The olfactory thalamocortical system and odor reversal learning examined using an asymmetrical lesion paradigm in rats.

The olfactory thalamocortical system was disrupted bilaterally in rats using (a) unilateral mediodorsal thalamic (MD) lesions plus contralateral bulbectomy and transection of the anterior commissure (AC), (b) unilateral MD lesions plus contralateral lesions of the frontal cortex, or (c) bilateral MD lesions. Rats were trained on an odor discrimination task and on the reversal of that task. Experimental groups performed as well as controls on the initial discrimination task but made more errors on the reversal problem. Rats with asymmetrical disruption of the olfactory thalamocortical system performed as poorly as those with bilateral MD lesions. These outcomes indicate that odor reversal learning deficits in rats with bilateral MD lesions stem from interruption of the olfactory thalamic-neocortical system and also provide evidence that the AC mediates significant interhemispheric transfer of olfactory information.