Evidence for both reaching and grasping activity in the medial parieto-occipital cortex of the macaque

In humans, the caudal pole of the superior parietal lobule is involved in the control of both reaching and grasping movements, whereas in monkey it is reported to be involved only in the control of reaching. Using single-unit recordings from trained macaque monkeys, we investigated whether area V6A, a visuomotor area located in the caudal part of the posterior parietal cortex, is involved in both components of prehension, the hand transport towards the visual target and the grip formation to secure the grasp. In Experiment 1, neural activity was recorded in V6A while two monkeys performed two instructed-delay reaching tasks (reach-to-point and reach-to-grasp) under controlled conditions in darkness. Fourty-five of 93 tested neurons (48%) were modulated during reach-to-point and 62% (52/84) during reach-to-grasp. In 63% of cells (51/81) neural activity was significantly different between reach-to-point and reach-to-grasp tasks, suggesting that grip formation could influence neural activity. In Experiment 2, two monkeys performed natural reach-to-grasp movements in fully lit environment; V6A neural activity and arm-hand movements were recorded by a digital camcorder and analysed frame-by-frame using a digital video technique. Thirty of the 58 tested neurons (52%) were modulated during natural prehension; about 30% of these neurons (8/30) were modulated only during the last phase of prehension, i.e. during finger flexion around the object to be grasped. This is the first direct demonstration that both reaching and grasping modulate neural activity in the caudal part of the posterior parietal cortex of the macaque. Our work suggests a strict functional homology between human and monkey superior parietal lobule.