A neurophysiological comparison of three distinct regions of the primate frontal lobe

We examined whether reported physiological distinctions between prefrontal (PF) and premotor (PM) areas reflect true dissimilarity or instead result from technical considerations such as differences among species, individuals, or behavioural, physiological and anatomical techniques. A Rhesus monkey performed a task involving 2 stimulus events on each trial: the first stimulus appeared briefly, then after a delay period, 2 peripheral stimuli appeared simultaneously. The monkey had to delay responding for an additional, self-timed instructed delay period, then contact a touch pad beneath the peripheral stimulus of the colour matching the first stimulus. PM and PF cortex differ in several respects. More PF than PM neurons discharge during the presentation of the first stimulus, and only PF neurons show phasic bursts of activity shortly after that stimulus disappears. During the instructed delay period, cells in PF begin activity earlier than PM neurons, whereas PM neurons continue their activity longer. Similarly, PF cells show greater discharge modulation early in the instructed delay period compared with PM cells, which discharge more intensely later. Cells with apparent selectivity for stimulus characteristics appear only in PF. Thus our results support the view that PM and PF have distinctive physiological properties that cannot be accounted for by technical considerations alone. In addition, we describe the activity cells in a cortical region, here termed the dorsomedial frontal cortex, which has a distinctive dysgranular cytoarchitecture and has physiological properties that ally it with PF.