Answers to important questions:
Question: What have researchers discovered about the severed brain hemisphere?
A: They discovered that brain hemispheres that have been surgically isolated can exhibit persistent, sleep-like, slow-wave activity for years after epilepsy surgery.
Q: Does this mean that the severed cerebral hemisphere is conscious?
A: There is no evidence to support this. EEG patterns are similar to unconscious states such as sleep or anesthesia, but the question remains unanswered.
Question: Why is this study important?
A: This deepens our understanding of brain dynamics after hemispherectomy and challenges scientists to rethink what brain activity means for consciousness.
Summary: A new study shows that the slow-motion brain activity seen during sleep can persist for years in surgically severed cerebral hemispheres of awake epilepsy patients. Using EEG recordings, researchers found that the isolated cortex exhibits patterns similar to deep sleep, anesthesia, or a vegetative state, indicating a lack or reduction of awareness.
These waves, observed years after surgery, raise fundamental questions about whether this neural tissue can still sustain any functional or conscious activity. The findings shed light on both the resilience of cortical dynamics and the philosophical challenge of explaining consciousness in disconnected brain systems.
Key data:
- Persistent slow waves: EEG recordings showed that years after surgery, sleep-like slow wave patterns persisted in the severed brain hemisphere.
- Reduced level of consciousness: Patterns resemble brain activity observed during deep sleep or anesthesia, suggesting limited or absent consciousness.
- Philosophical implications: The results reopen the debate over whether isolated brain tissue can support any kind of subjective consciousness.
Source: PLOS
In an article published in PLOS Biology by Universita degli Studi di Milano, Italy, and colleagues , it was observed that sleep-like slow-wave patterns persist for years in surgically severed nerve tissue from awake epilepsy patients.
The presence of slow waves in an isolated hemisphere alters consciousness. However, it is still unclear whether they play a functional or plastic role.
Hemispherectomy is a surgical procedure used to treat severe cases of epilepsy in children. The goal of this procedure is to separate the affected nerve tissue, possibly an entire hemisphere, from the rest of the brain to prevent seizures from spreading.
The isolated cerebral cortex—the outer layer of neural tissue in the brain—is not surgically removed and its blood supply is intact. Because it is isolated from sensory and motor pathways, it cannot be assessed behaviorally, leaving open the question of whether there are internal states consistent with any form of consciousness.
More generally, the patterns of activity that can keep large parts of the cortex unconnected in awake humans are not understood.
To answer these questions, Mesmini and colleagues used electroencephalography (EEG) to measure activity in the insular cortex in 10 pediatric patients before surgery and during wakefulness for three years, focusing on non-epileptic background activity.
After surgery, significant slow waves appeared over the severed cortex. This provides further evidence that this pattern can persist for months or years after complete cortical resection. The persistence of these slow waves raises the question of whether they represent a function or simply reflect a reversal of cortical activity toward a standard mode.
The marked broadband slowing of the EEG resembles patterns observed in deep non-rapid eye movement (NREM) sleep, general anesthesia, and the vegetative state. The results suggest an absence or reduced likelihood of dream experiences in the insular cortex. Overall, the EEG evidence is consistent with an absent or reduced state of consciousness.
According to the authors, any conclusion about the presence or absence of consciousness based solely on physical features of the brain, such as prominent slow EEG waves, should be approached with caution, especially in behaviorally inaccessible neural structures. The slowing observed at the skull level should be characterized in greater detail using intracranial recordings in cases where clinical findings require invasive postoperative monitoring.
Michel A. Colombo says: “This is just the beginning of clarifying the problem of consciousness in inaccessible systems. During the review process, we encountered different perspectives, which highlighted the complexity of the problem.”
Marcello Massimini adds: “This pattern could provide clues as to why patients with brain injuries have sleep-like brain activity and how it relates to their level of consciousness.”
Anil K. Seth added: “This has been an exciting and deeply satisfying scientific journey. It began years ago with philosophical discussions about the possibility of ‘islands of consciousness’ in completely isolated neural systems, and now this brilliant collaboration has shed important experimental light on this question of the great experimenter Selene.”
Tim Bay concluded: “The study of consciousness encompasses many troubling issues where it is unclear what to say about the possibility of subjective experience. As a philosopher, it was very rewarding to explore a new frontier in the science of consciousness with this brilliant team of scientists and clinicians.”
About this consciousness research news
Author: Claire Turner
Source: PLOS
Contact: Claire Turner – PLOS
Image: The image is credited to StackZone Neuro
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
Abstract
Hemispheretomy induces persistent, sleep-like slow waves in the isolated cortex of awake individuals.
Hemispherectomy is a neurosurgical procedure for the treatment of refractory epilepsy, in which a significant portion of the cerebral cortex, possibly an entire hemisphere, is severed from cortical and subcortical connections. Although this procedure prevents epileptic seizures from spreading, it raises important questions.
Given the complete isolation of sensorimotor pathways, it is still unclear whether the unpaired cortex sustains any form of inaccessible consciousness. In general, the patterns of activity that comprise the bulk of the unpaired cortex in awake humans are still poorly understood.
We address these issues by investigating for the first time the electroencephalographic (EEG) status of the isolated cortex during wakefulness before and after surgery in 10 pediatric patients, focusing on non-epileptic background activity.
After surgery, the isolated cortex showed notable slower oscillations (<2 Hz) and more pronounced broadband spectral roll-off, indicating a redistribution of power toward lower frequencies.
This broadband slowing of the EEG resulted in a marked decrease in the spectral exponent, a validated marker of consciousness, reaching deep anesthesia and characteristic of the vegetative state.
Compared to a reference sample of children throughout the sleep-wake cycle, the spectral exponent of the contralateral cortex corresponded to wakefulness, while that of the isolated cortex corresponded to deep NREM sleep.
The findings of significant slow oscillations and broadband slowing support the findings of absent or reduced consciousness in the temporally isolated cortex. Furthermore, the persistence of unperturbed sleep patterns years after surgery offers valuable insight into the long-term electrophysiological effects of cortical disconnection in the human brain.
