A new study published in iScience provides evidence that the human brain emits extremely faint light signals that not only pass through the skull but also appear to change in response to mental states. Researchers found that these ultraweak light emissions could be recorded in complete darkness, and they appeared to shift in response to simple tasks like closing the eyes or listening to sound. The findings suggest that this faint brain light may carry information about brain activity—possibly opening the door to a new way of studying the brain (photoencephalography).
All living tissues release tiny amounts of light during normal metabolism, known as ultraweak photon emissions. This happens when excited molecules return to a lower energy state and emit a photon in the process. The light is incredibly faint—about a million times weaker than what we can see—and falls within the visible to near-infrared range. In contrast to bioluminescence, which involves specific chemical reactions like those used by fireflies, ultraweak photon emissions happen constantly in all tissues, without special enzymes or glowing compounds.
The brain emits more of this faint light than most other organs because of its high energy use and dense concentration of photoactive molecules. These include compounds like flavins, serotonin, and proteins that can absorb and emit light. Photon emission rates also seem to rise during oxidative stress and aging and may reflect changes in cell health or communication.
The research team, led by Hayley Casey, Nirosha Murugan, and colleagues at Algoma University, Tufts University, and Wilfrid Laurier University, wanted to know if these faint light emissions could be used to monitor brain activity. Unlike other imaging methods that require stimulation—such as strong magnetic fields or infrared light—measuring UPEs is entirely passive. That means it doesn’t introduce anything new to the brain.
The researchers proposed that UPEs might offer a new way to monitor brain function safely and without interference, similar to how EEG tracks electrical brain waves without applying energy. They also wanted to test whether UPEs reflect mental states like resting with eyes closed or responding to sound, and whether these signals match known changes in electrical brain rhythms.
The researchers recruited 20 healthy adult participants and measured both UPEs and brain electrical activity while the participants sat in a dark room. The setup included photomultiplier tubes placed near the occipital and temporal regions of the head, where the brain processes visual and auditory information. A third sensor recorded background light. At the same time, participants wore a cap with electroencephalography sensors to record electrical brain rhythms.
Participants went through a ten-minute recording session that included five conditions. First, they sat with eyes open and then with eyes closed. Next, they listened to a simple repeating auditory stimulus, followed by another eyes-closed period, and finally another eyes-open period. The aim was to see whether brain UPEs responded to known manipulations of brain activity, particularly the shift in alpha rhythms that occurs when people close their eyes.
Photon emissions were recorded in short time intervals and analyzed for variability, frequency content, and stability over time. The team compared the results to background signals and examined correlations with electrical brain rhythms recorded at the same time.
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