Floating Microelectrode Arrays

Spatiotemporal Distribution of Location and Object Effects in Primary Motor Cortex Neurons during Reach-to-Grasp

Reaching and grasping typically are considered to be spatially separate processes that proceed concurrently in the arm and the hand, respectively. The proximal representation in the primary motor cortex (M1) controls the arm for reaching, while the distal representation controls the hand for grasping. Many studies of M1 activity therefore have focused either on reaching to various locations without grasping different objects, or else on grasping different objects all at the same location. Here,werecordedM1neurons in the anterior bank and lip of the central sulcus as monkeys performed more naturalistic movements, reaching toward, grasping, and manipulating four different objects in up to eight different locations. We quantified the extent to which variation in firing rates depended on location, on object, and on their interaction—all as a function of time. Activity proceeded largely in two sequential phases: the first related predominantly to the location to which the upper extremity reached, and the second related to the object about to be grasped. Both phases involved activity distributed widely throughout the sampled territory, spanning both the proximal and the distal upper extremity representation in caudal M1. Our findings indicate that naturalistic reaching and grasping, rather than being spatially segregated processes that proceed concurrently, each are spatially distributed processes controlled by caudal M1 in large part sequentially. Rather than neuromuscular processes separated in space but not time, reaching and grasping are separated more in time than in space.