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Summary: A new study reveals that sleep-like slow-wave brain activity can persist for years in surgically disconnected brain hemispheres of awake epilepsy patients. Using EEG recordings, the researchers found that the isolated cortex exhibits patterns similar to deep sleep, anesthesia, or vegetative states, suggesting absent or reduced consciousness.
These waves, observed years after surgery, raise fundamental questions about whether such neural tissue can maintain any functional or conscious activity. The findings highlight both the resilience of cortical dynamics and the philosophical challenge of defining consciousness in disconnected brain systems.
Key facts:
Persistent slow waves: EEG recordings showed long-lasting sleep-like slow wave patterns in disconnected hemispheres years after surgery. Reduced consciousness: The patterns resembled brain activity seen during deep sleep or anesthesia, suggesting limited or absent consciousness. Philosophical implications: The results reignite the debate over whether isolated brain tissue can sustain some form of subjective consciousness.
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Sleep-like slow wave patterns persist for years in surgically disconnected neural tissue from awake epilepsy patients, according to a study published Oct. 16 in the open-access journal PLOS Biology by Marcello Massimini of the Universita degli Studi di Milano, Italy, and colleagues.
The presence of slow waves in the isolated hemisphere impairs consciousness; However, it is still unclear whether they serve any functional or plastic function.
Hemispherotomy is a surgical procedure used to treat severe cases of epilepsy in children. The goal of this procedure is to achieve maximum disconnection of diseased neural tissue, potentially spanning an entire hemisphere, from the rest of the brain to prevent the spread of seizures.
The disconnected cortex (the brain’s outer layer of neural tissue) is not surgically removed and retains its vascular supply. Because it is isolated from sensory and motor pathways, it cannot be evaluated behaviorally, leaving open the question of whether it retains internal states consistent with some form of consciousness.
More generally, the patterns of activity that large portions of the disconnected cortex can maintain in awake humans remain poorly understood.
To address these questions, Massimini and colleagues used electroencephalography (EEG) to measure activity in the isolated cortex during wakefulness before and up to three years after surgery in 10 pediatric patients, focusing on nonepileptic background activity.
After surgery, prominent slow waves appeared over the disconnected cortex. This is novel evidence that this pattern can last for months and years after complete cortical disconnection. The persistence of slow waves raises the question of whether they play some functional role or simply reflect a regression to a default mode of cortical activity.
The pronounced broadband EEG slowing resembled patterns seen in conditions such as deep non-rapid eye movement (NREM) sleep, general anesthesia, and the vegetative state. The findings indicate an absent or reduced probability of dream-like experiences in the isolated cortex. In general, EEG evidence is compatible with a state of absent or reduced consciousness.
According to the authors, any inference about the presence or absence of consciousness, based solely on physical properties of the brain, such as prominent EEG slow waves, should be approached with caution, particularly in neural structures that are not behaviorally accessible. Slowing observed at the scalp level should be further characterized with intracranial recordings in cases where clinical results require postoperative invasive monitoring.
Michele A. Colombo says: “This is just the beginning to shed light on the problem of consciousness in inaccessible systems. During the review process, we were faced with different perspectives, which reveals the complexity of this problem.”
Marcello Massimini adds: “This pattern may provide clues as to why sleep-like brain activity arises in patients with brain injuries and how it relates to their level of consciousness.”
Anil K. Seth adds: “This has been an exciting and deeply satisfying scientific journey. It began years ago with philosophical debates about the possibility of ‘islands of consciousness’ in completely isolated neural systems, to now arriving at this wonderful collaboration that has shed important experimental light on this clinically important topic.”
Tim Bayne finally states: “The study of consciousness involves many perplexing cases in which it is not clear what to say about the possibility of subjective experience. As a philosopher, it has been deeply rewarding to explore a new frontier in the science of consciousness with this wonderful team of scientists and doctors.”
Key questions answered:
A: They found that surgically isolated brain hemispheres can show persistent, sleep-like slow wave activity for years after epilepsy surgery.
A: There is no evidence to support awareness; EEG patterns resemble unconscious states such as sleep or anesthesia, but the question remains open.
A: It deepens our understanding of brain dynamics after hemispherotomy and challenges scientists to reconsider what brain activity means for consciousness.
About this consciousness research news.
Author: Claire Turner
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Contact: Claire Turner – PLOS
Image: Image is credited to 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. More biology
Abstract
Hemispherotomy produces persistent sleep-like slow waves in isolated cortex of awake humans
Hemispherotomy is a neurosurgical procedure to treat refractory epilepsy, which involves disconnecting a significant portion of the cortex, potentially spanning an entire hemisphere, from its cortical and subcortical connections. While this intervention prevents the spread of seizures, it raises important questions.
Given the complete isolation of sensorimotor pathways, it is unclear whether the disconnected cortex retains some form of inaccessible consciousness. More generally, the patterns of activity that large portions of the deafferent cortex can sustain in awake humans remain poorly understood.
We addressed these questions by exploring for the first time the electroencephalographic (EEG) status of the isolated cortex during wakefulness before and after surgery in 10 pediatric patients, focusing on nonepileptic background activity.
After surgery, the isolated cortex exhibited prominent slow oscillations (<2 Hz) and a more pronounced broadband spectral decay, reflecting a redistribution of power toward lower frequencies.
This broadband EEG deceleration resulted in a marked decrease in the spectral exponent, a validated marker of consciousness, reaching values characteristic of deep anesthesia and vegetative state.
When compared to a pediatric reference sample across the sleep-wake cycle, the spectral exponent of the contralateral cortex aligned with wakefulness, whereas that of the isolated cortex was consistent with deep NREM sleep.
The findings of prominent slow oscillations and broadband slowing tentatively support inferences of absent or reduced consciousness in isolated cortex. Furthermore, the persistence of unihemispheric sleep-like patterns years after surgery provides unique insights into the long-term electrophysiological effects of cortical disconnections in the human brain.
