Encoding of reach and grasp by single neurons in premotor cortex is independent of recording site

Neural activity has been studied during reaching and grasping separately, yet little is known about their combined representation. To study the functional organization of reaching and grasping in the premotor cortex (PM), we trained two monkeys to reach in one of six directions and grasp one of three objects. During prehensile movements, activity of proximal (shoulder and elbow) muscles was mainly modulated by reach direction, whereas distal (finger) muscles were also modulated by grasp type. Using intracortical microstimulation, we identified spatially distinct PM sites from which movements of proximal or distal joints were evoked. In contrast to muscles, modulation of neural activity by reach direction was similar for single units recorded in proximal and distal sites. Similarly, grasp type encoding was the same for units recorded in the different sites. This pattern of encoding reach and grasp irrespective of recoding site was observed throughout the task: before, during, and after prehension movements. Despite the similarities between single units within different sites, we found differences between pairs of units. Pairs of directionally selective units recorded by the same electrode in the same proximal site preferred similar reach directions but not grasp types, whereas pairs of object-selective units recorded in the same distal site tended to prefer the same grasp type but not reach direction. We suggest that the unexpected "mixing neurons" encoding reach and grasp within distal and proximal sites, respectively, provide a neural substrate for coordination between reach and grasp during prehension.