Reduction of latent inhibition by D-amphetamine in a conditioned suppression paradigm in humans.

The sensitivity of latent inhibition (LI) to amphetamine has been tested in humans with a paradigm close to the conditioned emotional response suppression currently used in experimental animals. The conditioned stimulus (CS) was a tone, the unconditioned stimulus (US) a strong white noise, and the response a transient delay in a regular sequence of hand movements in the resolution of the Tower of Toronto puzzle. The aim of this study was to verify whether the previously reported, disruptive effect of CS preexposure on conditioning really represents LI, by examining its sensitivity to amphetamine. Three groups of healthy volunteers received placebo, 5 or 10 mg of dexamphetamine sulphate, respectively, in a double-blind experimental design. The preexposure, conditioning and test phases were carried out under either amphetamine or placebo. The non preexposed groups treated with amphetamine were not different from the non preexposed placebo group, indicating that amphetamine did not affect conditioning. Among the preexposed groups, those receiving 10 mg of amphetamine showed normal rates of conditioning, whereas those treated with either 5 mg of amphetamine or placebo showed LI. Similar results have been reported in experimental animals. This sensitivity to amphetamine suggests that the present paradigm may be used to study LI in humans.