USE-DEPENDENT ALTERATIONS OF MOVEMENT REPRESENTATIONS IN PRIMARY MOTOR CORTEX OF ADULT SQUIRREL-MONKEYS

This study was undertaken to document plastic changes in the functiona l topography of primary motor cortex (M1) that are generated in motor skill learning in the normal, intact primate. Intracortical microstimu lation mapping techniques were used to derive detailed maps of the rep resentation of movements in the distal forelimb zone of M1 of squirrel monkeys, before and after behavioral training on two different tasks that differentially encouraged specific sets of forelimb movements. Af ter training on a small-object retrieval task, which required skilled use of the digits, their evoked-movement digit representations expande d, whereas their evoked-movement wrist/forearm representational zones contracted. These changes were progressive and reversible. In a second motor skill exercise, a monkey pronated and supinated the forearm in a key (eyebolt)-turning task. In this case, the representation of the forearm expanded, whereas the digit representational zones contracted. These results show that M1 is alterable by use throughout the life of an animal. These studies also revealed that after digit training ther e was an areal expansion of dual-response representations, that is, co rtical sectors over which stimulation produced movements about two or more joints. Movement combinations that were used more frequently afte r training were selectively magnified in their cortical representation s. This close correspondence between changes in behavioral performance and electrophysiologically defined motor representations indicates th at a neurophysiological correlate of a motor skill resides in M1 for a t least several days after acquisition. The finding that cocontracting muscles in the behavior come to be represented together in the cortex argues that, as in sensory cortices, temporal correlations drive emer gent changes in distributed motor cortex representations.