Revisiting claims of extracranial biophoton detection from the human brain

Abstract

Ultraweak photon emission is the spontaneous emission of extremely low levels of light from a broad range of biological systems. Recent studies have reported that UPE measured extracranially can serve as a potential non-invasive biomarker of brain activity. Here, we show that this interpretation suffers from serious problems. First, when observed under properly dark conditions, the UPE from the head is much weaker than what is reported in certain papers on 'brain UPE' from human heads. Signals detected in these studies are overwhelmingly dominated by background light. Second, photons at wavelengths < 600 nm are strongly attenuated by scalp and skull tissues, and longer wavelengths fall largely outside the effective spectral sensitivity of the photomultiplier tubes (PMTs) used. As a consequence, even if UPE from the head is detected under properly background-free conditions, it is likely to be dominated by emission from the scalp rather than from the brain, certainly as long as PMTs are used. Our results emphasize the importance of careful experimental design to make genuine progress on this important question.

Figures (4)

• Figure 1: (A) Schematic of Casey et al.'s casey2025 experimental setup. (B) PMT recordings from left(L) Occipital and right(R) Temporal of the head, as well as the background, reported by Casey et al. (C) PMT recordings from the head, reported by Casey et al. The reported photon counts are much higher than the known UPE intensity from all examined living systems, which are in the range 10-10$^3$ counts per cm$^2$ per sec UPEreview. Moreover, the signal from the head is lower than the background, which is fundamentally questionable.
• Figure 2: The quantum efficiency diagram of (A) the PMT (Hamamatsu R4220-P) we used in our experiment and (B) the PMTs (SENS-TECH DM0090C) that Casey et al.casey2025 have used in their study. Note that the quantum efficiencies at wavelengths above 600 nm fall below 5%. The diagrams are taken from the respective data sheets PMT.
• Figure 3: (A) Schematic of our experimental setup in which a participant is seated in a darkened environment while a PMT is positioned at a fixed distance of 5 cm from the forehead to monitor UPE. (B) PMT photon count rates recorded in the dark tent during control measurements with the room lights off (dark counts) and with the room lights on at minimal intensity while introducing controlled apertures of 5 mm and 10 mm in the dark enclosure, demonstrating the sensitivity of the measurements to ambient light leakage. (C) PMT counts recorded from the forehead with the room lights off and during controlled light-leakage conditions in which the room lights were on and a 10 mm aperture was introduced in the dark enclosure, illustrating the sensitivity of the measurement to ambient light.
• Figure 4: Light transmission in the 450–880 nm wavelength range through human skin (A), skull bone (B,D), and brain tissue (C). Figures a, b, and c are taken from hart2016new, and Figure d is taken from Litscher2014LASERTHERAPIE.