Spatial organization of precentral cortex in awake primates. II. Motor outputs.

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Abstract
By means of low-current (< 30 .mu.A) intracortical microstimulation (ICMS), the output organization of the forelimb control area in the precentral cortex of awake monkeys [Macaca arctoides] was investigated. Movement about a joint was selected as the index of response. Cortical zones were designated by the joint about which movements occurred. Cortical zone controlling movements of a more distal joint was partly enclosed by a zone controlling a more proximal joint. This resulted in a nested organization with the finger zone in the center, successive enclosures by the wrist and elbow zones, and a shoulder zone at the perimeter. Zones for contiguous joints had extensive overlap, whereas those of noncontiguous joints had minimal or no overlap. This overlapping, nested organization spanned areas 4 and 6 with no apparent discontinuity. The same movement about a joint was in general represented in more than one cortical location within a joint zone. All possible movements about a joint were represented within its control zone. Lowest threshold ICMS loci were predominantly distributed over the caudal portion (area 4) of the precentral cortex. In general, loci in distal joint zones had lower ICMS thresholds than did proximal ones, irrespective of rostrocaudal position.