How do you know if someone who can’t move, speak, or respond is still conscious? For families of coma patients, accident victims, or someone under anesthesia, it’s the lights and sounds of machinery, the warmth of their body, and the rise and fall of their chest that give us our only indications of life when the conscious mind is absent. We call out to them, hoping for some small sign of recognition, wishing they could open their eyes or squeeze our hand.
A new study published in eLife offers an important way to answer that question, one that may change how consciousness is measured in hospitals around the world.
The question of whether someone is conscious has certainly been argued countless times over centuries, debated in lectures, and analyzed by doctors, philosophers, and theologians in countries around the world. In the 19th century, the idea that someone going under anesthesia using ether and chloroform would erase their consciousness, or would it “merely silence their body,” was a hotly debated topic between doctors, with religious leaders joining the argument, claiming that removing pain interfered with the moral purpose of suffering.
What PCI Reveals About Consciousness
In 2013, an Italian doctor, Dr. Marcello Massimini, a professor of psychology at the University of Milan, Italy, collaborating with a team of physicians and researchers, developed a technique called the Perturbational Complexity Index or (PCI) from a mathematical model from the Integrated Information Theory “which posits that conscious systems must exhibit both high integration and differentiation of information,” or another way of putting it, “consciousness needs both connection and complexity”
PCI was created to go beyond traditional methods of accessing consciousness, such as physical reactions, following commands, behavioral responses, or opening the eyes. Because patients with brain injuries or severe paralysis are unable to respond, a way was needed to measure consciousness from the brain itself, rather than from external stimuli.
A simple (enough) explanation of how PCI works is that it measures the brain’s response when it is stimulated or “perturbed” with Transcranial Magnetic Stimulation (TMS) using a magnetic coil placed over a patient’s head. The coil sends a small pulse into the brain’s cortex, and the reaction is recorded using a high-density EEG (Electroencephalogram), a device that measures electrical activity in the brain. That reaction is calculated using the Lempel–Ziv complexity, which measures how “surprising” or “unpredictable” the sequence of information received is. The response is assigned a numerical score of 0 or 1 based on the reaction. A conscious brain produces a large, complex, long-lasting response. An unconscious brain produces a weak, simple, short-lived response.
PCI is based on the idea that consciousness relies on the brain’s ability to connect and process information in complex patterns. In contrast, unconscious brains respond in a more repetitive, reflexive way. When stimulated, an unconscious brain’s response often begins strong, quickly fades, and doesn’t spread to other areas like an echo. It follows a steady pattern much like the cadence of a metronome.
Active brains under deep anesthesia, like propofol and xenon, will produce large slow waves that rise and fall in a predictable manner; those in a coma often display repeating low-complexity activity like delta waves.
People with high PCI scores are more likely to be conscious — whether awake, dreaming, in REM sleep, experiencing psychedelics, ketamine anesthesia, or certain forms of “covert consciousness.” Low PCI scores appear during deep anesthesia, non-REM sleep, coma, vegetative state, or severe brain damage.
PCI has already improved how doctors diagnose consciousness disorders and helps reduce misdiagnosis in coma, as well as accidental awareness during surgery.
(a) Consciousness is a continuum and can be explored with drug-induced coma of various depths (Xenon, Propofol >Ketamine > Wakefulness). We hypothesize a correspondence between the variations in complexity found with PCI and the dynamics of spontaneous activity across the spectrum of consciousness. (b) We sketch various patterns of spatio-temporal activity reflecting changes in perturbational complexity from left to right. In (c), we show the conceptual shapes of corresponding manifolds of brain activity responsible for different sizes of the functional repertoire (number of wells) and associated with consciousness. (d) The brain is modeled as a network of neural masses coupled by an empirical connectome. This whole-brain model serves as a platform to simulate resting state activity (bottom left) and cortical stimulation (top left, example of firing rate time series with applied stimulus). Dynamical properties of the simulations are studied and compared with data features of human empirical recordings of spontaneous activity (bottom right, EEG during wakefulness and under three anesthetics) and stimulation (top right, TMS-EEG protocol performed in the same conditions). Image by Martin Breyton
A New Discovery: Consciousness Without Stimulation
A new study, “Spatiotemporal brain complexity quantifies consciousness outside of perturbation paradigms,” published in eLife, suggests that consciousness may be measurable without stimulating the brain at all, using only a simple resting-state EEG.
The authors reasoned that if PCI and spontaneous brain complexity reflect the same underlying neural dynamics, then resting EEG (recorded while the brain is doing nothing in particular) should contain similar information. Their hypothesis was correct.
The study found that resting-state EEG could distinguish conscious from unconscious states just as accurately as PCI — and in some cases, even better. By analyzing how complex, flexible, and varied the brain’s natural electrical activity was, the researchers were able to perfectly separate conscious and unconscious states without any external stimulation. Metrics such as resting complexity, fluidity, and functional repertoire reliably identified awareness.
Real-World Applications
This finding has major real-world implications. A simple, inexpensive EEG could help detect hidden consciousness in people who cannot move or respond, improving care for coma, traumatic brain injury, and critical illness. It could also enhance anesthesia safety by reducing the risk of awareness during surgery and offering a more precise way to monitor sedation in the ICU.
Because EEG is widely available, these tools could be used in small hospitals, rural communities, and low-resource settings, making advanced consciousness assessment more accessible than ever.
Resting EEG measures could also improve diagnoses for vegetative state, minimally conscious state, and locked-in syndrome — conditions where small differences in brain activity can change a patient’s care plan and future.
Beyond clinical applications, this approach provides a new biomarker for neurological research related to Parkinson’s disease, Alzheimer’s, epilepsy, dementia, aging, and more. It may also support future closed-loop brain therapies that automatically adjust stimulation based on the brain’s current state.
Overall, the study points toward a future where consciousness can be measured in a simpler, safer, and more accessible way, bringing clarity to some of the most difficult and emotional decisions in medicine.
Listen to the full details of this study here.
Martin Breyton, Jan Fousek, Giovanni Rabuffo, Pierpaolo Sorrentino, Lionel Kusch, Marcello Massimini, Spase Petkoski, Viktor Jirsa (2025) Spatiotemporal brain complexity quantifies consciousness outside of perturbation paradigms eLife 13:RP98920 https://doi.org/10.7554/eLife.98920.3
Unless noted all media by Chris Denny/Adobe
