Splitting the brain: Complete corpus callosotomy in adulthood profoundly disrupts the functional architecture of interhemispheric brain networks
Tyler Santander1 (email@example.com), Jessica Simonson1, Selin Bekir1, Henri Skinner1, Theresa Paul2, Lena Hopf3, Anna Rada3, Friedrich Woermann3, Christian Bien3, Barry Giesbrecht1, Olaf Sporns4, Michael Gazzaniga1, Lukas Volz2, Michael Miller1; 1University of California, Santa Barbara, 2University of Cologne, 3Bielefeld University, 4Indiana University
Corpus callosotomy is a major intracranial intervention that severs the largest fiber tract in the human brain as a last-resort treatment for medically-intractable epilepsy. A rich history of experimental psychology has revealed striking insights into the ‘split-brain’ phenomenon, describing two cerebral hemispheres operating independently, without conscious awareness of the other. However, we know comparatively little about the impact of callosotomy on the human brain’s functional architecture: extant literature is limited to pediatric patients and/or single case studies. Here we investigate, for the first time, multiple adult callosotomy patients using modern network neuroscience methods. Five patients (2 partial splits, 3 full splits) underwent resting-state fMRI at least one-year post-op; large-scale patterns of intrinsic functional connectivity (FC) were assessed using seed-based and parcellation-based techniques, including edge timeseries and multiresolution modularity. For comparison, healthy adult controls were taken from the HCP 100 Unrelated Subjects sample. We find that full callosotomy dramatically diminishes interhemispheric FC, whereas partial splits retain relatively-normal levels of FC—even when only 1 cm of splenium is spared. Similarly, multivariate patterns of edge co-fluctuations suggest the left and right hemispheres are broadly-decoupled following full callosotomy, and the topological organization of functional modules becomes strongly lateralized. Curiously, the visual network demonstrates nominal interhemispheric FC when resting-state is collected with eyes open—but not with eyes closed, perhaps indicating that synchronized external sensory inputs can yield the appearance of interhemispheric coupling in the absence of anatomical pathways. Together, these results provide a novel perspective on the split-brain and a functional basis for ‘disconnected’ cognition.
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April 13–16 | 2024