Odour–place paired-associate learning and limbic thalamus: Comparison of anterior, lateral and medial thalamic lesions

Several subregions in the limbic thalamus have been suggested as the key locus for diencephalic amnesia, including the anterior thalamic nuclei, intralaminar nuclei and mediodorsal nuclei. There is, however, no consensus as to a single critical site and recent research has suggested instead that different thalamic areas may contribute to diencephalic amnesia in subtly different ways. This study compared the effects of lesions to anterior (AT), lateral (LT) and posteromedial (MT) aggregates of thalamic nuclei on Gilbert and Kesner's [Gilbert, PE, Kesner, RP. Role of the rodent hippocampus in paired-associate learning involving associations between a stimulus and a spatial location. Behav Neurosci 2002;116(1):63–71; Gilbert, PE, Kesner, RP. Localization of function within the dorsal hippocampus: the role of the CA3 subregion in paired-associate learning. Behav Neurosci 2003;117(6):1385–94] paired-associate task, in which rats were postoperatively trained to form an arbitrary association between odours and spatial locations in a circular open field. Both AT and LT lesions, but not MT lesions, severely impaired odour–place paired-associate learning. Probe trials revealed that the rats were not using specific location information after acquisition training. All groups were able to learn non-associative odour and place discrimination tasks quickly, with only the AT group showing delayed acquisition. This study provides the first direct comparison of different thalamic lesions on paired-associate learning and new evidence on the importance of the LT region in learning and memory. The results support the notion that injury to both the AT and LT subregions of the thalamus may each be major contributors to diencephalic amnesia. There is need for traditional models of memory function to take greater account of the contributions of thalamic nuclei.