Hemispherotomy, typically performed on pediatric epilepsy patients with intractable seizures, involves severing communication pathways between one cerebral hemisphere and the rest of the brain. Crucially, the disconnected hemisphere’s cortex remains physically intact with an intact blood supply but is cut off from sensory input and motor output. This surgical disconnection is intended to halt the spread of seizures but also isolates the affected hemisphere, making it behaviorally inaccessible. Until now, the intrinsic activity of this isolated cortex during wakefulness remained a profound mystery.

Using scalp electroencephalography (EEG), the researchers analyzed brain activity from ten children who underwent hemispherotomy, spanning periods from immediately before surgery to as long as three years post-operation. Surprisingly, the disconnected hemisphere exhibited pronounced slow-wave activity—characteristic of deep non-rapid eye movement (NREM) sleep—despite the patients being awake. This slow-wave pattern endured over months and years, indicating a lasting alteration in cortical state rather than a transient postoperative phenomenon.

Slow-wave EEG patterns are typically associated with unconscious or poorly conscious states such as deep sleep, general anesthesia, or vegetative states. Their presence in awake patients’ isolated cortex may suggest an absence or severe impairment of internal awareness in the disconnected tissue. Nevertheless, the other, intact hemisphere showed standard wakeful EEG signatures with faster rhythms indicative of active cognition and environmental engagement.

This dichotomy between hemispheres—the isolated cortex locked in a slow-wave, sleep-like mode, versus the connected hemisphere sustaining normal awake patterns—offers a striking example of how brain networks rely on integrated subcortical inputs to maintain conscious states. The researchers emphasize that without activating subcortical structures such as the thalamus, the cortex defaults into this sleep-like baseline, aligning with longstanding theories about the neuronal origins of consciousness and wakefulness.

Importantly, these findings contribute to an ongoing debate about whether behaviorally inaccessible brain regions can host any meaningful conscious experience. The persistence of sleep-like slow waves might indicate a form of cortical “shutdown” with little or no experiential content. However, the team urges caution in drawing direct conclusions about subjective awareness solely based on EEG patterns, especially in tissue completely isolated from sensory and motor pathways.

This work also raises intriguing questions about the potential functional or plastic roles that slow-wave activity might play in isolated brain tissue. Does this persistent sleep-like activity merely reflect a passive loss of input, or could it also support local homeostatic or regenerative processes within the cortex? The study opens new avenues for investigating the interplay between cortical dynamics, consciousness, and neuroplasticity in both health and disease.

The study further underscores the importance of intracranial recordings to complement scalp EEG, especially in clinical contexts requiring close postoperative monitoring. Such invasive methods could provide finer spatial resolution and deeper insights into the electrophysiological states of isolated cortical tissue, ultimately enhancing understanding of patient outcomes and guiding rehabilitative strategies.

Aside from its clinical implications, this research holds profound philosophical significance by challenging notions of consciousness as a unitary, holistic brain state. The emergence of a “sleep-like” island within an otherwise fully awake brain adds complexity to models of subjective experience and highlights the brain’s modular organization with distinct functional compartments.

Researchers Michele A. Colombo, Marcello Massimini, and collaborators articulate the delicate balance and complexity involved in assessing consciousness in neural substrates with no overt behavioral output. Their work elegantly bridges clinical neurology, cognitive neuroscience, and philosophical inquiries into the nature of awareness, contributing valuable empirical data to this challenging field.

The research journey was marked by intense interdisciplinary dialogue and exchange, reflecting the complexity of studying consciousness in isolated neural systems. This multicultural, multi-institutional collaboration spans Italy, Australia, Canada, and the United Kingdom, combining expertise in neuroscience, philosophy, and clinical neurology.

In conclusion, this study provides the first long-term EEG evidence that disconnected cortical regions in hemispherotomy patients reside in a persistent, sleep-like slow-wave state despite wakefulness. These findings have significant ramifications for understanding brain function, consciousness, and rehabilitation strategies following major brain surgery. As research advances, it may unveil new insights into the mechanisms governing cortical state transitions and the elusive neural correlates of conscious experience.

Subject of Research: People

Article Title: Hemispherotomy leads to persistent sleep-like slow waves in the isolated cortex of awake humans

News Publication Date: October 16, 2025

Web References: http://dx.doi.org/10.1371/journal.pbio.3003060

References: Colombo MA, Favaro J, Mikulan E, Pigorini A, Zauli FM, Sartori I, et al. (2025) Hemispherotomy leads to persistent sleep-like slow waves in the isolated cortex of awake humans. PLoS Biol 23(10): e3003060.

Image Credits: Michele A. Colombo (CC-BY 4.0)

Keywords: hemispherotomy, epilepsy surgery, slow-wave sleep, EEG, cortical disconnection, consciousness, pediatric neurology, brain plasticity, neural oscillations, isolated cortex

Postictal symptoms comprise a spectrum of neurological impairments occurring after the termination of seizures, ranging from speech difficulties and cognitive disorientation to erratic motor behaviors, including aimless wandering. These episodes, which can last from several minutes to hours, pose significant practical dangers, such as patients inadvertently placing themselves in life-threatening situations. Traditionally, these symptoms have been attributed directly to the preceding seizures, yet until now, the precise causal neurophysiological underpinnings remained elusive. Professor Michael Wenzel and his team at the UKB sought to disentangle this complexity by focusing on the aftermath of seizures rather than the seizures themselves.

At the heart of their investigation lies the hippocampus, a critical brain structure situated within the temporal lobe and heavily implicated in the pathophysiology of epilepsy. Employing cutting-edge methodologies—including cellular-resolution fluorescence microscopy, electrophysiology, and optogenetics—the researchers monitored the living brain networks of a mouse model over extended periods. This approach allowed an unprecedented view of neuronal communication and network dynamics in situ and across multiple temporal scales. What emerged was a striking finding: the identification of slow, spreading waves of neuronal depolarization distinct from classical epileptic seizures, a phenomenon long associated with other neurological conditions such as migraine and acute brain trauma.

Spreading depolarization (SD) involves the breakdown of the neuronal membrane potential, causing a temporary failure of neural networks. These waves propagate slowly through tissue, effectively paralyzing affected neural circuits for durations stretching from minutes to hours. Strikingly, the hippocampus and adjacent temporal lobe regions displayed heightened susceptibility to these events during epileptic activity, a vulnerability not equally shared by the neocortex—the brain’s sensory and motor processing hub. This spatial specificity may explain the predominance of postictal symptoms in temporal lobe epilepsy patients and indicates that the scope of impact extends far beyond the immediate seizure event.

Crucially, the velocity and temporal profile of spreading depolarization waves mean they elude detection through conventional clinical electroencephalograms (EEGs), which routinely filter out such slow potentials due to bandwidth constraints. This invisibility factor has left decades of clinical data potentially misinterpreted, whereby the seizures themselves were blamed for postictal manifestations that may, in fact, be sequelae of associated SD episodes. Professor Wenzel emphasizes that this oversight necessitates a reevaluation of diagnostic protocols and could revolutionize EEG-based epilepsy monitoring.

The translational aspect of this research gains strength through the inclusion of human data drawn from deep brain electrode implantations designed for pre-surgical diagnostics in refractory epilepsy patients. Utilizing technologically advanced microwire bundles, the team extended the EEG bandwidth traditionally employed in clinical settings, thereby capturing previously hidden slow potential shifts indicative of SD within deep brain regions. This remarkable demonstration establishes a direct link between seizure events and local SD in human epilepsy, bolstering the hypothesis that SD waves are a pivotal neurobiological substrate of postictal symptomatology.

These findings prompt significant reconsiderations regarding the pathophysiological narrative of epilepsy and raise critical questions about therapeutic targets. If spreading depolarizations drive postictal phenomena rather than seizures alone, interventions tailored to mitigate or prevent SD could markedly improve patient outcomes. Furthermore, the variability of SD susceptibility among brain regions suggests that personalized approaches might better address individual symptom profiles and seizure foci, particularly within the temporal lobe.

Beyond the immediate clinical implications, this discovery touches upon broader neuroscientific themes, such as network excitability, neuronal resilience, and the interplay between pathological brain states and inherent protective mechanisms. It also invites reflection on the common mechanistic threads shared by diverse neurological disorders where spreading depolarization features prominently, such as migraines and ischemic stroke, potentially enriching the conceptual framework for multiple fields.

The research consortium, which included partners from Forschungszentrum Jülich, RWTH Aachen University, the University of Veterinary Medicine Hannover, and the University of California, underscores the collaborative and interdisciplinary nature of this endeavor. Their collective expertise and technological innovation have unveiled a critical dimension of epilepsy biology previously hidden in the slow rhythm of brain waves, inviting both excitement and cautious optimism for future applications.

As the authors advocate, the next steps involve deeper exploration of the causal relationships between SD and various postictal symptoms beyond wandering, as well as the impact of these waves on cognitive and behavioral functions. Investigations will also need to address how widespread and frequent SD events are among different epilepsy types and severity levels, and whether interventions can disrupt the deleterious cycle they engender.

Moreover, these revelations bear on the design and interpretation of past and ongoing epilepsy studies. The conventional EEG’s blind spot concerning SD raises the possibility that previous conclusions relating postictal effects solely to seizure activity may be incomplete or partially inaccurate. This necessitates a paradigm shift in both clinical and research methodologies, including the advocacy for expanding EEG spectral windows to incorporate SD detection routinely.

Altogether, this pioneering study redefines our understanding of epileptic pathology by spotlighting the critical role of spreading depolarization waves in generating postictal neurological dysfunction. It challenges entrenched assumptions and opens new frontiers for diagnostic refinement and therapeutic innovation. As the field moves forward, recognizing and addressing the hidden currents of SD could transform the landscape of epilepsy care, sparing many patients from the confusion and hazards of postictal cognitive impairment and behavioral disturbances.

Subject of Research: Hippocampal spreading depolarization as a neurobiological mechanism underlying postictal symptoms in temporal lobe epilepsy

Article Title: Hippocampal spreading depolarization as a driver of postictal ambulation

News Publication Date: 17-Sep-2025

Web References:
http://dx.doi.org/10.1126/scitranslmed.adv3260

Image Credits:
University Hospital Bonn (UKB) / Prof. Michael Wenzel’s research group

Keywords:
Epilepsy, Temporal lobe epilepsy, Postictal symptoms, Spreading depolarization, Hippocampus, EEG, Neurobiology, Seizure-associated waves, Neurophysiology, Optogenetics, Fluorescence microscopy, Electrophysiology