Third tier ventral extrastriate cortex in the New World monkey, Cebus apella

Abstract.

The ventral extrastriate cortex adjacent to the second visual area was studied in the New World monkey Cebus apella, using anaesthetised preparations. The visuotopic organisation and myeloarchitecture of this region demonstrate the existence of a distinct strip of cortex, 3–4 mm wide, with an ordered representation of the contralateral upper visual quadrant, up to 60° eccentricity. This upper-quadrant representation is probably homologous to the ventral subdivision of the third visual complex (V3v) of Old World monkeys, also known as the ventral posterior area. The representation of the horizontal meridian in V3v forms its posterior and medial border with V2, while the upper vertical meridian is represented anterior and laterally, forming a congruent border with the fourth visual area (V4). Central visual fields are represented in posterior and lateral portions of V3v, in the inferior occipital sulcus, while the periphery of the visual field is represented anteriorly, on the tentorial surface. Cortex anterior to V3v, at the ventral occipitotemporal transition, had neurones that had poor visual responses. No representation of the lower quadrant was found adjacent to V3v in ventral cortex. However, we observed cells with perifoveal receptive fields centred in the lower quadrant immediately dorsal to V3v, around the junction of the inferior occipital and lunate sulci. These observations argue against the idea that V3v is an area restricted to the ventral cortex in New World monkeys and support the conclusions of previous anatomical studies in Cebus that showed a continuity of myeloarchitecture and connectional patterns between ventral and lateral extrastriate cortices. Together, these data suggest that V3v may be part of a larger area that extends into dorsolateral extrastriate cortex, overlapping to some extent with the caudal subdivision of the dorsolateral area described in other New World monkeys.