Differential effect of thalamic and cortical lesions on memory systems in the rat

The behavioral effects of lesions of anterior thalamic and medio-dorsal thalamic nuclei, posterior cingulate (retrosplenial) and posterior parietal associative cortex were studied in rats performing a eight-arm radial maze, in which three/eight arms were baited. Lesions were made after rats reached a training criterion. Rats were tested 2 weeks later, in the same experimental apparatus. In the first experiment, performance was assessed by number of errors and time per trial, retention by comparing performance during training with retraining sessions (using specific scores for reference and working memory) and by an evaluation of the 'reminiscence' defined as daily improvement in performance. After the reacquisition of the task, the rats were tested in experiment II, in modified situations for an evaluation of flexible aspects of memory processing in this spatial task. Results showed dissociation between the effects of the different lesions, according to the specific demands of the different tasks. Posterior parietal lesions produced significant, but relatively mild deficits, in all situations, in accordance with the well-established function of this cortical region in spatial tasks. In contrast, after cortical lesion in the retrosplenial region, performance deficit was only observed on priming and attention to contextual change. Lesions of the thalamic associative nuclei also induced task-specific deficits. Medio-dorsal lesions induced mild and reversible deficits in complex tasks only, with preservation of working memory, of priming effect, and of a novel acquisition. Rats with anterior thalamic lesions had massive deficits across tasks, probably due to basic difficulties with reference and working memory, demonstrated no benefit from a priming session but considerable interference from the previous training to a new one. These results are discussed within framework of the specificity of different cerebral regions for behavioural adaptation and plasticity.