DIFFERENTIAL RELATION OF DISCHARGE IN PRIMARY MOTOR CORTEX AND PREMOTOR CORTEX TO MOVEMENTS VERSUS ACTIVELY MAINTAINED POSTURES DURING A REACHING TASK

The activity of cells in primary motor cortex (MI) and dorsal premotor cortex (PMd) were compared during reaching movements in a reaction-ti me (RT) task, without prior instructions, which required precise contr ol of limb posture before and after movement. MI neurons typically sho wed strong, directionally tuned activity prior to and during movement as well as large gradations of tonic activity while holding the limb o ver different targets. The directionality of their movement- and postu re-related activity was generally similar. Proximal-arm muscles behave d similarly. This is consistent with a role for MI in the moment-to-mo ment control of motor out put, including both movement and actively ma intained postures, and suggests a common functional relation for MI ce lls to both aspects of motor behavior. In contrast, PMd cells were gen erally more phasic, frequently emitting only strong bursts of activity confined mainly to the behavioral reaction time before movement onset . PMd tonic activity during different postures was generally weaker th an in MI, and showed a much more variable relation with their movement -related directional tuning. These results imply that the major contri bution of PMd to this RT task occurred prior to the onset of movement itself, consistent with a role for PMd in the selection and planning o f visually guided movements. Furthermore, the nature of the relative c ontribution of PMd to movement versus actively maintained postures app ears to be fundamentally different from that in MI. Finally, there was a continuous gradient of changes in responses across the rostrocaudal extent of the precentral gyrus, with no abrupt transition in response properties between PMd and MI.