Amphetamine-induced disruption of latent inhibition depends on the nature of the stimulus.

It is well documented that latent inhibition (LI), i.e. slower conditioning to a stimulus that had been repeatedly pre-exposed without consequences, compared to a non-pre-exposed stimulus, is prevented by amphetamine. Recently, we found that the effects of amphetamine on LI, as assessed in an off-baseline conditioned emotional response (CER) procedure, depend on the nature of the pre-exposed stimulus, irrespective of reinforcer intensity. Because these results contrast with a recent finding that a reduction in reinforcer intensity reversed amphetamine-induced attenuation of LI in an on-baseline CER procedure, the present study investigated the effects of amphetamine on LI as a function of the nature of the pre-exposed stimuli and shock intensity, using an on-baseline CER procedure. The effects of amphetamine on post-shock suppression of drinking as well as on activity, were monitored throughout the stages of the CER procedure. Experiment 1 used a 5 s steady light as the pre-exposed and conditioned stimulus, and two shock intensities in conditioning, and Experiment 2 used a 10 s flashing light and two shock intensities. Amphetamine disrupted LI with a steady light at both low and high shock intensities, but failed to disrupt LI with a flashing light at both shock intensities. In addition, the drug disrupted LI in Experiment 3, which increased the duration of the steady light to 10 s and used only low shock intensity, but failed to affect LI in Experiment 4 which used the flashing light on the background of darkness or of light, and only high shock intensity. The effects of amphetamine on LI were not related to its effects on behavioural suppression after footshock, or on activity.