Comprehensive mapping of the thalamic commissures in the rodent brain

Brain commissures are antiparallel white matter fiber bundles that connect both hemispheres across the midsagittal plane at different anatomical levels. In the mammalian brain, the best-known commissures are the corpus callosum, the anterior and the posterior commissures. Recent studies have identified a new white matter pathway, the thalamic commissures (TCs), that connect the cortex to the contralateral thalamus in rodents and primates. However, the specific cortical regions that project via the TCs and their target thalamic nuclei are still unknown. To thoroughly and accurately map this projectome, we utilized the mouse Allen Brain Institute connectivity atlas. Our results indicate that not all cortical regions project via the TCs, with most of these regions concentrated in the rostral portion of the brain. In addition, our data suggest that the TCs projections are very numerous, although less robust than the ipsilateral counterpart. Furthermore, these projections had a high degree of bilateral symmetry. We did not find any projections from the thalamus to the contralateral cortex, suggesting that the TCs are not reciprocal. Our findings reinforce the concept of the thalamus as an interhemispheric connectivity hub and suggest that the TCs might play a role in interhemispheric functional modulation.