Further physiological observations on the ventralis intermedius neurons in the human thalamus

1. During the course of stereotaxic thalamotomy for 56 cases with tremor mainly due to Parkinson's disease and essential tremor, extracellular recordings were made from the thalamic ventralis intermedius (Vim) nucleus under local anesthesia. These procedures have been justified as an essential technique to achieve the best therapeutic results by a selective coagulation. These physiological observations provide important information about the functional organization of the ventrolateral thalamic mass in humans. 2. Using Leksell's stereotaxic apparatus, a pair of semimicroelectrodes was introduced simultaneously to the thalamic ventral lateral region from the prefrontal area. The Vim nucleus was identified tentatively by characteristic high background activity which contrasted to that found in its rostral part and by superimposed large amplitude spontaneously active units. 3. In this high activity zone, 135 units (approximately 1/5 of the recorded units) responded to natural stimulation applied to contralateral body parts. Among them, approximately 90% responded to a passive or active movement of a joint. Several lines of evidence suggested that probably muscle receptors were responsible. 4. The rest of units (approximately 10%) responded to light touch applied to contralateral skin surface. Convergent responses between kinesthetic and tactile units were never encountered. Also, kinesthetic and tactile neurons were geographically separated. The latter were found always at the end of our oblique trajectory, following the kinesthetic neurons. 5. Neurons with sensory responses were clustered mostly within the confines of the Vim nucleus, probably extending caudally to the ventrocaudalis externus anterior of Hassler. Evidence for a somatotopic representation in the Vim nucleus was obtained. 6. Electrical stimulation of the appropriate peripheral nerve produced responses of the same thalamic unit(s) that responded to natural stimulation. The latency to upper limb nerve stimulation was between approximately 10 and 20 ms. It was almost fixed in a given case. 7. It is concluded that the Vim nucleus receives kinesthetic afferent input from the contralateral body parts (mainly from the muscle receptor) and may be concerned with muscle sense. This may explain why a small, selective coagulation of the physiologically identified Vim has such a constant effect on several different kinds of tremor.