The effects of selective lesions within the anterior thalamic nuclei on spatial memory in the rat

Groups of rats received cytotoxic lesions centred in either the anterior medial thalamic nucleus (AM), the anterior ventral and anterior dorsal thalamic nuclei (AV/AD), or all three nuclei combined (ANT.T). These lesions were made by injecting N-methyl-d-aspartate acid (NMDA). These rats, and a group of surgical controls (SHAM), were trained on a rewarded forced-alternation task in a T-maze. While the selective AM and AV/AD lesions prduced an initial acquisition impairment, only the animals with combined lesions (ANT.T) showed a persistent deficit throughout the 16 acquisition sessions. Subsequent testing with a cross-maze confirmed that the SHAM, AV/AD, and AM groups were able to use allocentric cues, while the ANT.T group were impaired. In contrast none of the three anterior thalamic groups were impaired on a subsequent egocentric discrimination and reversal task run in the same apparatus. A final test using the eight arm radial-maze, revealed marked deficits in the ANT.T group as well as milder deficits in the AV/AD group. The results from these experiments help to confirm the importance of the anterior thalamic nuclei for allocentric tasks, but suggest that no region is pre-eminently important. The findings also help to account for other studies which have reported that anterior thalamic lesions have seemingly mild effects on tests of spatial memory.