Tactile discrimination capacity in relation to size and organization of somatic sensory cortex in primates: I. Old-World prosimian, Galago; II. New-World anthropoids, Saimiri and Cebus.

Living primates vary considerably in brain organization, in sensorimotor and cognitive abilities, and in natural behavioral repertoires. Comparative studies of primary (SI) and secondary (SII) somatic sensory cortex of primates reveal major differences in the size and in the complexity of topographic projection patterns. The separate projections of the glabrous hand to SI cytoarchitectonic areas 3b and 1 described in the Old World (OW) anthropoid Macaca and in New World (NW) anthropoids Cebus, Saimiri, and Aotus are lacking in NW Saguinus and in the prosimian Galago. The relationship between the size and complexity of SI organization and tactile abilities is explored in this study of four species of primates--Galago, Macaca, Cebus, and Saimiri. These species were trained to discriminate between pairs of objects differing either in cross-sectional diameter (size) or surface roughness (texture). The course of acquisition of such tactile discrimination in normal Macaca and the nature of deficits following SI or SII removals are known. Selective lesions of either cytoarchitectonic area 1 or 2 in Macaca affect only texture or size discriminations, respectively. Removal of area 3b in SI, or of SII, in Macaca affects both size and texture capacities. The single projection of the glabrous hand to area 3b-1 of Galago led to our expectation that the capacity of Galago to discriminate textures would be more similar to an area 1-lesioned than to a normal Macaca. The substantial and persistent differences between Macaca and Galago on texture, but not size, tasks lend support to the view that the evolution of a second projection of the glabrous hand to area 1 in Macaca contributes to increased texture discrimination capacity. The similarity in multiple projection patterns of the glabrous hand to areas 3b and 1 in Macaca, Saimiri, and Cebus led us to expect greater correspondence in texture discrimination capacity between these three anthropoids than to Galago. Contrary to expectations, Saimiri and Cebus showed a tactile capacity more similar to Galago than to Macaca. Furthermore, the texture discrimination capacity of Cebus actually improved substantially after removal of area 1. This provides further evidence, together with the single SI hand area in NW Saguinus and Galago, that the separate cutaneous projections to area 1 in OW and NW primates are not homologous but evolved independently and possibly serve different tactile functions.