Dorsomedial parieto-frontal circuit is not just for reaching: Superior parieto-occipital cortex influences primary motor cortex during grasp planning in humans


Neural control of goal-directed hand actions is thought to rely on two specialized parieto-frontal pathways: 1) the dorsomedial stream (namely, superior parieto-occipital cortex, SPOC, in humans and the dorsal premotor cortex, PMd) involved in programming arm transport during reaching and 2) the dorsolateral stream (namely, anterior intraparietal sulcus, aIPS, and ventral premotor cortex, PMv) involved in programming hand grip during grasping - both of which send output to primary motor cortex, M1. Recent evidence has argued that macaque dorsomedial stream, specifically V6A (the putative homologue of human SPOC), codes both hand transport and grip. In humans, functional imaging studies also show evidence that SPOC differentiates grasping versus touching, but it is not clear whether these brain areas also code the details of the grip. We used dual-site transcranial magnetic stimulation (dsTMS) to test the functional interactions between SPOC-M1 in the left hemisphere during different hand actions. Subjects performed an event-related delayed movement task toward a single peripherally located object (consisting of a small cylinder attached atop a larger cylinder). For each trial, after visual presentation of the object, one of three hand movements was instructed: 1) grasp the top cylinder (precision grip); 2) grasp the bottom cylinder (whole hand grasp); or 3) reach-to-touch the side of the object (without preshaping the hand). We found facilitatory influences of left SPOC on left M1 for grip formation at two specific time intervals (50 and 150 ms) after a cue to select a manual response. Consistent with findings reported in the monkey, these results suggest that the human dorsomedial parieto-frontal stream may play a critical role in the planning of all phases of reach-to-grasp action. Critically, they challenge the view that the reach and grasp components are processed independently.

Acknowledgments: NSERC: CIHR