Microelectrode multiunit mapping studies of parietal cortex in owl monkeys indicate that the classical “primary” somatosensory region (or “SI”) including the separate architectonic fields 3a, 3b, 1, and 2 contains as many as four separate representations of the body rather than one. An analysis of receptive field locations for extensive arrays of closely placed recording sites in parietal cortex which were later related to cortical architecture led to the following conclusions: (1) There are two large systematic representations of the body surface within “SI”. Each is activated by low threshold cutaneous stimuli; one representation is coextensive with Area 3b and the other with Area 1. (2) While each of these representations contain regions of cortex with topological or “somatotopic” transformations of skin surface, the representations have many discontinuities where adjoining skin surfaces are adjoining in the representations. Thus, the representations can be considered as composites of somatotopically organized regions, but cannot be accurately depicted by simple continuous homunculi. Lines of discontinuity often cut across dermatomes and seldom follow dermatomal boundaries, i.e., neither cutaneous representation constitutes a systematic representation of dermatomal skin fields. (3) While the two cutaneous fields are basically similar in organization and are approximate mirror images of each other, they differ in important details, i.e., lines of discontinuity in the representations and the sites of representations of different specific skin surfaces differ significantly in the two representations. (4) The two cutaneous representations also differ in size and in the relative proportions in each representation differ, they cannot both be simple reflections of overall peripheral innervation density. (5) All or part of Area 2 contains a systematic representation of deep body structures.
These conclusions are consistent with a view of the anterior parietal region as containing functionally distinct fields at least partially related to different subsets of receptor populations and coding or representing different aspects of somatic sensation. We suggest that the “SI” region of primates be redefined as a parietal somatosensory strip, the Area 1 representation as the posterior cutaneous field, and, for reasons of probable homology with “SI” of other mammals, the Area 3b representation as SI proper.