COMPARISON OF CEREBELLAR AND MOTOR CORTEX ACTIVITY DURING REACHING - DIRECTIONAL TUNING AND RESPONSE VARIABILITY

1. The responses of 262 motor cortex cells and 223 cerebellar neurons were recorded during whole-arm reaching movements toward target lights in eight evenly distributed directions radiating from a common centra l starting position. The reaching movements were followed by a 2-s tar get hold period where a fixed arm posture was actively maintained to s tabilize the hand over the target light.2. Cerebellar neurons had a hi gher mean tonic discharge rate while holding over the starting positio n (22.9 imp/s) than did motor cortex cells (12.5 imp/s). The mean popu lation response curve describing the changes in activities with moveme nt direction was likewise shifted toward higher frequencies in the cer ebellum compared with the motor cortex, but the amplitude of the two c urves was about equal. Therefore, the baseline discharges of cerebella r neurons were higher, but their changes in activity during movement w ere similar to those of motor cortical cells. 3. Motor cortex neurons were more strongly related to active maintenance of different arm post ures than were cerebellar units. This was shown by a larger posture-re lated population response curve in the motor cortex (half-wave amplitu de of cosine function was 11.2 imp/s, compared with 7.0 imp/s for cere bellar neurons), which represented the average response curve calculat ed from all the cells of the population. Furthermore, the motor cortex population had a higher percentage of single cells with tonic respons es while the hand was held over different targets (tonic and phasic-to nic cells composed 57% of motor cortex population, compared with 38% o f cerebellar population). Proportionately more cerebellar cells were p hasically related to the movements. 4. The majority of motor cortex ce lls (58%) showed reciprocal changes relative to the center-hold time a ctivity where the activity increased for movements in the preferred di rection and decreased for movements in the opposite direction. Most of the remaining cells (40%) showed graded changes where the activity in creased gradually as reaching was directed closer to the preferred dir ection. In contrast, the most common cerebellar response pattern was g raded (38%). Only 26% were reciprocal and 18% were nondirectional. The remaining 2% of motor cortical cells and 18% of cerebellar neurons co uld not be readily assigned to any of these three response classes. 5. Sector widths were calculated to measure the dispersion of individual cerebellar and motor cortical cell activities about the eight movemen t directions. Sector widths calculated from the absolute activities we re always broader for cerebellar neurons (i.e., the cells were more br oadly tuned). This was in part due to the higher spontaneous activitie s of cerebellar neurons and to a large tonic component that was unmodu lated with movement direction. Both formed an offset for the direction -related responses. When this unmodulated component was subtracted, al l but the reciprocal cerebellar cells were still more broadly tuned th an motor cortical cells. 6. Significant variation of activity during r eplication of the same movement direction occurred in 38-51% of cerebe llar neurons, depending on the epoch of the trial, as opposed to 8-14% of motor cortical cells. Furthermore, the preferred directions calcul ated for each block of single replications of the eight movement direc tions revealed a much greater variability among cerebellar neurons tha n among motor cortex cells. The movement kinematics were, however, sim ilar during recordings of cerebellar and motor cortical neurons. 7. Th ere was a large degree of overlap among the onset latencies of cerebel lar and motor cortical neurons that were active before any detectable movement. This suggests a parallel and simultaneous mode of recruitmen t for cerebellar and motor cortical neurons in whole-arm reaching move ments.