Stimulus pre-exposure effect Definition

Latent inhibition (LI) is the reduced efficacy of a previously exposed, inconsequential stimulus to generate a conditioned response when paired with reinforcement, compared with a novel stimulus. LI is extremely robust, appearing across many different learning paradigms and mammalian species, including humans.

While a variety of behavioral tasks are used to demonstrate LI in rodents, all of them share a basic procedure. In the first stage, pre-exposure, animals from each of two groups are placed in an environment that will later serve as the conditioning-test apparatus. Subjects in the "stimulus pre-exposed'' (PE) group are repeatedly exposed to a stimulus (e.g., tone), which is not followed by a significant consequence. Subjects in the "nonpre-exposed" (NPE) group spend an equivalent amount of time in the apparatus without receiving the stimulus. Either immediately or a certain time after the pre-exposure time is completed, all subjects enter the conditioning stage of the procedure, in which the PE stimulus is paired with a reinforcer over a number of trials. Performance is assessed by examining some behavioral index of conditioned responding, either during the conditioning stage or in a third, test stage. LI is manifested in poorer performance of the PE when compared with the NPE group.

In terms of psychological processes underlying LI, it is believed that the pairing of stimulus-no event in the pre-exposure stage results in reduced attention to, or salience of, the stimulus, which subsequently interferes with the generation of the conditioned response resulting from the stimulus-reinforcement association in conditioning (Fig. 1).

LI is a phenomenon of ► selective attention in the sense that it reflects a modulating effect of past experience on the current performance. Specifically, it reflects the ability of organisms to ignore stimuli that had been irrelevant in the past, in spite of their current relationship with a reinforcer. Since selective attention deficit is a hallmark cognitive deficit of ► schizophrenia and a central target for treatment, research that examined the effects of psychoactive drugs on LI in rodents has focused primarily on the use of LI to develop animal models of deficient attention in schizophrenia and the identification of ► anti-psychotic activity. The link between LI and schizophrenia is supported by the presence of LI abnormalities in schizophrenia patients.

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