  We address the question of color-space interactions in the brain, by
proposing a neural field model of color perception with spatial context for the
visual area V1 of the cortex. Our framework reconciles two opposing perceptual
phenomena, known as simultaneous contrast and chromatic assimilation. They have
been previously shown to act synergistically, so that at some point in an
image, the color seems perceptually more similar to that of adjacent neighbors,
while being more dissimilar from that of remote ones. Thus, their combined
effects are enhanced in the presence of a spatial pattern, and can be measured
as larger shifts in color matching experiments. Our model supposes a
hypercolumnar structure coding for colors in V1, and relies on the notion of
color opponency introduced by Hering. The connectivity kernel of the neural
field exploits the balance between attraction and repulsion in color and
physical spaces, so as to reproduce the sign reversal in the influence of
neighboring points. The color sensation at a point, defined from a steady state
of the neural activities, is then extracted as a nonlinear percept conveyed by
an assembly of neurons. It connects the cortical and perceptual levels, because
we describe the search for a color match in asymmetric matching experiments as
a mathematical projection on color sensations. We validate our color neural
field alongside this color matching framework, by performing a multi-parameter
regression to data produced by psychophysicists and ourselves. All the results
show that we are able to explain the nonlinear behavior of shifts observed
along one or two dimensions in color space, which cannot be done using a simple
linear model.
