A motor cortical contribution to the anticipatory postural adjustments that precede reaching in the cat

We tested the hypothesis that pyramidal tract neurons (PTNs) in the motor cortex contribute to the anticipatory postural adjustments (APAs) that precede the onset of a reach in the standing cat. We recorded the discharge activity of 151 PTNs in area 4 of the pericruciate cortex during reaches of both the contralateral and the ipsilateral limbs in an instructed delay task. A total of 70/151 PTNs were identified as showing an initial short-latency period of discharge following the Go signal. Linear regression analysis showed that in many of these PTNs the short-latency discharge was time-locked to the Go signal and temporally dissociated from the subsequent voluntary movement of the limb. The onset of the change in activity of most of those Go-related neurons that we could test (62/70) was temporally related to the onset of the change in the center of vertical pressure. In 33/70 PTNs, Go-related activity was observed only during contralateral reach, in 13/70 only during ipsilateral reach, and in 24/70 during movements of each limb; most of these latter cells (20/24) showed nonreciprocal changes in activity. Although 35/151 (23%) cells showed significant changes during the instructed delay period for reaches made with at least one of the limbs, only one neuron showed a significant reciprocal change during reaches with either limb. We suggest that the discharge characteristics of these PTNs are compatible with our hypothesis that the motor cortex contributes to the production of the APAs preceding movement.