We performed successive H(2)(15)O-PET scans on volunteers as they ate chocolate to
beyond satiety. Thus, the sensory stimulus and act (eating) were held constant while
the reward value of the chocolate and motivation of the subject to eat were manipulated
by feeding. Non-specific effects of satiety (such as feelings of fullness and autonomic
changes) were also present and probably contributed to the modulation of brain activity.
After eating each piece of chocolate, subjects gave ratings of how pleasant/unpleasant
the chocolate was and of how much they did or did not want another piece of chocolate.
Regional cerebral blood flow was then regressed against subjects' ratings. Different
groups of structures were recruited selectively depending on whether subjects were
eating chocolate when they were highly motivated to eat and rated the chocolate as
very pleasant [subcallosal region, caudomedial orbitofrontal cortex (OFC), insula/operculum,
striatum and midbrain] or whether they ate chocolate despite being satiated (parahippocampal
gyrus, caudolateral OFC and prefrontal regions). As predicted, modulation was observed
in cortical chemosensory areas, including the insula and caudomedial and caudolateral
OFC, suggesting that the reward value of food is represented here. Of particular interest,
the medial and lateral caudal OFC showed opposite patterns of activity. This pattern
of activity indicates that there may be a functional segregation of the neural representation
of reward and punishment within this region. The only brain region that was active
during both positive and negative compared with neutral conditions was the posterior
cingulate cortex. Therefore, these results support the hypothesis that there are two
separate motivational systems: one orchestrating approach and another avoidance behaviours.