Default-mode network activity is retained in the isolated hemisphere of people after hemispherotomy.

Hemispherotomy is a surgical intervention for severe drug-resistant epilepsy that fully disconnects the lesional brain hemisphere from the rest of the nervous system. This creates a rare opportunity to explore whether consciousness can be sustained without external input. The default-mode network, linked to introspective mental activity, is considered a necessary, though not sufficient, condition for consciousness. In this prospective case-control study, resting-state functional MRI data from 26 individuals post-hemispherotomy and 24 healthy controls were analysed. Default-mode network activity was assessed using seed-to-voxel correlations based on the peak activity in the precuneus of controls, delineated by group independent component analysis. In connected hemispheres, typical default-mode network patterns were preserved and did not differ significantly from controls. Isolated hemispheres also showed preserved positive default-mode network connectivity, though negative connectivity was significantly reduced compared to controls. In two patients with both pre- and post-surgical scans, the individual default-mode network remained detectable but was divided between the isolated and connected hemispheres. These findings suggest that default-mode network activity persists in isolated hemispheres, meeting a necessary condition for consciousness. However, the presence of default-mode network activity alone does not confirm conscious awareness, encouraging further investigation into the neural correlates of consciousness in isolated hemispheres.