Positive contrast in the rat: a test of the additivity theory.

Rats were trained to lever press for food on a multiple variable-interval variable-interval schedule, then shifted to a multiple variable-interval extinction schedule. For six subjects (group L), schedule components were signalled by the presence or absence of a flashing light emitted from an alternate, "signal" lever. For four subjects (group T), schedule components were signalled by two distinct auditory tones. Contrary to the predictions of an additivity theory based on the summation of response classes, contacts on the signal lever did not increase after the schedule shift in group L. However, nine of the ten subjects in the study demonstrated positive contrast effects on the operant lever. In a subsequent test for stimulus control, enhancement and suppression by the discriminative stimuli were found in these same nine subjects. An additivity theory based on the summation of excitatory processes, rather than response classes, appears to account for these data.