Visuotopic organization and extent of V3 and V4 of the macaque.

The representation of the visual field in areas V3 and V4 of the macaque was mapped with multiunit electrodes. Twelve Macaca fascicularis were studied in repeated recording sessions while immobilized and anesthetized. V3 is a narrow strip (4-5 mm wide) of myeloarchitectonically distinct cortex located immediately anterior to V2. It contains a systematic representation of the central 35-40 degrees of the contralateral visual field; the representation of the upper quadrant is located ventrally in the hemisphere and that of the lower quadrant, dorsally. There is a small gap between the dorsal (V3d) and ventral (V3v) portions of V3. The representation of the horizontal meridian is adjacent to that in V2 and forms the posterior border of both V3d and V3v. Most or all of the anterior border of V3d consists of the representation of the lower vertical meridian. The entire anterior border of V3v consists of the representation of the upper vertical meridian. V4 is a strip of myeloarchitectonically distinct cortex 5-8 mm wide, immediately anterior to V3. It contains a coarse, but systematic, representation of approximately the central 35-40 degrees of the contralateral visual field. The representation of the upper visual field is located ventrally in the hemisphere. Most of the representation of the lower visual field is located dorsally. The posterior border of V4 corresponds to the representation of the vertical meridian, and the representation of the horizontal meridian is located at or near its anterior border. In both V3 and V4, the representation of the central visual field is magnified relative to that of the periphery. In both areas, the size of receptive fields increases with increasing eccentricity; however, at a given eccentricity, the receptive fields of V4 are larger than those of V3.