CHANGES IN MOTOR CORTEX ACTIVITY DURING REACHING MOVEMENTS WITH SIMILAR HAND PATHS BUT DIFFERENT ARM POSTURES

1. Neuronal activity was recorded in the motor cortex of a monkey that performed reaching movements with the use of two different arm postur es. In the first posture (control), the monkey used its natural arm or ientation, approximately in the sagittal plane. In the second posture (abducted), the monkey had to abduct its elbow nearly to shoulder leve l to grasp the handle. The path of the hand between targets was simila r in both arm postures, but the joint kinematics and kinetics were dif ferent. 2. In both postures, the activity of single cells was often br oadly tuned with movement direction and static arm posture over the ta rgets. In a large proportion of cells, either the level of tonic activ ity, the directional tuning, or both, varied between the two postures during the movement and target hold periods. 3. For most directions of movement, there was a statistically significant difference in the dir ection of the population vector for the two arm postures. Furthermore, whereas the population vector tended to point in the direction of mov ement for the control posture, there was a poorer correspondence betwe en the direction of movement and the population vector for the abducte d posture. These observed changes are inconsistent with the notion tha t the motor cortex encodes purely hand trajectory in space.