The latent inhibition model of schizophrenic attention disorder. Haloperidol and sulpiride enhance rats' ability to ignore irrelevant stimuli.

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. The development of LI reflects a process of learning to ignore, or tune out, irrelevant stimuli. Two experiments investigated the effects of haloperidol (0.02, 0.1, and 0.5 mg/kg) and sulpiride (100 mg/kg) administration on LI. The investigation was carried out using a conditioned emotional response (CER) procedure consisting of three stages: (1) preexposure, in which the to-be-conditioned stimulus, tone, was repeatedly presented without reinforcement; (2) conditioning, in which the preexposed stimulus was paired with shock; and (3) test, where LI was indexed by animals' suppression of licking during tone presentation. The three stages were conducted 24 hr apart. In the preexposure stage, only ten nonreinforced stimulus preexposures were given, a procedure known to be insufficient to yield LI in normal animals. In both experiments, LI was indeed absent in the placebo animals. In marked contrast, animals treated with haloperidol (experiment 1) as well as with sulpiride (experiment 2) exhibited LI. These results demonstrate that both typical and atypical neuroleptics enhance animals' capacity to ignore irrelevant stimuli. The implications of this finding for an animal model of schizophrenia and for a novel screening test for antipsychotic drugs are discussed.