RELATIONSHIP OF CEREBELLAR PURKINJE-CELL SIMPLE SPIKE DISCHARGE TO MOVEMENT KINEMATICS IN THE MONKEY

The simple spike discharge of 231 cerebellar Purkinje cells in ipsilat eral lobules V and VI was recorded in three monkeys trained to perform a visually guided reaching task requiring movements of different dire ctions and distances. The discharge of 179 cells was significantly mod ulated during movement to one or more targets. Mean simple spike rate was fitted to a cosine function for direction tuning, a simple linear function for distance modulation, and a multiple linear regression mod el that included terms for direction, distance, and target position. O n the basis of the fit to the direction and distance models, there wer e more distance-related than direction-related Purkinje cells. The sim ple spike discharge of most direction-related cells modulated at only one target distance. The preferred directions for the simple spike tun ing were not uniformly distributed across the workspace. The discharge of most distance-related cells modulated along only one movement dire ction. On the basis of the multiple Linear regression model, simple sp ike discharge was also correlated with target position, in addition to direction and distance. Approximately half of the Purkinje cells had simple spike activity associated with only a single parameter, and onl y a small fraction of the cells with all three. The multiple regressio n model was extended to evaluate the correlations as a function of tim e. Considerable overlap occurred in the timing of the simple spike cor relations with the parameters. The latency for correlation with moveme nt direction occurred mainly in a 500-ms interval centered on movement onset. The correlations with target position also occurred around mov ement onset, in the range of -200-500 ms. Distance correlations were m ore variable, with onset latencies from -500 to 1,000 ms. These result s demonstrate that the simple spike discharge of cerebellar Purkinje c ells is correlated with movement direction, distance, and target posit ion. Comparing these results to motor cortical discharge shows that th e correlations with these parameters were weaker in Purkinje cell simp le spike discharge, and that, for the majority of Purkinje cells, the simple spike discharge was significantly related to only a single move ment parameter. Other differences between simple spike responses and t hose of motor cortical cells include the nonuniform distribution of pr eferred directions and the extensive overlap in the timing of the corr elations. These differences suggest that Purkinje cells process, encod e, and use kinematic information differently than motor cortical neuro ns.