandwidth. We also discuss currently unavailable data that could be useful for biologically accurate models.Peer reviewe

A psychophysical investigation of human visual perceptual memory. A study of the retention of colour, spatial frequency and motion visual information by human visual short term memory mechanisms.

The aim of this thesis was to investigate how visual information is organised in perceptual short term memory, with special interest in colour, spatial frequency and velocity. Previous studies of VSTM have indicated the existence of specific memory mechanisms for visual attributes such as orientation, spatial frequency, velocity, contrast and colour. The retention of information in visual short term memory for these basic visual attributes can be disrupted by the presentation of masking stimuli during inter-stimulus intervals (ISIs), which are outside the range of traditional sensory masking. We exploited this memory masking effect in order to examine the organisation of visual information in VSTM. Four groups of experiments were conducted in which participants carried out a delayed discrimination paradigm that employed a two-alternative forced choice (2-AFC) procedure in conjunction with a method of constant stimuli. The fidelity of VSTM was measured by performance markers such as discrimination thresholds and point of subjective equalities. We have found selective memory masking effects, which serve as further evidence in favour of the modular organisation in VSTM, namely, that human visual perceptual memory is based upon multiple, tuned channels in case of colour, spatial frequency and speed, similar to those found in the earliest stages of visual processing for spatial frequency. Moreover, each of these storage mechanisms are tuned to a relatively narrow range of stimulus parameters that are closely linked to visual discrimination mechanisms. These findings add further support to the view that low-level sensory processing mechanisms form the basis for the retention of colour, spatial frequency and velocity information in perceptual memory. We also found evidence for the broad range of transfer of memory masking effects across spatial location, which indicates more long range, long duration interactions between channels that are likely to rely upon contributions from neural processes located in higher visual areas. In conclusion, the experiments presented in this thesis provide significant insight into the organization of visual information in perceptual short term memory.Federation of Ophthalmic and Dispensing Optician