Medial Dorsal Thalamic Lesions and Working Memory in the Rat

Pigmented rats of the DA strain with either radio frequency or ibotenic acid lesions of the thalamic nucleus medialis dorsalis were postoperatively given non-spatial and spatial tests of working memory. In the non-spatial task, delayed non-matching to sample, rats with both types of thalamic lesions showed acquisition impairmnts. The subgroup of rats with nucleus medialis dorsalis lesions that were able to reach the acquisition criterion did, however, perform normally when the retention interval was extended to 60 seconds and when proactive interference was increased. A final, control variant of this task was included to address the possibility of a sensory deficit. In the spatial task, delayed forced alternation, rats were tested with differing retention intervals and with both spaced and massed trials. Damage to nucleus medialis dorsalis had no effect on acquisition or on spaced trials, but a slight deficit was found in the animals with radio frequency lesions under the massed trial condition. Much clearer deficits were, however, present in those animals in which the lesion extended appreciably into the anterior thalamic nuclei. The findings indicate that while cellular damage to nucleus medialis dorsalis may disrupt learning, some impairments in tests of spatial working memory attributed to this nucleus may reflect damage to the adjacent anterior thalamic nuclei