On the mechanism of the post-asymptotic CR decrement phenomenon

Three experiments with 49 dogs explored the decrease in CR magnitude that sometimes occurs when CS-US pairings are continued beyond those needed to reach maximum CR magnitude. The first experiment confirmed the existence of the phenomenon, obtaining less conditioned excitation after 300 CS-US trials than after 18 CS-US trials. The second experiment demonstrated that the phenomenon is not dependent upon paired CS-US presentations because 18 CS-US pairings yielded little excitation if preceded by, followed by, or intermixed with 282 US presentations. The third experiment indicated that in contrast to the decremental effects on excitatory conditioning, inhibitory conditioning was faciliated by large numbers of prior US exposures, suggesting explanation of the post-asymptotic decrement phenomenon by opponent-process theory.