VISUAL AREAS IN THE DORSAL AND MEDIAL EXTRASTRIATE CORTICES OF THE MARMOSET

To define the number and limits of the visual areas in the primate ext rastriate cortex, the visuotopy of the dorsal convexity and medial wal l was studied by electrophysiological recordings in five marmosets ana esthetised with sufentanil and nitrous oxide and paralysed with pancur onium bromide. We identified five visuotopic representations in and ar ound the densely myelinated zone between visual area 2 (V2) and the po sterior parietal cortex. Most of the densely myelinated zone is formed by the homologue of the owl monkey's dorsomedial area (DM); thus, we also termed this area DM in the marmoset. Within DM, the lower quadran t representation is continuous, with central vision represented latera lly, peripheral vision medially, the horizontal meridian caudally, and the vertical meridian rostrally. In contrast, the upper quadrant repr esentation is split, with the central portion represented at the later al edge of DM on the dorsal surface, and the periphery along the midli ne. Two other visual field representations, corresponding to the dorso intermediate area (DI) and to a new subdivision termed the dorsoanteri or area (DA), are also densely myelinated but can be distinguished fro m DM based on the separation of the bands of Baillarger and visual top ography. In addition, a homologue of the medial visual area (M) was id entified. Our results reveal a highly complex visuotopy in primate cor tex, with local discontinuities in representation and borders between areas that are often not coincident with either the horizontal or the vertical meridian. The topography of the dorsal extrastriate cortex in the marmoset strongly suggests that both visual area 3 (V3) and the p arietoococipital area (PO) of other primates are portions of a single visuotopic representation, DM, and calls into question the existence o f visual areas with partial or quadrantic representations of the visua l field. (C) 1995 Wiley-Liss, Inc.