The effects of primary olfactory cortical and thalamic lesions on olfactory continuous non-matching to sample and discrimination in the rat

Forty-two male rats were pretrained on the olfactory continuous delayed non-matching to sample (CDNMTS) task. They were then matched for performance and randomly assigned to one of the following six treatment groups: excitotoxic lesions of pyriform cortex, later entorhinal cortex, lateral internal medullary lamina (L-IML), mediodorsal nucleus (MDn), nonspecific nuclei, and sham control. After recovery from surgery, the rats were retrained on the olfactory CDNMTS. Delay effects were examined by manipulating retention intervals (RI), with five RIs (4, 6, 9, 13.5, and 20.25 s) randomly mixed within each session. The number of odor stimuli used in each session varied from 8 to 2. All the rats were then trained on olfactory discrimination task, a task with similar procedural requirements as olfactory CDNMTS except that the stimulus-response contingency was fixed. Only L-IML and pyriform groups were impaired on the olfactory CDNMTS. Increasing RIs or reducing number of odor stimuli used in each session significantly decreased performance accuracy for all the rats. There was no differential effects. All subjects learned the two-odor discrimination task at equivalent rates. The normal performance of discrimination task, indicated that the olfactory CDNMTS deficits could not be attributed to olfactory sensory dysfunction, to difficulties in learning the procedural aspects of the task, or to an inability to suppress responding. The present findings demonstrate that excitotoxic lesions of L-IML disrupt olfactory working memory but not reference memory, which is consistent with previous studies of radiofrequency lesions of L-IML. Pyriform cortex, a major primary olfactory cortex, apparently is not necessary for olfactory discrimination, although it is critical for olfactory CDNMTS. Taken together, it suggests that either pyriform cortex alone is critical for olfactory memory or there exist parallel pathways for olfactory memory processing