Premovement neuronal activity in the primary motor cortex is associated with the initiation of ipsilateral hand movements in monkeys

The primary motor cortex (M1) is believed to be a cortical center for the execution of limb movements. Although M1 neurons mainly project to the spinal cord on the contralateral side, some M1 neurons project to the ipsilateral side via the uncrossed corticospinal pathway. Moreover, some M1 neurons are activated during ipsilateral forelimb movements. However, the extent to which M1 neurons are involved in ipsilateral movement execution has not been determined. Therefore, we investigated the involvement of M1 neurons in the initiation of ipsilateral and contralateral hand movements by examining trial-by-trial correlations between premovement neuronal spikes and hand movement reaction times in monkeys. Overall, the activity of M1 neurons was more strongly correlated with the reaction times for contralateral hand movements than those for ipsilateral hand movements. However, the activity of some M1 neurons was correlated with reaction times for ipsilateral hand movements, and these correlations were as strong as those between the activity of other M1 neurons and reaction times for contralateral hand movements. This finding suggests that one subset of M1 neurons sends motor commands for ipsilateral hand movements to the same extent as another subset of M1 neurons sends motor commands for contralateral hand movements.