Conditioned Excitation and Conditioned Inhibition Acquired through Backward Conditioning

Historically, there has been little consensus regarding the outcome of backward conditioning trials in Pavlovian conditioning. One view states that few US→CS trials will produce an excitatory association, but with additional trials the excitatory association will wane and eventually give way to an inhibitory association (Heth, 1976). An alternative view is that the initial association remains intact over further training trials, but the subject additionally learns the backward temporal relationship between the CS and the US (Barnet & Miller, 1996). Toward testing these views, we conducted four parametric experiments using conditioned suppression by rats to examine the development of excitatory and inhibitory response potentials as a function of the number of trials. In all experiments, animals received a low (4), moderate (16), or high (96) number of backward conditioning training. In Experiments 1 and 2, conditioned inhibition was assessed with summation and retardation tests, respectively, and more inhibition was found with more backward pairings. In Experiment 3, first-order excitatory responding was observed only with low levels of training. In Experiment 4, robust second-order excitatory responding was seen following low and high levels of US→CS training. The results are discussed in terms of Heth's views and the temporal coding hypothesis, a recent model of Pavlovian conditioned responding.