A cortical evoked potential that reflects the conditioned, positive incentive value of the stimulus. III. Manipulative motivation and the value of the reward.

Changing animals from a schedule on which they had to bar-press after a tone cue to get a food pellet, to a schedule on which the food pellet was presented automatically after the tone cue and the manipulandum was not available, had no effect on the latency, amplitude, or shape of the M-wave evoked by the tone cue. The amplitude of the evoked wave did, however, decrease more rapidly, as a test session progressed, with the latter schedule than with the former schedule. Failure to fast animals before test sessions markedly reduced the amplitude of the M-wave. Variation in the type of food pellet used caused marked variations in the amplitude of the evoked response. In all cases, there was a high degree of correlation between M-wave amplitude and a behavioral indication of the animal's interest in the reward. It was concluded that the M-wave reflects the conditioned incentive value of the cue, or in other words, the level of interest of the animal in the reward.