Abstract
In macaque V4, it has been proposed that there are several subdivisions based on patterns of connectivity and visual topography (Zeki, 1971; Maguire and Baizer, 1984). Zeki (1983) reported that color selective cells were more frequently found in the anterior bank of lunate sulcus (V4 proper), as compared with the cortex on the prelunate gyrus (V4A). However, this finding was based on qualitative observation and there has been no study to confirm this finding. Here, we studied the functional organization of V4 using intrinsic signal optical imaging in two alert macaque monkeys and examined whether there is any functional subdivision in V4 in relation to color, luminance, and orientation preferences. Intrinsic signals were obtained through a chronic chamber over the portion of V4 representing foveal and parafoveal visual space ([[lt]]1.5° and 3–7° eccentricity, respectively). To reveal feature representation, a patch (0.25–8°) or full-screen stimulus filled with isoluminant color-varying or achromatic luminance-varying gratings was presented while the monkey was performing a fixation task. Visual activations in V4 showed a stimulus-size dependence: small grating patches activated the cortex more strongly than the full-screen gratings, consistent with the presence of strong surround suppression. In the foveal V4, color/luminance and orientation mapped in the different cortical locations. Color and luminance-sensitive regions were located posteriorly, close to the lunate sulcus (within 2 mm), alternating with a band-like appearance (color/luminance domains). The width of these domains was approximately 0.5 mm. In a region anterior to these domains, orientation-sensitive regions were observed (orientation domains). These two types of domains were largely separated but with some overlap. Within the color/luminance domains, different colors activated separate sub-domains. This functional compartmentalization was also observed in the parafoveal V4. These results indicate that there are distinct functional subdivisions in V4, which appear to correspond to the subdivisions proposed previously (V4 proper/V4A).
NIH Grant EY11744, Vanderbilt VVRC, Vanderbilt CICN, Vanderbilt Discovery Grant.