Decline of the monkey's limbic and prefrontal activity during task repetition

In order to investigate the neural mechanism underlying the motivation, we measured the regional cerebral blood flow (rCBF) by positron emission tomography in a monkey that was repeatedly engaged in cognito-behavioural tasks. The rCBF of the anterior cingulate area, medial parts of prefrontal area, and hippocampal region decreased significantly with the succession of tasks within a day. The activity was restored abruptly when the reward was then increased, but decreased again on further repetition of the tasks. These findings suggest that the limbic and prefrontal areas are the regions involved in motivational function in the brain.