Unit activity in prefrontal cortex during delayed-response performance: neuronal correlates of transient memory.

SINCE THE ORIGINAL investigations of Jacob-sen (22, 23) in primates, the granular cortex of the frontal lobe, also called pre-hontal cortex, has been thought to play a role in short-term memory. This is an inference from the deficits that lesions of this cortical region cause on performance of delayed-response tests. A delayed-response .llv consists of tl trial typica one of two possible visual tation of lc presen cues, xi ensuing period it, a ch of enforced delay and, oice of motor response i at the end n accord wi of th the cue. The temporal separation between cue and response is the principal element making the delayed-response procedure a test of an operationally defined short-term memory function. However, the neural processes i nvolvetl in delayed-response performance , as the psychological processes participating in short-term memory, are probably many and complex. The precise role of the prefrontal cortex in these is not yet clear. In contrast to the abundant documen ta-tion of the effects of frontal ablations, the literature 011 the neuroelectrical correlates of delayed-response behavior is brief and recent. In 1969, Stamm and Rosen (43) showed steady potential shifts in the cortex of monkeys during a delayed-response test. Their most significant finding was a slow negative potential on the surface of the l&frontal cortex at the beginning of the delay period, suggesting that at that time there is in that region an increase of neu-ronal excitability. Kubota and Niki (27) have more recently reported firing changes of units irl t 1~ periprincipal area of the llrefrontal cortex in relation to the performance of a delayed alternation task calling for successive alternate presses of two levers with interspersed 5see delays. Some units are more active immediately before lever-pressing than during the delays; others show the opposite pattern. A previous investigation showed that bilateral or unilateral cooling of the pre-frontal cortical convexity produces marked and reversible deficits of performance in animals trained at the direct-method version of the delayed-response test (16). It is because of these results that the same behav-ioral test was adopted for the present study: a microelectrode exploration of the pre-frontal cortex of the monkey performing the delayed-response task. To the three successive periods that corn stitute a single trial in the delayed-response task-cue period, delay, and response period-three fundamental processes of short-term memory function may be attributed: the acquisition, the retention, and the corollary use of information. Therefore, …