Hemispherotomy: cortical islands of deep sleep in awake humans

Abstract

Hemispherotomy is a surgical procedure that disconnects a large portion of the cerebral cortex from cortical and subcortical inputs in patients with severe refractory epilepsy. Whether the disconnected cortex - inaccessible to behavioral assessment - supports consciousness remains unknown. Functional MRI studies have indicated preserved resting-state networks within the disconnected hemisphere, raising the possibility that it may represent an ‘island of awareness’. However, these networks can also persist in unconscious states, such as anesthesia and deep sleep. Here we assess the capacity of the disconnected cortex to support consciousness by exploring its electrophysiological state, before and after hemispherotomy, in ten awake pediatric patients. After surgery, the disconnected cortex–but not the contralateral cortex–entered a state dominated by slow oscillations (<2 Hz) resembling those observed during deep sleep; further, the spectral exponent, a previously validated marker of consciousness indexing the 1/f-like decay of the power spectral density, assumed values typically found in unconscious brain-injured and anesthetized adults. When compared to a reference pediatric sample, spectral exponent values were compatible with wakefulness in the contralateral cortex but attained levels typical of deep sleep over the disconnected cortex, suggesting that the disconnected cortex is not an island of awareness.