Thalamocortical convergence in the secondary motor cortex supports adaptive decision-making

The secondary motor cortex (M2) of rodents appears as a highly integrative brain region, with functions beyond motor processing. It indeed features prefrontal-like thalamocortical connectivity, with prominent innervation from thalamic nuclei involved in adaptive decision-making, namely the mediodorsal thalamic nucleus (MD) and the ventral anterior/ventral lateral thalamic nuclei (VA/VL). To study the functional connectivity between the M2 and the thalamus, we performed tracing experiments and behavioral assays in rats. Through a classic retrograde tracing approach, we identified convergent projections from the MD and the VAVL in the anterior portion of M2 (see figure, A). We next examined the behavioral impact of post-training M2 and VAVL lesions in a three-armed instrumental bandit task. This task aims to examine decision-making in uncertain environments, where a trade-off between exploitation (securing a gain) and exploration (favoring the acquisition of new knowledge) is necessary. M2 lesions were found to cause a reduction in performance (see figure, B) reminiscent of that produced by MD lesion. VA/VL lesions on the other hand had no effect on this task but in a separate task we instead found an impairment in the ability to track action-outcome contingency (see figure, C). Collectively these data thus suggest complementary roles for distinct thalamic nuclei which may be integrated at the level of M2. Overall, our findings suggest a role for M2 and VA/VL in adapting to changes in action-outcome contingencies, possibly in relation to a specific thalamocortical connectivity which will be assessed as a perspective of the present work.