Brain mechanisms of satiety and taste in macaques.

Flavor is the primary reinforcer of eating. As satiety is induced, the reinforcement of flavor is lost. Since flavor derives largely from taste, one might expect gustatory responsiveness to decline with increasing satiety. However, no such loss of sensitivity occurs in humans, even as the reinforcing value of taste declines with satiety. Thus, we explored the effect of satiety on taste responses at several levels of the macaque's nervous system to determine where is influence began. Taste-evoked activity in the NTS and primary taste cortex was unaffected by the induction of satiety through or administration of glucose. Taste cortex projects to amygdala and orbitofrontal cortex (OFC). In amygdala, satiety reduced responsiveness by 58%; in OFC, neurons were fully suppressed. Both amygdala and OFC project to the hypothalamus, where taste responsiveness was also suppressed. Thus, the neural impact of food is reduced not in area devoted to quality analysis, but in those concerned with motivation and reinforcement.