Spatial tuning of reaching activity in the medial parieto-occipital cortex (area V6A) of macaque monkey.

We recorded neural activity from the medial parieto-occipital area V6A while three monkeys performed an instructed-delay reaching task in the dark. Targets to be reached were in different spatial positions. Neural discharges were recorded during reaching movements directed outward from the body (towards visual objects), during the holding phase (when the hand was on the target) and during inward movements of the hand towards the home button (which was near the body and outside the field of view). Reach-related activity was observed in the majority of 207 V6A cells, during outward (78%) and inward (65%) movements as well as during the holding phase (62%). Most V6A reaching neurons (84%) were modulated in more than one phase of the task. The reach-related activity in V6A could depend on somatosensory inputs and/or on corollary discharges from the dorsal premotor cortex. Although visual and oculomotor inputs are known to have a strong influence on V6A activity, we excluded the possibility that the reach-related activity which we observed was due to visual stimulation and/or oculomotor activity. Reach-related activity for movements towards different locations was spatially modulated during outward (40%) and inward (47%) reaching movements. The position of the hand/arm in space modulated about 40% of V6A cells. Preferred reach directions and spatial locations were represented uniformly across the workspace. These data suggest that V6A reach-related neurons are able to code the direction of movement of the arm and the position of the hand/arm in space. We suggest that the V6A reach-related neurons are involved in the guidance of goal-directed arm movements, whether these actions are visually guided or not.