Consistent features of the representation of the hand in area 3b of macaque monkeys.

Multiunit microelectrode recordings were used to explore the responsiveness and somatotopic organization of the representation of the hand in area 3b of anesthetized macaque monkeys. Major findings were as follows: Recording sites throughout the hand representation were activated by low-threshold cutaneous stimulation. Simple, punctate mechanical stimuli were highly effective in activating neurons. Neurons had small, restricted receptive fields. Representations of nearly all skin surfaces of the hand were demonstrated in individual monkeys. The basic topographic pattern found in all monkeys included the following: a large sequential representation of the glabrous digits from thumb to little finger from lateral to medial in cortex, and from proximal to distal hand parts in cortex extending down the caudal bank of the central sulcus; moderately large representations of radial and ulnar pads of the palm in respective lateral and medial cortical locations in the hand representation; and a relatively small, fragmented representation of the dorsal hand and dorsal digits, with the fragments interspersed within the representation of the glabrous hand. The proportions of the proximal, middle, and distal glabrous digits varied, so that the representation of the distal phalanx sometimes approached the dorsal border of area 3b with area 1. A comparison of the present findings with previous results from macaque monkeys indicates that the above-described features have been revealed under a variety of recording and anesthetic conditions. Consistencies in previous and present results strongly support the conclusions that the hand representation in area 3b of macaque monkeys is activated by cutaneous receptors throughout; is composed of neurons with relatively simple, small, cutaneous receptive fields; includes all skin surfaces of the hand; and is somatotopic for the glabrous skin with small, discontinuous, intercalated representations of fragments of the dorsal skin.