Comparison of differential neuronal responsiveness for different regions of the prefrontal cortex in a conditional visual discrimination task

To investigate neuronal processing during monkeys' performance of a visual conditional discrimination task, recordings were made from four areas of prefrontal cortex (ventromedial, orbitofrontal, dorsolateral and anterior cingulate) where lesions have been shown to produce impairment of such tasks. Of 1911 recorded neurons, 573 (31%) responded to elements of the task. This proportion was less than the 50% previously reported as responsive in temporal cortex under the same conditions, suggesting sparser encoding in prefrontal than temporal cortex. Of the responsive prefrontal neurons, 165 (29%) responded differently on the different types of trial, so signalling various types of information relevant to task performance and cognition. In line with recent lesion findings, in the dorsolateral region the incidence of such differentially responsive neurons was only an eighth that in the other regions. The relatively high incidence of neuronal responses that encoded a potential instruction cue rather than specific individual stimulus arrangements was consistent with the animals solving the task by using such information, though other neuronal responses could have enabled the task to have been solved by rote learning. Compared to temporal neurons, prefrontal responses more frequently coded information relating to the planned behavioural response rather than perceptual aspects of the task. Population differential response latencies were long (> approximately 225 ms) in prefrontal cortex. A comparison of such differential latencies between temporal and prefrontal cortex indicated that potential information flow was likely to be primarily from temporal to prefrontal cortex rather than vice versa.