Activity in the Precentral Motor Areas After Presentation of Targets for Delayed Reaching Movements Varies with the Initial Arm Position.

We studied single unit activity in the primary motor and premotor cortex during preparation for unrestrained arm movements that the monkey made to a right or left visual target. 'Set-related' activity occurring during an instructed delay period could not only vary with the target location, but also with the initial arm posture which was changed via a motor-driven forearm support between two positions. Modulation with arm displacement of some cells was contingent upon prior instruction, i.e. it occurred only if a target for the upcoming movement was presented, and for the majority of neurons it was different from the effects caused by the same passive arm shift when being applied before the visual instruction cue. Furthermore, responses to presentation of the same target were found to depend on the starting position of the hand. These findings suggest that both kinaesthetic and visual information interact in motor cortical cells in an appropriate manner for elaborating the movement trajectory.