Function of prefrontal cortex in timing behavior of monkeys

Abstract

Three groups of monkeys—with prefrontal lobectomy, cingulectomy, and normal controls—were trained on the DRL operant conditioning schedule. Lever presses by the subjects resulted in food reward, provided no response was made during a predetermined delay period. Subjects were first trained on a 10-sec delay to the criterion of 50% rewarded responses in three consecutive sessions. The delay was then lengthened by steps of 5 sec to a maximum of 70 sec. All subjects met criterion on delays as long as 25 sec. On longer delay settings subjects in each group failed to meet criterion. During the course of training, subjects reduced their response rates and responded with unimodal interresponse time distributions during criterion sessions, with mean interresponse times near the delay settings. The findings that prefrontally ablated and cingulectomized monkeys were unimpaired in reducing their response rates and in developing timing responses are contrary to the hypotheses that frontal lobes are essential to the inhibition of responses, recent memory, or temporal patterning of behavior.