A new view of dorsal visual cortex in macaques revealed by submillimeter fMRI

Abstract

Macaque dorsal occipital cortex is generally thought to contain one elongated third-tier area, V3d, extending along most of the rostral V2d border. In contrast, our submillimeter retinotopic fMRI maps (0.6 mm isotropic voxels, achieved by implanted phased-array receive coils) consistently show three sectors anterior to V2d. The dorsal (mirror-image) sector complies with the traditional V3d definition, and the middle (non-mirror image) sector with V3A. The ventral (mirror-image) sector bends away from V2d, as does VLP in marmosets and DLP in owl monkeys, and represents the entire contralateral hemifield like V3A. Its population receptive field size, however, suggests that this ventral sector is another fourth-tier area caudal to V4d. Hence, contrary to prevailing views, the retinotopic organization of cortex rostral to V2d differs substantially from widely accepted models. Instead, it is evolutionarily largely conserved in Old and New World monkeys.