Multisensory convergence in calcarine visual areas in macaque monkey.

The neural substrates of multisensory perception and integration are still obscure, especially at the cortical level. Alternative viewpoints emphasize (1) 'bottom-up' processes, where different modalities converge in higher order multisensory areas, or (2) 'top-down' projections from multimodal to unimodal areas. In this anatomic study, we use anterograde tracer injections in parietal (8 monkeys) and auditory (3 monkeys) association areas, and demonstrate direct projections to areas V1 and V2 in the calcarine fissure (i.e. the peripheral visual field representation). The laminar signature, with terminations in layers 1 and/or 6, could be consistent with feedback-type connections. A subset of connections from parietal areas, however, branch to both V1 and a ventral extrastriate area (TEO or TEp). Thus, the direct connections to early visual areas V1 and V2 may well operate in conjunction with polysynaptic pathways in a densely parallel network.