The Postcentral Somatosensory Cortex

Abstract

Many of our present day concepts of the organization of somatosensory cortex stem from the landmark study of Woolsey, Marshall and Bard, first published in 1937 (7) and then more completely in 1942 (21). These investigators documented important features of the overall organization of the postcentral parietal cortex in Macaca mulatta by recording evoked potentials from the surface of the brain. The basic procedure was to determine the portion of the body surface (the receptive field), where movements of hairs or punctate stimulations of glabrous surfaces with a cat’s vibrissa were capable of evoking responses at a given recording site and to repeat the procedure for a grid of closely spaced recording sites. By removing parts of the brain, they were even able to explore cortical surfaces buried in the central sulcus and on the medial wall of the cerebral hemisphere. These studies led to several important conclusions: (a) the region of postcentral cortex activated by tactile somatic stimuli includes the separate architectonic fields 3 (3a and 3b), 1 and 2 of Brodmann; (b) the cortex is activated almost exclusively from the contralateral half of the body; (c) there is an orderly representation of body parts, with a medial to lateral sequence across cortex from tail to tongue along the body; (d) the cortical organization does not exactly reflect the body surface, so that there are displacements in the cortical map; one clear example given was the separation in cortex of the representation of the face from the caudal head; and (e) the skin surfaces with the greatest tactile acuity have the largest cortical projection areas.