Facilitation of latent inhibition by haloperidol in rats

Latent inhibition (LI) is a behavioral paradigm in which prior exposure to a stimulus not followed by reinforcement retards subsequent conditioning to that stimulus when it is paired with reinforcement. Two experiments investigated the effects of 0.1 mg/kg haloperidol administration on LI as a function of number of CS pre-exposures. The investigation was carried out using a conditioned emotional response (CER) procedure consisting of three stages: pre-exposure, in which the to-be-conditioned stimulus, tone, was repeatedly presented without reinforcement; conditioning, in which the pre-exposed stimulus was paired with shock; and test, where LI was indexed by animals' suppression of licking during tone presentation. The three stages were conducted 24 h apart. In Experiment 1, 40 CS pre-exposures were given. LI was obtained in both the placebo and haloperidol conditions, but the effect was much more pronounced under the drug. Experiment 2 used ten CS pre-exposures. LI was not obtained in the placebo animals but was clearly evident in animals injected with haloperidol. The implications of these findings for the effects of neuroleptics on learning are discussed.