Prefrontal unit activity of macaque monkeys during auditory and visual reaction time tasks

During a simple reaction time task using auditory or visual stimuli, a total of 96 single units were recorded from the dorsolateral prefrontal cortex of macaque monkeys. These monkeys were trained to depress a lever for a fixed period which produced a tone burst or a small spot of light. After a variable period, the stimulus intensity changed, and then, the monkey released the lever. Eighty-one cue-related units were classified into 3 types according to their decay time; that is, phasic, tonic and mixed. Phasic units (n = 19) showed a transient increase of discharge rate with a relatively short peak latency (70-300 ms). Of these, 17 units responded exclusively to either visual or auditory stimuli and two to both. Tonic units (n = 55) showed enhanced or suppressed activity, with longer latencies, which was sustained as long as the cue period continued. The temporal pattern of the discharge in 23 tonic units was found to be similar for both the auditory and visual cues. Seven mixed-type units showed combined phasic and tonic patterns. Lever release-related units (n = 15) were activated only during the period of lever release with no distinction in cue modality. It is suggested that the dorsolateral prefrontal cortex receives sensory inputs fairly discretely on the phasic-type neurons and that these sensory activities are transmitted to the tonic-type neurons which lead to an initiation of the lever release behavior.