Summary: A new study reports that sleep-like slow-wave electrical activity can persist for years in surgically disconnected brain tissue of awake epilepsy patients. Using scalp EEG recordings, researchers observed that the isolated hemisphere displays patterns similar to deep non-REM sleep, anesthesia, or vegetative states—patterns that are typically associated with absent or greatly reduced awareness.
These enduring slow waves, detected long after hemispherotomy, raise fundamental questions about whether disconnected cortical tissue can support any form of functional processing or subjective experience. The results both underscore the robustness of cortical dynamics and highlight the conceptual challenges of assessing consciousness when behavioral access is impossible.
Key Facts:
- Persistent slow waves: Scalp EEG showed long-lasting, sleep-like slow oscillations in the surgically disconnected hemisphere for months to years after operation.
- Patterns linked to reduced awareness: The EEG signatures resembled those seen in deep NREM sleep, general anesthesia, and the vegetative state, consistent with absent or markedly reduced conscious processing.
- Philosophical and clinical implications: The findings reopen debate about whether isolated brain tissue can sustain any subjective awareness and inform how clinicians interpret post-surgical brain activity.
Source: PLOS
Sleep-like slow-wave patterns persist for years in surgically disconnected neural tissue of awake epilepsy patients, according to a study published October 16th in the open-access journal PLOS Biology by Marcello Massimini (Università degli Studi di Milano) and colleagues.
While the appearance of slow waves over the isolated cortex suggests impaired consciousness, it remains unclear whether those oscillations have any residual functional or plastic role in the disconnected tissue.
Hemispherotomy is a neurosurgical treatment for severe, drug-resistant epilepsy, most often performed in children. The procedure aims to disconnect the malfunctioning hemisphere from the rest of the brain to stop seizures from spreading. Importantly, the disconnected cortex is left in place and remains vascularized, but it is cut off from sensory inputs and motor outputs, making behavioral assessment impossible.
Because the isolated cortex cannot be probed through behavior, questions persist about the internal states it may sustain. More generally, little is known about the range of spontaneous activity patterns that a large deafferented cortical region can maintain while the person is awake.
To investigate, Massimini and colleagues analyzed non-epileptic background activity recorded with scalp electroencephalography (EEG) in 10 pediatric patients, both before and up to three years after hemispherotomy. The team focused specifically on spontaneous electrical dynamics in the disconnected hemisphere while patients were awake.
After surgery, the disconnected cortex consistently exhibited prominent slow oscillations (<2 Hz) and a shift of EEG power toward lower frequencies, reflected in a steeper broadband spectral decay. This broadband slowing produced a substantial reduction in the spectral exponent, a quantitative EEG marker linked in previous work to levels of conscious processing.
When compared with a pediatric reference dataset spanning wakefulness and sleep, the spectral properties of the intact (contralateral) cortex matched wakeful activity, while the disconnected hemisphere’s spectral exponent aligned with deep NREM sleep and states associated with reduced awareness. These similarities suggest the isolated tissue is much more likely to be in a sleep-like, low-awareness electrophysiological regime than in an active, wake-like state.
The persistence of unihemispheric slow-wave activity for months or years after complete disconnection provides unique insight into the long-term electrophysiological consequences of cortical disconnection in humans. At the same time, the authors caution that inferring consciousness purely from EEG features—however informative—remains uncertain, especially for brain structures that cannot be behaviorally evaluated.
The study authors recommend that scalp-level slowing be complemented, when clinically indicated, with intracranial recordings to better characterize the electrophysiological state of isolated cortex in cases that require invasive postoperative monitoring.
Michele A. Colombo commented that this research opens only the first chapter in understanding consciousness in systems that are inaccessible to behavioral testing, emphasizing the conceptual complexity encountered during peer review. Marcello Massimini noted that these sleep-like patterns may help explain why slow-wave activity appears in patients with brain lesions and how it relates to their awareness. Anil K. Seth described the project as a rewarding scientific journey that grew out of philosophical questions about “islands of awareness” and matured into an important collaborative investigation. Tim Bayne added that exploring such puzzling cases is valuable for advancing consciousness science.
Key Questions Answered:
A: They observed that hemispheres surgically disconnected by hemispherotomy can display persistent, sleep-like slow-wave activity measurable with scalp EEG, lasting months to years after surgery.
A: The EEG patterns are characteristic of low-awareness states such as deep NREM sleep and anesthesia, and the evidence does not support the presence of normal wakeful awareness in the isolated cortex; however, the question cannot be closed definitively without additional invasive data.
A: It advances understanding of post-hemispherotomy brain dynamics, informs clinical interpretation of postoperative EEG, and provokes deeper inquiry into how electrophysiological signatures relate to subjective awareness when behavior is unavailable.
About this consciousness research news
Author: Claire Turner
Source: PLOS
Contact: Claire Turner – PLOS
Image credit: Neuroscience News
Original Research: Open access. “Hemispherotomy leads to persistent sleep-like slow waves in the isolated cortex of awake humans” by Marcello Massimini et al., PLOS Biology. DOI: 10.1371/journal.pbio.3003060
Abstract
Hemispherotomy leads to persistent sleep-like slow waves in the isolated cortex of awake humans
Hemispherotomy disconnects much of one cerebral hemisphere from its cortical and subcortical inputs and outputs as a treatment for refractory epilepsy. While this intervention reduces seizure spread, it leaves the disconnected cortex structurally intact and perfused but behaviorally inaccessible, prompting questions about the internal states that such tissue can maintain.
This study examined, for the first time, the electroencephalographic state of the isolated cortex during wakefulness in 10 pediatric patients, analyzing non-epileptic background EEG before and after surgery. After hemispherotomy, the isolated hemisphere showed clear slow oscillations (<2 Hz) and a redistribution of spectral power toward lower frequencies, producing a steeper broadband spectral decay and a reduced spectral exponent—values that overlap with those observed in deep anesthesia and the vegetative state.
Compared with a pediatric reference across the sleep–wake cycle, the intact hemisphere displayed spectral features consistent with wakefulness, whereas the isolated hemisphere’s metrics were consistent with deep NREM sleep. These electrophysiological patterns support the provisional inference of absent or reduced awareness in the disconnected cortex and reveal persistent unihemispheric sleep-like dynamics that can endure for years after surgical disconnection.