Sensorimotor unit activity related to intention in the pulvinar of behaving Cebus Apella monkeys

Previous observations made in our laboratory in a waking behaving Cebus Apella monkey revealed that neurons of the Pulvinar nucleus of the thalamus discharged preferentially in relation to intentional movements of the limbs and eyes. We give here a description of further observations made in two waking, behaving Cebus Apella monkeys trained to perform two tasks: in the first to make saccadic eye movements to eccentrically placed visual targets; in the second, to make projection movements of the arm and hand to touch targets within arm's reach. The electrical activity of thalamic neurons was recorded extracellularly and records were made simultaneously of the horizontal eye movements and of tasks events. Four-hundred-sixty-five neurons were studied: of these, the activity of 272 could be correlated with behavioral events, while the remaining 193 could not be correlated or classified in this manner. The cells identified were classed in five groups, as follows: (1) neurons active during attentive fixation of a target, but which did not respond to our ordinary visual test stimuli; (2) neurons active during projection movements of the arm or manipulation with the hand, but which were not active during casual movements of the hand or arm, and which were not activated by passive somatic sensory stimuli; (3) those active before, during or after evoked saccadic movements of the eyes, but which were not activated by our testing visual stimuli; (4) neurons active during tracking movements of the eyes, or during projection movements of the arm, alone, but which discharged maximally when these two events occurred simultaneously; and, (5) neurons active during both saccadic movements of the eyes and during projection movements of the arm. We regularly observed, for each of these classes of neurons of the Pulvinar, that optimal correlated activity depended upon the intentional nature of the associated behavioral events, and the animal's attention to them. We conclude that there exists at the level of the Pulvinar a neural correlate of certain evolving behavioral events, and particularly of intentional activity such as the projection of the arm or the direction of gaze towards targets of interest in the immediately surrounding visual environment. The regions of the Pulvinar containing neurons with these properties are reciprocally related to association areas of the Neocortex known to contain neurons with similar properties. It can then be concluded from both anatomical and electrophysiological observations that the Pulvinar is an essential part of the system controlling these complex behavioral events.