Taste and olfactory processing in the brain and its relation to the control of eating.

Taste processing in primates through the nucleus of the solitary tract and the primary taste cortex is shown to represent the identity and intensity of taste inputs. In contrast, in the secondary taste cortex in the orbitofrontal area, single neurons respond to the taste of a food only if hunger is present. The neurons here reflect the reward value of taste. They show sensory-specific satiety. In addition to neurons with best responses to sweet, salt, bitter, and sour, there are separate representations of the taste of protein ("umami") and of astringency (e.g., tannic acid). In the orbitofrontal cortex, olfactory inputs converge onto neurons with taste inputs, forming representations of flavor. Neurons in this region may also respond to the sight of food and to its texture. The olfactory representation for some neurons reflects the taste association of odors; and olfactory sensory-specific satiety is represented in this part of the brain. Rapid learning of visual-to-taste associations is also a feature of the neural processing that occurs in the orbitofrontal cortex.