Life, for the most part, is bathed in light. The sun immerses the planet in energy that supports the vast majority of ecosystems that call Earth home.
But life also generates its own light—and not just the bioluminescence of glowworms and lamp-headed anglerfish or the radiation produced by heat. In a phenomenon scientists refer to as ultraweak photon emissions (UPEs), living tissues emit a continuous stream of low-intensity light, or biophotons. Scientists think that this light comes from the biomolecular reactions that generate energy, which create photons as by-products. The more energy a tissue burns, the more light it gives off—which means, of our body’s tissues, our brain should glow brightest of all.
In a new study published in the journal iScience, researchers detected biophotons emitted by the human brain from outside the skull for the first time. What’s more, emissions of biophotons from the brain changed when participants switched between different cognitive tasks—though the relationship between brain activity and biophoton emissions was far from straightforward. The study authors think this may be hinting at a deeper role these particles of light might be playing in the brain.
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On some level, all matter emits photons. That’s because everything has a temperature above absolute zero and radiates photons as heat, often with longer wavelengths (infrared light) than can be seen with our eyes. UPEs are orders of magnitude more intense than this thermal radiation, with wavelengths in the visible or near-visible light range of the electromagnetic spectrum. As living cells generate energy through metabolism, they create oxygen molecules with excited electrons as by-products. When these worked-up electrons return to a lower energy state, they emit photons through a process called radiative decay.
Researchers studying biological tissues, including neurons in petri dishes, can detect this as a weak but continuous stream of light—from a few photons to several hundred photons per square centimeter each second. “Scaling this up to humans, we wanted to know if those photons might be involved in some information processing or propagation [in the brain],” says senior author Nirosha Murugan, a biophysicist at Wilfrid Laurier University in Ontario.
Scientists have been proposing that biophotons play a role in cellular communication for at least a century. In 1923 Alexander Gurwitsch conducted experiments where he showed that photon-blocking barriers placed between onion roots could prevent the plant from growing. In the past few decades, a handful of studies have added weight to the possible role biophotons play in cellular communication, which influences an organism’s growth and development.
With this work in mind, Murugan and her team wanted to see if they could detect hints of this phenomenon at the level of the human brain. First, they needed to see if they could measure UPEs emitted by a working brain from outside the skull. In a blacked out room, 20 participants wore head caps studded with electroencephalography (EEG) electrodes to measure the brain’s electrical activity. Photon-amplifying tubes to detect UPEs were positioned around their head. The photon detectors were clustered over two brain regions: the occipital lobes in the back of the brain, which are responsible for visual processing, and temporal lobes on each side of the brain, which are responsible for auditory processing. To distinguish brain UPEs from background levels of photons in the room, the team also set up separate UPE detectors facing away from the participants.
“The very first finding is that photons are coming out of the head—full stop. It’s independent, it’s not spurious, it’s not random,” Murugan says.
Next, she wanted to see if the intensity of these emissions would change depending on what sort of cognitive task people were performing. Because the brain is such a metabolically expensive organ, she reasoned that UPE intensity should increase when people were engaged in tasks that required more energy, such as visual processing. This is roughly what happens to neurons in a dish—more neural activity means more UPE emissions.
But while biophotons coming from participants’ heads could be easily distinguished from background levels of photons in the room, increased EEG activity in a given brain region didn’t result in higher levels of biophotons being captured by the closest detector. Clearly, something changes when you move from a few cells on a petri dish to a living brain. “Maybe [UPEs] are not getting picked up by our detector because they could be getting used or absorbed or scattered within the brain,” Murugan suggests. The researchers did find, however, that changes in the UPE signals came only when participants changed cognitive tasks, such as opening or closing their eyes, suggesting some link between brain processing and the biophotons it emits.
This leaves researchers with more questions than answers about what these UPEs are doing in the brain. “I think this is a very intriguing and potentially groundbreaking approach [for measuring brain activity, though] there are still many uncertainties that need to be explored,” says Michael Gramlich, a biophysicist at Auburn University, who was not involved in the new study. “The essential question to address,” he says, is whether “UPEs are an active mechanism to alter cognitive processes or if UPEs simply reinforce more traditional mechanisms of cognition.”
Daniel Remondini, a biophysicist at the University of Bologna in Italy, points to another open question: “How far can these photons travel inside biological matter?” The answer could shed some light on the lack of clear relationship between brain activity and photon detections in different regions, he says.
To answer these new questions, Murugan and her team want to use more precise sensor arrays to find where in the brain these photons are coming from. Scientists at the University of Rochester are also developing nanoscale probes to determine whether nerve fibers can transmit biophotons.
Even if our brain’s steady glow doesn’t play a role in how it works, the technique of measuring biophotons alongside electrical signals—what Murugan and her colleagues call photoencephalography—could still one day be a useful way to noninvasively measure brain states. “I suspect the technique will become widely adopted in the coming decades even if the theory that UPEs support cognition proves not to be true,” Gramlich says.
