Hierarchy of Hue Maps in the Primate Visual Cortex

Many stimulus attributes are represented in the cortex in the form of a map, such as the retinotopic map in the visual cortex and the frequency map in the auditory cortex where nearby cortical locations are associated with similar values of a given stimulus or action attribute. Given the computational advantage of maps for processing information,1 it is likely that a cortical region with a mapped representation of an attribute plays a much more important role in processing the given attribute than do regions without such mapped representations. Color is an important attribute for catarrhine primates. A theoretical work has predicted the existence of color maps in primate visual cortex,2 but no clear evidence for it was present until we carried out an imaging study in the visual area V2 of macaque monkeys.3 In that study, we first located modules in V2 that were highly sensitive to color stimuli as reported by Roe and Ts’o.4 Such modules in V2 and other areas are called “highly color sensitive modules” below. We then imaged intrinsic optical signals at these modules to determine the spatial distribution of their responses to various color/gray gratings. Different colors elicited largely overlapping responses across the highly color sensitive modules, but the response peaks shifted systematically as a function of the stimulus color. Most interestingly, the spatial shift of these response peaks followed the perceptual hue order of the stimuli; nearby peaks were associated with perceptually close hues (Figure 1). Therefore, our results suggested the existence of a “hue map” in each highly color sensitive module in V2. Furthermore, we found that these hue maps were located at the cytochrome oxidase (CO) thin stripes. Some previous studies have suggested an important role for CO thin stripes in processing color information,4-6 although other studies did not find any evidence for this suggestion (see Shipp and Zeki6 for a review). Those previous studies were largely focused on the behavior of individual neurons and were unable to map the organization of neurons tuned to various colors. Given that we found V2 hue maps only at the highly color sensitive modules and CO thin stripes, and the computational advantage of maps for processing information,1 our results suggest an important role for these V2 compartments in processing stimulus hue. Visual signals from the eyes reach area V2 through area V1. To determine whether hue maps in V2 originate in V1, we also imaged the responses to various uniform colors at the highly color sensitive modules in monkey V1. Similar to our findings in V2, the spatial shift in response peaks followed the order of stimulus hue (Figure 2, top image). However, hue maps in V1 were smaller than those in V2 by an order