Structural and functional brain rewiring clarifies preserved interhemispheric transfer in humans born without the corpus callosum

Significance Individuals subjected to surgical transection of the corpus callosum (“split-brains”) fail to transfer information between the cerebral hemispheres, a condition known as “disconnection syndrome.” On the other hand, subjects born without the callosum (callosal dysgenesis, CD) typically show preserved interhemispheric communication. To clarify this paradox, which has defied neuroscientists for decades, we investigated CD subjects using functional and structural neuroimaging and neuropsychological tests. Results demonstrated the existence of anomalous interhemispheric tracts that cross through the midbrain and ventral forebrain, linking the parietal cortices bilaterally. These findings provide an explanation for the preserved cross-transfer of tactile information between hemispheres in CD. We suggest that this condition is associated with extensive brain rewiring, generating a new circuitry that provides functional compensatory interhemispheric integration.