AUDITORY CONDITIONAL DISCRIMINATION DEFICITS WITHOUT DELAYS IN RATS WITH LESIONS OF EITHER FRONTAL-CORTEX OR MEDIAL THALAMUS

An auditory match-to-position (AMTP) task was developed to compare the effects of medial thalamic (L-IML) and prefrontal cortical (MW) lesio ns on the ability of rats to perform conditional discriminations with and without memory delays. Both lesions affected AMTP when there was n o delay, and these impairments were not exacerbated when memory delays were imposed. The MW group performed normally, whereas the L-IML grou p was impaired in learning a go/no-go discrimination based on the same auditory stimuli as those used for AMTP. These findings are consisten t with other evidence that the medial prefrontal cortex is critical fo r conditional associative learning when the same response alternatives are presented repeatedly across trials. The impairments observed for AMTP when there was no memory delay suggest that the effects of these lesions on the rate of forgetting may have been masked by the effects of these lesions on nonmnemonic aspects of the AMTP task.