Primate frontal cortex: effects of stimulus and movement.

We compared neuronal activity in the dorsal premotor cortex (PMd), ventral premotor cortex (PMv), and prefrontal (PF) cortex of two rhesus monkeys. The behavioral design was a variant of the instructed delay task which established that: (1) a given visual stimulus could, on different trials, instruct different limb movements and (2) several different visual stimuli could instruct the same movement. Neurons in all frontal areas displayed the often replicated activity patterns that occur during instructed delay tasks, including phasic increases after instruction stimuli (signal-related activity), tonic discharge during an instructed delay period (set-related activity), and phasic premovement discharge (movement-related activity). For signal-, set-, and movement-related activity, the majority of neurons in PMd (51-64%), but only a minority in PF (16-18%) and PMv (32-40%), showed activity levels that significantly depended on the action instructed by that stimulus rather than simply the characteristics of the stimulus per se. Thus, most PMd activity, including the aspects that most resembled a sensory response, reflected factors in addition to the signal. Taken together with the results of related studies, it seems most likely that these other factors are dominated by the motor instructional significance of the stimulus. In addition, many neurons (17-37%) in all examined areas showed activity that significantly depended on which of various stimuli guided the same movement. This finding shows that, in those frontal areas, neuronal activity can be affected by both the action to be taken and the events guiding that action.