Odor memory and odor learning in rats with lesions of the lateral olfactory tract and mediodorsal thalamic nucleus.

Rats with posterior transection of the lateral olfactory tract (LOT), lesions of the mediodorsal thalamic nucleus (MD), or with combined lesions of these structures were tested for retention of a preoperatively learned multiple odor discrimination task, acquisition of 3 simple 2-odor discrimination problems, and reversal of the last problem of the series. Compared to controls, rats with transection of the LOT had no deficits in any task. Those with MD lesions had no deficits in retention but moderate deficits in acquisition of novel odor discriminations and a severe deficit in reversal learning. Rats with combined LOT and MD lesions had no retention of the preoperatively learned task but their performance on the remaining problems was similar to that of rats with only MD lesions. These results suggest that neither the olfactory thalamocortical projections nor those to the periamygdaloid or entorhinal cortex are essential for storage and/or recall of an olfactory procedural memory task. Performance on the postoperative acquisition tests indicate that the thalamocortical but not the limbic olfactory projections play an important role in olfactory discrimination learning.