ASSESSING THE MAGNITUDE OF THE ALLOCENTRIC SPATIAL DEFICIT ASSOCIATEDWITH COMPLETE LOSS OF THE ANTERIOR THALAMIC NUCLEI IN RATS

The behavioural effects of complete lesions of the anterior thalamic n uclei (ANT), the anterior thalamic nuclei plus the lateral dorsal nucl eus (ANT + LD), and fornix (FX) were compared using a series of tests of spatial memory. All three lesion groups were found to have an equal ly severe and long-lasting impairment in the acquisition of a T-maze a lternation task when compared with the control animals (COMB SHAM). In Experiment 2, the control animals were able to perform the alternatio n task when the test trial was started from a different location to th e sample trial, so demonstrating that they were able to use allocentri c cues in order to differentiate the most recently visited arm. In con trast, all the lesion groups performed close to chance level. In fact, for this condition the ANT + LD group was significantly worse than th e FX group. In contrast, none of the lesion groups was impaired on an egocentric discrimination and subsequent reversal task (Experiment 3). The control animals came from two different control procedures, a sur gical control sub-group (SHAM) and a group of animals that received in jections of N-methyl-D-aspartic (NMDA) into the fornix (NMDA SHAM). Th ere were no differences in the performance levels of the NMDA SHAM gro up compared with the surgical control group in any of the experiments conducted, so showing that the anterior thalamic lesion effects were n ot due to non-specific damage to the fornix by NMDA. This series of ex periments demonstrated that complete lesions of the anterior thalamic region impair the ability to process allocentric information, and prov ide evidence for a contribution from the lateral dorsal thalamic nucle us. (C) 1997 Elsevier Science B.V.