Anticipatory postural adjustment before bimanual unloading reactions: the role of the motor cortex in motor learning.

The role of the motor cortex in forming a learned coordination (stabilization of the forearm on unloading) was studied in humans. Subjects maintained a 1-kg weight with the right (postural) forearm, the weight being attached via an electromagnet. Unloading of the postural arm was initiated by the subjects by lifting a similar load with the left arm. In control experiments, lifting of the load did not lead to unloading of the postural arm. Changes in motor cortex excitability were studied by transcranial magnetic stimulation applied to the representation area of the right biceps muscle in the motor cortex at the beginning and end of the experiments. Repeated unloading tests showed progressive decreases in the amplitude of the movement of the unloaded forearm, which were accompanied by increases in the anticipatory inhibition of the electromyogram of the biceps muscle of the unloaded arm (learning). Muscle responses to transcranial magnetic stimulation during the learning process showed no significant changes. Analysis of normalized muscle responses to transcranial magnetic stimulation (response/baseline) showed that these increased at the end of training and reached a significantly higher level than seen at the beginning of training. These results lead to the conclusion that the motor cortex plays a fundamental role in inhibiting synergies and coordinations which would interfere with the formation of the new coordination during motor learning.