A functional analysis of the olfactory thalamocortical system using an asymmetrical lesion paradigm

The central segment of the mediodorsal nucleus of the thalamus (MD) in the rat receives an olfactory input from the olfactory cortex and, in turn, projects to the lateral orbital and agranular insular cortex. Prior studies found that bilateral lesions of MD produce deficits in olfactory reversal learning tasks, but not on simple odor detection or two-odor discrimination tasks (Slotnick and Kaneko, 1981; Slotnick and Risser, 1990). Because such lesions also destroy non-olfactory components of the thalamus and produce deficits in a variety of non-olfactory tasks, it is unclear whether olfactory deficits stem specifically from disruption of the olfactory component of MD. To investigate this, the olfactory thalamocortical system of the rat was disrupted bilaterally using asymmetrical lesions. Subjects were then tested for odor detection, discrimination and reversal learning. Specifically, experimental rats received one of the following lesion combinations: (1) bilateral lesions of MD (BiMD group); (2) a unilateral bulbectomy and contralateral lesion of MD (RLBulb group); (3) a unilateral bulbectomy and contralateral lesion of MD combined with transection of the anterior commissure (RLBulbAC group); or (4) a unilateral lesion of the cortical projection targets of MD and a contralateral lesion of MD (RLCort group). In general, rats in each group made few errors on the odor detection and discrimination tasks. However, rats in the RLBulbAC, RLCort and BiMD groups made 2 to 3 times more errors than controls on the odor reversal tasks. Rats with similar lesions that spared the olfactory components of the system performed at control levels. In this study the characteristic reversal deficit of rats with bilateral MD lesions was reproduced with asymmetrical lesions whose only commonality was disruption of the olfactory thalamocortical system. This outcome indicates that olfactory deficits obtained from bilateral damage to MD are due to disruption of the olfactory component of this nucleus. Finally, rats in the RLBulb group made more errors than controls on the reversal task, but did not perform as poorly as the RLBulbAC group. These data demonstrate that functional olfactory information can be transferred interhemispherically via the anterior commissure.