MICROSTIMULATION OF PRIMATE MOTOR THALAMUS - SOMATOTOPIC ORGANIZATIONAND DIFFERENTIAL DISTRIBUTION OF EVOKED MOTOR-RESPONSES AMONG SUBNUCLEI

1. The functional organization of motor responses to microstimulation throughout the primate ''motor'' thalamus including nucleus ventralis lateralis, pars oralis (VLo); nucleus ventralis posterior lateralis, p ars oralis (VPLo); nucleus ventralis lateralis, pars caudalis (VLc); a nd portions of ventralis anterior (VA) and area X, was systematically studied in awake monkeys. A total of 2,021 sites were examined for the ir response to microstimulation. Of these, 1,123 were histologically v erified as to their location within the motor thalamus. At or near eac h site, isolated neurons were examined for their responses to somatose nsory examination and active movement (n = 1,272). This study was carr ied out as pari of a larger study examining the responses of neurons i n the motor thalamus to somatosensory examination, torque-induced limb per turbations, and active movement in a visuomotor step-tracking tas k. 2. Microstimulation at less than or equal to 40 mu A evoked movemen ts in the contralateral limbs, trunk, or face. Evoked movements of the limb were generally maximal about a single joint. 3. There was a diff erential response to microstimulation between subnuclei of the motor t halamus. Tn order of decreasing frequency, the percentages of sites wi thin each subnucleus from which movements were evoked were as follows. VPLo, 93% (449 of 483); VLo, 21% (57 of 272); VLc, 11% (15 of 140); V A, 1% (1 of 85), and reticular nucleus, 0% (0 of 65). In VPLc, 44% (34 of 78) of sites examined were microexcitable. However, these were alm ost all within 500 mu m of the border of VPLo, suggesting they may hav e occurred as a result of current spread to adjacent VPLo. Although ar ea X was not sampled in its entirety, it did nor appear to be microexc itable. 4, Microexcitable responses had a somatotopic organization, si milar to that for neuronal responses to sensorimotor examination, with leg responses found most laterally and arm and face responses found p rogressively more medially. 5. Zones in VPLo generally ranging from 50 0 to 1,500 mu m were found in which microstimulation resulted in the s ame motor response. These microexcitable zones resemble those describe d for the striatum and were termed thalamic microexcitable zones (TMZ) , TMZs also resemble cortical efferent zones in that both are somatoto pically organized, may affect a single muscle or group of muscles, hav e low thresholds for microstimulation with sharp boundaries that lie a djacent to other microexcitable zones with the opposite effects, aid a re of approximately the same dimension. 6. This study suggests that a fundamental unit of motor organization, i.e., single muscle or joint, is preserved at the thalamic level in the form of TMZs, and that these fundamental units of organization may contribute to the modular organ ization of the cortex.