Properties of delay-period neuronal activity in the primate prefrontal cortex during memory- and sensory-guided saccade tasks.

The dorsolateral prefrontal cortex (DLPFC) is involved in visuospatial short-term (or working) memory. Its cellular basis has been widely examined using the delayed-response paradigm in nonhuman primates. Sustained delay-period activity in DLPFC neurons with directional difference (i.e. directional delay-period activity) has been thought to represent visuospatial short-term (or working) memory. However, little is known about the activity of these neurons during a delay period when the sensory input remains. To address this issue, we examined neuronal activity in the DLPFC while macaque monkeys performed a memory-guided saccade (MGS) task and a delayed visually guided saccade (VGS) task. The MGS task required a memory-guided saccade for a remembered target location. The VGS task had the same temporal sequence as the MGS task, but the sensory stimulus remained during the delay period. We found that most of the DLPFC neurons with directional delay-period activity showed sustained activation during the 'delay' period in the VGS task only ('V-neurons', 49%), or in both tasks ('MV-neurons', 46%). Neurons showing directional delay-period activity in the MGS task only ('M-neurons') were only 5% of the DLPFC neurons with directional delay-period activity. These findings indicate that most DLPFC neurons that are active during the delay period are also active when the sensory stimulus remains, suggesting that DLPFC neurons driven by mnemonic information are also driven by sensory input. Such sustained representation of information should have potential utility in flexible cognitive controls of behaviour.