Involvement of Subcortical Structures in the Preparation of Self-Paced Movement

Although it has long been suggested that the basal ganglia and thalamus are involved in movement planning and preparation, there was little direct evidence in humans to support this hypothesis. Deep brain stimulation (DBS) is a well-established treatment for movement disorders such as Parkinson's disease, tremor, and dystonia. In patients undergoing DBS surgery, we recorded simultaneously from scalp contacts and from electrodes surgically implanted in the subthalamic nucleus (STN) of 13 patients with Parkinson's disease and in the “cerebellar” thalamus of 5 patients with tremor. The aim of our studies was to assess the role of the cortico-basal ganglia-thalamocortical loop through the STN and the cerebello-thalamocortical circuit through the “cerebellar” thalamus in movement preparation. The patients were asked to perform self-paced wrist extension movements. All subjects showed a cortical readiness potential (RP) with onset ranging between 1.5 to 2s before the onset of movement. Subcortical RPs were recorded in 11 of 13 with electrodes in the STN and in 4 of 5 patients with electrodes in the thalamus. The onset time of the STN and thalamic RPs were not significantly different from the onset time of the scalp RP. The STN and thalamic RPs were present before both contralateral and ipsilateral hand movements. Postoperative MRI studies showed that contacts with maximum RP amplitude generally were inside the target nucleus. These findings indicate that both the basal ganglia and the cerebellar circuits participate in movement preparation in parallel with the cortex.