[Comparison between responses of monkey prefrontal neurons to signal and to indifferent stimuli]

Unit activities of the principal sulcus area were recorded in three monkeys (Macaca mullata). Two of them were trained to perform visual discrimination task, and the other had not received any training. The sequence of the task is as follows: (1) a cue of presentation of red (in R Model) or blue (in B Model) light (1.3s); (2) a delay period (2.1s); (3) a reaction period of presentation of white light (4.88s). The monkey should draw a bar after the onset of the white light of R Model to avoid a electric shock, and should not draw the bar after the onset of the white light of B Model in which the monkeys would not receive the electric shock. Of 127 prefrontal units recorded from the two trained monkeys when they were performing the task, 80 units (63%) were related to the events of the task. Ninety six units were recorded for the test of responses to click and flash in the intertrial interval, and 23 units (24%) responded to click and/or flash. Fifteen out of 23 units were event-related. Twelve out of 103 units responded to the red, blue and white flash in the intertrial interval. A hundred units were recorded from the untrained monkey a only 4 units showed nonspecific responses to the indifferent stimuli. The results showed that after training much more prefrontal neurons involved in processing of the information of signal stimuli than those in response to the indifferent stimuli. In other words, the prefrontal neurons possess plasticity and change their activities during the training of visual discrimination task.