Persistent discharges in the prefrontal cortex of monkeys naive to working memory tasks.

Neurons in the prefrontal cortex and a network of interconnected brain areas discharge in a persistent fashion after the offset of sensory stimulation. Such persistent discharges are thought to constitute a neuronal correlate of working memory. The information content of neuronal discharges and its anatomical localization across the surface of the prefrontal cortex has been a matter of debate. Discrepant results by different laboratories may be due to the effects of different training regiments and tasks used in memory tasks. In order to address how training in a memory task alters neuronal responses, we performed recordings in monkeys that were never trained in memory tasks, but passively viewed visual stimuli. We have found that a population of prefrontal neurons responded to visual stimuli and also exhibited significantly elevated responses during "delay" intervals of the task. For a population of these neurons, persistent discharges were selective for the location and feature of the preceding stimulus. These discharges were typically disrupted by the appearance of a subsequent stimulus. Our results suggest that some prefrontal neurons represent the location and identity of visual stimuli in a persistent fashion, even when the latter are not behaviorally important or required to be kept in memory.