Inhibitory deficits in rats rendered “mentally retarded” by early brain damage

Abstract

Young rats with lesions to the parietal cortex, globus pallidus, or median raphe have previously been reported to be deficient in learning a wide variety of laboratory tasks. The purpose of the current study was to determine whether weanling rats with similarly placed lesions (or with hippocampal or amygdaloid lesions) would subsequently manifest a global deficit in behavioral inhibition (as evidenced by abnormalities on activity, extinction, spontaneous alternation, go/no-go discrimination, passive avoidance, spatial reversal, and visual reversal tests), and whether the degree of this inhibitory deficit would correlate with an independent assessment of learning ability (as gauged by performance on three distinct detour problems). The parietal and median raphe groups showed losses on at least six of the seven tests of inhibition, whereas the remaining groups showed losses on no more than three tests. However, the correlations between overall inhibition scores and learning scores did not exceed.43. Based on a key cluster analysis, two factors were extracted from the data on the inhibition tests. Inhibition related to Factor I (which was strongly associated with learning ability) appeared to underlie the suppression of an initially preferred response in the face of nonreinforcement or punishment. Inhibition related to Factor II appeared to underlie the suppression of an approach response to the negative cue in approach-avoidance and go/no-go discrimination situations. These results are discussed in relation to certain theories of intelligence and mental retardation.