Information About Color and Orientation in the Primate Visual Cortex - High-Throughput Image Reconstruction and Analysis - page 229

CHAPTER 10
Information About Color and
Orientation in the Primate
Visual Cortex
Youping Xiao and Ehud Kaplan
10.1 Introduction
A major goal of neuroscience is to understand how the outside world is repre-
sented in the brain. In primates, visual information plays a major role in guiding
behavior, and the largest part of the sensory cortex is devoted to the analysis of
visual information [1]. The visual cortex has thus attracted much research, and
a great deal is now known about the representation of several important aspects
of the visual world, such as orientation or size, although less is known about the
functional architecture of the representation of other aspects, such as color. The
availability of imaging techniques, together with the fact that the thin cortical sheet
is suitable for the visualization of spatial activity patterns, have made it possible
in the past two decades to gain new insights into this important issue.
Psychophysical studies have suggested that the various attributes of visual stim-
uli such as color, form, or motion are represented by separate channels in our brain
[2]. Consistent with this hypothesis, many studies suggest that the primate cortex
comprises several areas that are specialized in representing one or a few particular
attributes. For instance, patients with cerebral achromatopsia [3, 4], a deficit in
color perception, have damage in cortical areas that seem to correspond to the
color-preferring regions as uncovered by functional imaging studies (''V4'' accord-
ing to [5]; ''V8'' according to [6]). These patients seem to be relatively normal
with respect to other visual functions, suggesting that the affected color-preferring
regions, or the so-called ''color centers,'' are parts of a subpathway specialized for
color perception [7]. Studies in macaque monkeys have also found areas that are
specialized for processing of color information [8].
However, it had been intensely debated whether or not these attributes are
represented by different compartments within early visual areas. These areas, in-
cluding V1 and V2, process the visual inputs from the eyes, and pass them along
to the higher visual areas that are putatively specialized for different functions.
Whether the representation of different attributes is spatially separated in early
visual areas has to be determined before we have a full understanding of where
and how the specialized channels for various attributes are constructed.
An early influential study [9] has suggested that information about color is
represented in the primary visual cortex, V1, by cells in specialized regions that
are rich in the mitochondrial enzyme cytochrome oxidase (CO). These regions are
referred to as the CO ''blobs.'' Livingstone and Hubel [9] further suggested that
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