The organization and connections of anterior and posterior parietal cortex in titi monkeys: do New World monkeys have an area 2?

We used multiunit electrophysiological recording techniques to examine the topographic organization of somatosensory area 3b and cortex posterior to area 3b, including area 1 and the presumptive area 5, in the New World titi monkey, Callicebus moloch. We also examined the ipsilateral and contralateral connections of these fields, as well as those in a region of cortex that appeared to be similar to both area 7b and the anterior intraparietal area (7b/AIP) described in macaque monkeys. All data were combined with architectonic analysis to generate comprehensive reconstructions. These studies led to several observations. First, area 1 in titi monkeys is not as precisely organized in terms of topographic order and receptive field size as is area 1 in macaque monkeys and a few New World monkeys. Second, cortex caudal to area 1 in titi monkeys is dominated by the representation of the hand and forelimb, and contains neurons that are often responsive to visual stimulation as well as somatic stimulation. This organization is more like area 5 described in macaque monkeys than like area 2. Third, ipsilateral and contralateral cortical connections become more broadly distributed away from area 3b towards the posterior parietal cortex. Specifically, area 3b has a relatively restricted pattern of connectivity with adjacent somatosensory fields 3a, 1, S2 and PV; area 1 has more broadly distributed connections than area 3b; and the presumptive areas 5 and 7b/AIP have highly diverse connections, including connections with motor and premotor cortex, extrastriate visual areas, auditory areas and somatosensory areas of the lateral sulcus. Fourth, the hand representation of the presumptive area 5 has dense callosal connections. Our results, together with previous studies in other primates, suggest that anterior parietal cortex has expanded in some primate lineages, perhaps in relation to manual abilities, and that the region of cortex we term area 5 is involved in integrating somatic inputs with the motor system and across hemispheres. Such connections could form the substrate for intentional reaching, grasping and intermanual transfer of information necessary for bilateral coordination of the hands.