REACHING MOVEMENTS WITH SIMILAR HAND PATHS BUT DIFFERENT ARM ORIENTATIONS .1. ACTIVITY OF INDIVIDUAL CELLS IN MOTOR CORTEX

This study shows that the discharge of many motor cortical cells is st rongly influenced by attributes of movement related to the geometry an d mechanics of the arm and not only by spatial attributes of the hand trajectory. The activity of 619 directionally tuned cells was recorded from the motor cortex of two monkeys during reaching movements with t he use of similar hand paths but two different ann orientations, in th e natural parasagittal plane and abducted into the horizontal plane. N early all cells (588 of 619, 95%) showed statistically significant cha nges in activity between the two arm orientations [analysis of varianc e (ANOVA), P < 0.01]. A majority of cells showed a significant change in their overall level of activity (ANOVA, main effect of task, P < 0. 01) between arm orientations before, during, and after movement. Many cells (433 of 619, 70%) also showed a significant change in the relati on of their discharge with movement direction (ANOVA; task x direction interaction term, P < 0.01) during movement, including changes in the dynamic range of discharge with movement and changes in tile directio nal preference of cells that were directionally tuned in both arm orie ntations; Similar effects were seen for the discharge of cells while t he monkey main tained constant arm postures over the different periphe ral targets with the use of different arm orientations. Repeated data files from the same cell with the use of the same arm orientation show ed only small changes in tile level of discharge or in directional tun ing, suggesting that changes in cell discharge between arm orientation s cannot be explained by random temporal variations in cell activity. The distribution of movement-related preferred directions of the whole sample differed between arm orientations, and also differed strongly between cells receiving passive input predominantly from the shoulder or elbow. The electromyographic activity of most prime mover muscles a t the shoulder and elbow was also strongly affected by arm orientation , resulting in changes in overall level of activity and/or directional tuning that often resembled those of the proximal arm-related motor c ortical cells. A mathematical model that represented movements in term s of movement direction centered on the hand could not account for any of the arm-orientation-related response changes seen in this task, wh ereas models in intrinsic parameter spaces of joint kinematics and joi nt torques predicted many of the effects.