Dissociable effects of anterior and mediodorsal thalamic lesions on spatial goal-directed behavior.

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Goal-directed behaviors are thought to be supported by a neural circuit encompassing the prefrontal cortex, the dorsomedial striatum, the amygdala, and, as more recently suggested, the limbic thalamus. Since evidence indicates that the various thalamic nuclei contribute to dissociable functions, we directly compared the functional contribution of the mediodorsal thalamus (MD) and of the anterior thalamic nuclei (ATN) in a new task assessing spatial goal-directed behavior in a cross-maze. Rats sustaining lesions of the mediodorsal or the anterior thalamus were trained to associate each of the two goal arms with a distinctive food reward. Unlike control rats, both lesioned groups failed to express a bias for the goal arm corresponding to the non-devalued outcome following devaluation by sensory-specific satiety. In addition, MD rats were slower than the other groups to complete the trials. When tested for spatial working memory using a standard non-matching-to-place procedure in the same apparatus, ATN rats were severely impaired but MD rats performed as well as controls, even when spatial or temporal challenges were introduced. Finally, all groups displayed comparable breaking points in a progressive ratio test, indicating that the slower choice performance of MD rats did not result from motivational factors. Thus, a spatial task requiring the integration of instrumental and Pavlovian contingencies reveals a fundamental deficit of MD rats in adapting their choice according to goal value. By contrast, the deficit associated with anterior thalamic lesions appears to simply reflect the inability to process spatial information.