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

SINCE THE ORIGINAL investigations of Jacobsen (22, 23) in primates, the granular cortex of the frontal lobe, also called prehontal 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 shortterm 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 tation 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 neuronal 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 prefrontal 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 behavioral test was adopted for the present study: a microelectrode exploration of the prefrontal 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, it is reasonable to inquire whether neurons in the prefrontal cortex manifest changes of activity temporally related to these test periods. A question of particular interest 011 which the present study was focused is whether prefrontal neuron discharge during the clelay period differs from discharge in intertrial periods, even though the physical characteristics of the animal’s environment are identical in the two conditions. Differences of prefrontal neuron activity between these two conditions might be relatecl to a role of the prefrontal cortex in the Went ion of mnemonic information.