NEUROTOXIC LESIONS OF THE DORSOMEDIAL THALAMUS IMPAIR THE ACQUISITIONBUT NOT THE PERFORMANCE OF DELAYED MATCHING TO PLACE BY RATS - A DEFICIT IN SHIFTING RESPONSE RULES

This study examined the acquisition of a T-maze matching to place task by rats with neurotoxic lesions of the thalamic nucleus medialis dors alis. This test of spatial working memory also entails learning a task rule that is contrary to the animals' innate preference. The rats nex t performed the same matching task over different retention delays. Fi nally, they were trained on a reversal of the task rule, i.e., to nonm atch to place. Although the lesions produced a clear acquisition impai rment on the matching task, there was no evidence of a loss of working memory. A series of control tasks found no appreciable effect on a co nditioned cue preference task or on open field activity. The pattern o f results shows that medialis dorsalis lesions lead to a selective inc rease in perseverative behavior that can retard task acquisition. This perseverative deficit closely resembles that observed after prefronta l damage in rats, strongly indicating dysfunction in a common system.