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. ?? 1977.
Boyd, E. S., Boyd, E. H., & Brown, L. E. (1977). A cortical evoked potential that reflects the conditioned, positive incentive value of the stimulus. III. Manipulative motivation and the value of the reward. Electroencephalography and Clinical Neurophysiology, 42(3), 364–371. https://doi.org/10.1016/0013-4694(77)90172-9