Comparative characteristics of unit activity in the prefrontal and parietal areas during delayed performance in monkeys.

Neuronal mechanisms of prefrontal and parietal areas were compared in 3 monkeys during delayed performance. Spatioselective neurons were found in both areas in question. In the prefrontal cortex, they constitute 28% of all units sampled and in the parietal cortex they account for 21%. For the prefrontal area, spatial selectivity was particularly great during the delay (8%), and in the parietal area during the cue display (9%). During the delay, however, spatioselective parietal neurons accounted for 4% of all units sampled, i.e. their number was half that in the prefrontal area. The prefrontal cortex appears to play a major role in short-term memory proper, whereas the parietal area is more involved in assessing spatial relationships of emerging sensory stimuli. Spatioselective neurons of both areas were heterogeneous in their functions. Activity of some (11% in the prefrontal and 10% in the parietal area) was related only to the cue location. Activity of others (13% in the prefrontal cortex and 8% in the parietal cortex) was moreover coupled with the forthcoming movement. With lengthening of the delay, units related to the established temporal stereotype and some labile units which quickly rearranged to a new temporal task were recorded. Thus association area neurons reflect two concurrent processes linked with spatial and temporal memories. During cue displays, it is not only their spatial location that is described, but also a future motor act with its temporal and spatial properties programmed.