Unit activity: motiviation-dependent responses from midbrain neurons.

Single neurons in the midbrain tegmentum of rats showed clear discriminatory responses to three tones announcing either food or water or no reinforcement. Subjects were required to press a single lever and remain motionless for 2 seconds during which time unit activity from several brain sites was recorded. One of the tones, randomly designated, was sounded halfway through this period. Manipulation of drive states revealed that the highest activity was associated with tones announcing the reinforcement for which the animal was most strongly motivated. Thus, from a hungry animal sustained or increased firing rates were elicited by a " food" tone and differentially lowered rates by the other tones.