Neural correlates of motor performance in primary motor cortex

Abstract

Prior experiments have characterized the behavior of cells in the primary motor cortex that correlate with movement of the upper limb. The patterns of behavior of cells before, during, and after the application of an external forcefield to the hand of a primate performing a motor reaching task have been described. In these forcefields, forces that are proportional in magnitude and perpendicular in direction to the velocity of the hand are generated, resulting in disturbances in the motor trajectory. This research explored the motor disturbances and their neural correlates during an interference task in which a sequence of forcefields of opposite polarity were applied. The goals of the present research were to characterize the motor performance and neural correlates in a task that requires the sequential recall and utilization of previously learned motor memories for performing opposing tasks. Recent research has demonstrated that opposing tasks performed in close temporal proximity interfere with motor learning. This research aims to explore degradation in motor performance occurring when a series of previously learned motor tasks are performed in sequence, as evidence that interference occurs between temporally spaced motor memories. In addition, identification of neural correlates of this interference phenomenon are sough:. specifically performance of the recently described memory cell. Furthermore. in responding to forcefield-induced perturbations, motor performance was found to be substantially degraded during an early "transient" phase. with resumption of relatively improved levels of performance during subsequent steady-state phases. The neural behavior during these transient and steady-state phases was analyzed to glean some insight into the neural correlates of feedback control. In this task, single-cell activity from primary motor cortex was recorded while a primate performed a motor reaching task through a 7-stage session. This 7-stage task involves the application of 3 forcefields of alternating polarity, with the first and third being identical and the second or interference forcefield of opposite polarity. with four null field stages interposed between forcefields. Motor performance was markedly degraded in the interference forcefield. particularly during a transient phase immediately following the onset of the interference forcefield. Within each of the forcefields. the most marked degradation in motor performance was found to occur during a transient phase following the onset of the forcefield. Neural behavior was analyzed during time periods corresponding to the transient and steady-state phases of motor performance in each stage. Discovery of a subtle neural response in the transient phase led to the definition of a new metric, termed sharpness of tuning. which was used in the characterization of neural behavior in the current experimental paradigm. Distinct differences in patterns of neural firing rate and sharpness of tuning between the transient and steady-state phases were identified. and their implications on the neural correlates of motor control are discussed.