Amphetamine-induced disruption of latent inhibition is not reinforcer-mediated

Latent inhibition (LI) refers to retarded conditioning to a stimulus that had been repeatedly preexposed without consequences, as compared with a nonpreexposed stimulus. Amphetamine disrupts LI, and this effect was suggested to result from enhanced switching to respond according to the stimulus-reinforcer contingency. Recently, it has been argued that amphetamine disrupts LI by increasing the impact of the reinforcer. This implies that amphetamine should produce stronger conditioning in the nonpreexposed group, and that its influence on LI can be modified only by changing reinforcer parameters. We report two studies, using an off-baseline conditioned emotional response procedure in rats licking for water, that question both predictions. In the first study, a meta-analysis based on 23 replications of the effect of amphetamine on LI, using tone as the preexposed stimulus, showed that LI is significantly attenuated due to drug-induced increased suppression in the preexposed groups only. The second study included two experiments, each using two shock intensities but different preexposed stimuli. Amphetamine disrupted LI at both shock intensities when the stimulus was a steady light, but this effect disappeared when the stimulus was three flashing lights. Thus, the effect of amphetamine could not be modified by manipulating shock intensity, but was modifiable by manipulating the nature of the preexposed stimulus. The results are inconsistent with the hypothesis that amphetamine-induced disruption of LI is solely mediated by drug-induced changes in the effects of reinforcers.