Activity Properties and Location of Neurons in the Motor Thalamus That Project to the Cortical Motor Areas in Monkeys

Abstract

The activity of neurons in the motor nuclei of the thalamus that project to the cortical motor areas (the primary motor cortex, the ventral and dorsal premotor cortex, and the supplementary motor area) was investigated in monkeys that were performing a task in which wrist extension and flexion movements were instructed by visuospatial cues before the onset of movement. Movement was triggered by a visual, auditory, or somatosensory stimulus. Thalamocortical neurons were identified by a spike collision, and exhibited 2 distinct types of task-related activity: 1) a sustained change in activity during the instructed preparation period in response to the instruction cues (set-related activity); and 2) phasic changes in activity during the reaction and movement time periods (movement-related activity). A number of set- and moment-related neurons exhibited direction selectivity. Most movement-related neurons were similarly active, irrespective of the different sensory modalities of the cue for movement. These properties of neuronal activity were similar, regardless of their target cortical motor areas. There were no significant differences in the antidromic latencies of neurons that projected to the primary and nonprimary motor areas. These results suggest that the thalamocortical neurons play an important role in the preparation for, and initiation and execution of, the movements, but are less important than neurons of the nonprimary cortical motor areas in modality-selective sensorimotor transformation. It is likely that such transformations take place within the nonprimary cortical motor areas, but not through thalamocortical information channels.