The goal of this project is to elucidate the neural mechanisms that underlie cognitive information processing during the planning stages of spatial-motor behavior. Neurophysiological recordings of single neuronal activity were undertaken as monkeys performed multidirectional, sequenced reaching movements pre-instructed by visual signals. This design isolated neuronal activity related to movement planning from that related to movement execution. Three major subdivisions of the primate frontal lobe were studied in order to understand the functional role and specialization of each: The primary motor cortex (M1), dorsal premotor cortex (PMd) and supplementary motor area (SMA). In M1 and SMA, the majority of neurons discharged in relation to movement execution. The spatially tuned activity of these neurons conveys information as to movement kinematics such as movement direction and amplitude or to spatial attributes of the target location and movement end-point. In PMd the majority of neurons discharge during movement planning in response to the visual signals which instruct the impending movement. This represents the encoding and storage in working memory of information required to execute movements to remembered spatial targets. PMd neuronal activity was examined in relation to the memorization of instructions signaling a two-stage reaching movement and the proper sequence of its execution. In 25% of PMd neurons this activity was found to encode spatial attributes of remembered reaching movements independent of whether the first or second movement segment was instructed. These cells appear to reliably encode the spatial aspects of every movement segment into a specific part of space. In contrast, in 75% of PMd cells, neuronal responses preferentially encode the remembered instructional information related to one segment of a sequenced reaching movement, usually the first. For a random movement sequence, spatially tuned responses of PMd neurons differ if a constant visual signal instructs planning of the first or second movement segment. Thus, PMd cell activity encodes spatial attributes of impending movements into space and incorporates information as to the correct sequence of movements. For movement sequences instructed by visual signals described by a fixed spatial rule, e.g., targets opposite each other, spatially tuned PMd activity was generally enhanced in response to the first and suppressed in response to the second visual instruction. Since the same visual signals were used to instruct trials with either fixed or random spatial rules, the differential neuronal responses recorded in PMd are attributable to the spatial rule in effect. Thus, PMd neurons are involved in the preparation of reaching movements into space, emit a signal that encodes spatial attributes of either movement kinematics or target endpoints, signal the order of an intended movement sequence, and finally, discharge differentially during the preparation of movement sequences predicted by a fixed spatial rule.
