VISUOMOTOR FUNCTIONS OF CENTRAL THALAMUS IN MONKEY .1. UNIT-ACTIVITY RELATED TO SPONTANEOUS EYE-MOVEMENTS

The region in and around the thalamic internal medullary lamina (IML) in the cat contains neurons active with ocular saccades and responding to visual stimuli. Single-unit microelectrode recordings were made in the corresponding thalamic region of the alert monkey [Macaca nemestrina] to determine whether neurons with similar properties existed. The objective was to specify the functional characteristics of these thalamic cells in the monkey, since cell populations in the central thalamus form an important link between brain stem structures and cortical areas, and most neurophysiological information on these structures with regard to gaze mechanisms was obtained in primates. Observations on 164 thalamic units whose activity was related to the performance of spontaneous eye movements, head fixed, are reported. The animals were trained on a visual discrimination task but photic stimuli were used only for calibrating the eye-position recording and for inducing small saccades and smooth pursuit. The experiments were performed in dim red light and in total darkness. Three types of units were found: 67 saccadic burst units, 58 saccade pause-rebound units and 39 eye-position units. Of the burst units (62) had a directional preference. Most of the on-directions were contraversive, and it was in such units that the lead time of firing before saccades was the longest (up to at least 400 ms). Some of the burst units had a movement field, others fired more intensively and with a longer lead time, depending on the eccentricity of the eye position reached in orbit. The 5 units with no directional preference were the ones showing the best relation of burst duration with saccade duration. Three types of pause-rebound units were distinguished, depending on whether the saccadic pause or the postsaccadic burst was the most conspicuous event or the pause occurred after saccade offset. The 3 types were omnipausers, omnirebound cells and late pausers. Omnipausers and omnirebound cells had no directional preference but their typical firing patterns occurred very consistently with all saccades, even less than 2.degree.. In a few units, the rebound progressively faded away in total darkness. The relation of firing rate of eye-position units with eccentricity of the eyes in orbit was analyzed. Fluctuations in time and a hysteresis effect were found to affect this relation. The anatomical and functional characteristics of the thalamic region explored are reviewed. In the monkey as in the cat, the most important feature of the units studied was that their firing pattern anticipated changes in eye position, a property commonly shown by brain stem structures but not by cortical ones insofar as spontaneous eye movements are concerned. This suggests a participation in movement initiation. Certain types of neurons, such as pause-rebound units, may provide cortical visual areas with the signal that a saccade has just been accomplished and that new information has become available for processing.