Properties of delay-period neuronal activity in the monkey dorsolateral prefrontal cortex during a spatial delayed matching-to-sample task

The dorsolateral prefrontal cortex (PFC) has been implicated in visuospatia l memory, and its cellular basis has been extensively studied with the dela yed-response paradigm in monkeys. However, using this paradigm, it is diffi cult to dissociate neuronal activities related to visuospatial memory from those related to motor preparation, and few studies have provided evidence for the involvement of PFC neurons in visuospatial memory of a sensory cue, rather than in motor preparation. To extend this finding, we examined neur onal activities in the dorsolateral PFC while a rhesus monkey performed a s patial delayed matching-to-sample (SDMTS) task, which allows us to adequate ly access visuospatial memory independent of any sensorimotor components. T he SDMTS task required the subject to make a lever-holding NOGG response or a lever-releasing GO response when a visuospatial matching cue (white spot , one of four peripheral locations, 15 degrees in eccentricity) matched or did not match a sample cue (physically the same as the matching cue) that h ad been presented prior to a delay period (3 s). Thus, the SDMTS task requi res the subject to remember visuospatial information regarding the sample c ue location during the delay period and is suitable for accessing visuospat ial memory independent of any sensorimotor components, such as motor prepar ation, for directed movements. Of a total of 385 task-related neurons, 184 showed a sustained increase in activity during the delay period ("delay-per iod activity"). Most of these neurons (n = 165/184, 90%) showed positional delay-period activity, i.e., delay-period activity where the magnitude diff ered significantly with the position of the sample cue. This activity appea rs to be involved in visuospatail memory and to form a "memory field." To q uantitatively examine the properties of positional delay-period activity, w e introduced a tuning index (TI) and a discriminative index (DI), which rep resent the sharpness of tuning and the discriminative ability, respectively , of positional delay-period activity. Both TI and Dr varied among neurons with positional delay-period activity and were closely related to the time from the onset of the sample cue to the onset of positional delay-period ac tivity; positional delay-period activity with. sharper tuning and a greater discriminative ability had a slower onset. Furthermore, at the population level, both TI and DI were increased during the delay period in the neurona l population with a high DI value. These results extend previous findings t o suggest that integrative, convergent processes of neuronal activities for increasing the accuracy of visuospatial memory may occur in the dorsolater al PFC. Thus, a critical role of the dorsolateral PFC in visuospatial memor y may be to sharpen it to guide behaviors/decisions requiring accurate visu ospatial memory.