EFFECTS OF METHYLAZOXYMETHANOL-INDUCED MICRENCEPHALY ON TEMPORAL RESPONSE DIFFERENTIATION AND PROGRESSIVE RATIO RESPONDING IN RATS

Micrencephalic Sprague-Dawley rats were produced by an injection of 20 mg/kg methylazoxymethanol acetate on gestational Day 14. Brain weight s of the offspring were 70% of controls while weights of frontal corte x and hippocampus were approximately 58% (Ferguson, Racey, Paule, and Holson, 1993). Operant performance was measured with particular emphas is on assessment of time estimation. The temporal response differentia tion (TRD) and the progressive ratio (PR) tasks, previously used in th e NCTR operant test battery for monkeys, were chosen for evaluation. T he TRD schedule is notably different from other temporal tasks in that it requires subjects to initiate and maintain a lever press for 10-14 s. The PR task was included as a measure of motivation to work for fo od reinforcers. Micrencephalics acquired and performed both tasks comp arably to controls. During extinction, however, micrencephalics exhibi ted an increased TRD lever hold duration. This suggests an atypical re sponse perseveration, that is, perseverating the previously correct re sponse. Previously, frontal cortical alterations were suggested to con tribute heavily to micrencephalic-induced behavioral alterations (Ferg uson et al., 1993). This study provides further evidence that response perseveration, a hallmark of frontal cortical lesions, is expressed i n micrencephalic rats. (C) 1994 Academic Press, Inc.