Area V4 in Cebus monkey: extent and visuotopic organization.

We used electrophysiological mapping and myeloarchitectural criteria in order to define the location, extent and visual topography of the fourth visual area (V4) in anesthetized and paralyzed Cebus monkey. Based on these criteria, the borders of V4 with surrounding areas were defined both on the dorsal and ventral cortical surfaces. In addition, to better visualize the visuotopic organization and to evaluate its regularity, we constructed bidimensional maps and projected the recording sites onto them. Area V4 has an almost complete representation of the binocular visual field with the lower visual field represented dorsally (V4d) and the upper field ventrally (V4v). We found this representation to be more extensive than those previously described. The representation of the central portion of the visual field is largely expanded in comparison with that of the periphery. This emphasis in central vision could be related with the involvement of V4 in the ventral stream of visual information processing. Receptive field size increases with increasing eccentricity, while cortical magnification factor decreases. The cortical magnification factor measured along isopolar lines is, on average, 1.5-2.0 times greater than that measured along the isoeccentric lines, suggesting the existence of a small anisotropy in central and peripheral V4.