Neuronal activity related to anticipated reward in frontal cortex: does it represent value or reflect motivation?

It is thought that neuronal activity in orbitofrontal cortex (OFC) represents the value of anticipated reward; however activity in many other brain areas also seems to reflect expected reward value. For example, we have shown that in monkeys performing a memory-guided saccade task for a reward of variable size, activity in numerous areas of frontal cortex is stronger when the monkey anticipates a larger reward. The activity of these neurons might be related to the value of the expected reward or to the degree of motivation induced by expectation of the reward. Anticipation of a more valued reward leads to stronger motivation, as evidenced by measures of arousal, attention, and intensity of motor output. On the assumption that motivated behavior depends on influences arising in the limbic system and acting on the motor system, we hypothesized that neuronal signals representing reward value are unique to OFC, whereas signals arising from other frontal areas, those more closely tied the motor system, reflect the degree of motivation. To test this hypothesis, we recorded from single neurons in OFC and premotor cortex while two monkeys performed a task in which we dissociated value from motivation. Neuronal activity in premotor cortex reflected the monkey's degree of motivation, presumably related to the monkey's level of motor readiness and movement preparation, whereas neuronal activity in OFC represented the value of expected reward.