Revisiting claims of extracranial biophoton detection from the human brain

This paper challenges the validity of recent claims regarding extracranial ultraweak photon emission as a non-invasive brain biomarker by demonstrating that reported signals are likely dominated by background light and scalp emission rather than genuine brain activity due to experimental flaws and tissue attenuation.

The Gist

The Big Idea: "Is the Brain Glowing?"

Imagine a theory that says your brain is like a tiny, glowing lightbulb. Scientists have been trying to catch this "brain light" (called biophotons) shining through your skin and skull to see what your brain is doing without sticking electrodes inside your head.

Some recent studies claimed they successfully detected this light, suggesting it could be a new way to read thoughts or brain activity.

This new paper says: "Hold on a minute. We think those studies got it wrong."

The authors, a team of physicists from the University of Calgary, argue that the "brain light" those studies found wasn't actually coming from the brain at all. Instead, it was likely just leaking light from the room or glowing skin, not the brain itself.

Here is the breakdown of why they think this, using three simple analogies.

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1. The "Dark Room" Problem (The Light Leak)

The Claim: The previous studies said they were in a pitch-black room and detected thousands of photons (tiny particles of light) coming from the head.

The Reality Check:
Imagine you are trying to hear a cricket chirping in a forest at night. If a car drives by with its headlights on, you won't hear the cricket; you'll just hear the car.

The authors built a "super-dark" tent to test this. They found that even if a room looks dark to your human eyes, it might still have tiny cracks letting in light.

• The Experiment: They took a light detector (a super-sensitive camera) and put it in a dark tent.
• The Result: When they left a tiny 5mm gap (like a crack under a door), the detector went crazy, picking up thousands of "lights."
• The Comparison: The amount of light the detector picked up through that tiny crack was exactly the same amount that the previous studies claimed was coming from the human brain.

The Takeaway: The "brain light" they saw was probably just a tiny bit of light leaking into the tent, not light coming from the brain. It's like mistaking a car headlight for a firefly.

2. The "Foggy Window" Problem (The Skull Barrier)

The Claim: The brain emits light, and we can see it through the head.

The Reality Check:
Imagine your brain is a lightbulb inside a house. The "walls" of the house are your scalp and skull.

• Most of the light the brain naturally emits is blue or green (short wavelengths).
• Your skin and skull are like thick, foggy windows that block blue and green light almost completely.
• Only red or near-infrared light (long wavelengths) can get through the foggy window, and even then, very little of it makes it through.

The Takeaway: Even if the brain is glowing, the thick walls of your skull block the specific colors of light the brain usually emits. It's like trying to see a blue LED through a thick red blanket; the blue light just doesn't get through.

3. The "Wrong Glasses" Problem (The Detector Mismatch)

The Claim: The detectors used in the previous studies were sensitive enough to catch the brain light.

The Reality Check:
Imagine you are trying to catch fish in a pond.

• The fish you want (the brain light that gets through the skull) are Red Fish (wavelengths above 600nm).
• The fish the brain actually emits are Blue Fish (wavelengths below 600nm).
• The "net" (the detector) used by the previous studies was designed to catch Blue Fish. It is terrible at catching Red Fish.

The Math:
The authors did the math:

1. The skull blocks almost all the blue light.
2. The detector is blind to the red light that does get through.
3. To get the signal the previous studies claimed to see, the brain would have to be glowing 100 million times brighter than any living thing has ever been observed to glow.

The Takeaway: The equipment was looking for the wrong color of light. It was like using a net with holes too big to catch the tiny fish you were looking for, while the net itself was made of material that repelled the big fish.

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The Final Verdict

The authors conclude that the "brain light" reported in those studies was likely a combination of:

1. Background noise: Tiny amounts of light leaking into the "dark" room.
2. Skin glow: Light coming from the surface of the skin, not the brain deep inside.
3. Measurement errors: The brain signal was actually weaker than the background noise, which is physically impossible if the brain were the source.

What does this mean for the future?
The idea of reading the brain with light is still cool and scientifically interesting. However, to make it work, scientists need to:

• Build rooms that are truly, absolutely dark (no leaks).
• Use detectors that are sensitive to red/infrared light (the only color that gets through the skull).
• Be much more careful with their math.

Until these strict rules are followed, we can't say for sure that we are seeing the brain "glow." We are likely just seeing the room's light or the skin's glow.

Technical Summary

1. Problem Statement

Recent studies (specifically Casey et al. [10] and Dotta et al. [12]) have claimed that Ultraweak Photon Emission (UPE), or "biophotons," can be detected extracranially (through the scalp and skull) to serve as a non-invasive biomarker for brain activity. These studies reported photon count rates of 25,000–35,000 counts per second (kcounts/s) from the human head, suggesting a correlation with functional brain states.

The authors of this paper argue that these claims are scientifically unsound due to two primary issues:

1. Background Contamination: The reported signal intensities are orders of magnitude higher than the known UPE levels of biological systems ($10–10^3$ photons/cm²/s) and are likely dominated by undetected ambient light leakage.
2. Optical Attenuation & Detector Mismatch: The wavelengths of light that can physically penetrate the human scalp and skull (red/near-infrared, >600 nm) fall outside the high-sensitivity range of the Photomultiplier Tubes (PMTs) used in the previous studies. Conversely, the wavelengths the PMTs are most sensitive to (blue/green, <600 nm) are almost completely blocked by tissue.

2. Methodology

To test the validity of previous claims, the authors conducted a controlled experiment using a rigorous optical setup:

• Experimental Setup:
  • A human participant was seated inside a fully enclosed, opaque dark tent within a dark room.
  • A broadband PMT (Hamamatsu R4220-P) was positioned 5 cm from the participant's forehead, mimicking the geometry of the Casey et al. study.
  • The PMT's quantum efficiency curve was compared to the Sens-Tech DM0090C PMTs used by Casey et al., noting similar response characteristics (peak sensitivity in the blue-green range, steep decline >600 nm).
• Control Measurements:
  • Dark Count: The intrinsic background of the PMT was measured with the entrance capped, yielding a stable rate of ~25 counts/s.
  • Light Leakage Sensitivity: Controlled apertures (5 mm and 10 mm slits) were introduced into the dark tent while room lights were on. This tested how minute amounts of stray light affect the detector.
  • Head Measurement: The PMT was uncapped and pointed at the forehead in complete darkness to measure the actual UPE signal.
• Theoretical Analysis:
  • The authors analyzed tissue transmission data (skin, skull, brain) from Hart et al. [14] and Litscher et al. [15] to calculate the transmission of photons through the head.
  • They combined these transmission curves with the PMT quantum efficiency to determine the theoretical detection probability of "brain-derived" photons.

3. Key Results

A. Optical Leakage Dominates Signals

• Dark Counts: The intrinsic background was ~25 counts/s.
• Leakage Impact:
  • A 5 mm slit in the dark tent increased the count rate to ~1,000 counts/s.
  • A 10 mm slit increased the rate to ~40,000 counts/s.
• Comparison: The 40,000 counts/s observed with a 10 mm slit is comparable to the "brain UPE" signals reported by Casey et al. (25–35 kcounts/s).
• Observation: Under these leakage conditions, the room still appeared completely dark to the naked eye.
• Actual Head Signal: When measuring the forehead in complete darkness (no slits), the count rate was only ~80 counts/s. This is two orders of magnitude lower than the signals claimed by Casey et al.

B. Spectral Attenuation and Detector Mismatch

• Tissue Blocking: Human skin and skull bone strongly attenuate wavelengths < 600 nm. Transmission drops to effectively 0–2% for these wavelengths.
• Detector Insensitivity: The PMTs used in previous studies have peak sensitivity in the 350–500 nm range. Their quantum efficiency drops to <5% at 600 nm and <2% above 650 nm.
• The "Impossible" Detection:
  • The only wavelengths that can penetrate the skull are >600–650 nm (red/near-infrared).
  • However, the PMTs are least sensitive in this exact range.
  • Calculation: At the optimal transmission wavelength (~700 nm), the combined probability of a photon passing through the skull and being detected by the PMT is approximately 0.0004–0.0005.
  • Implication: To generate the reported extracranial signal of ~30,000 counts/s, the brain would need to emit $10^8$ photons/cm²/s. This is 5 to 7 orders of magnitude higher than established UPE levels in biological systems.

C. Signal vs. Background

• The authors noted that in Casey et al.'s data, the signal from the head was actually lower than the background measurement. This suggests the head was acting as a shield, blocking ambient light, rather than emitting light.

4. Key Contributions

1. Debunking Previous Claims: The paper provides empirical evidence that the high photon counts reported as "brain UPE" are artifacts of minute, imperceptible light leakage, not biological emission.
2. Quantifying Sensitivity Limits: It demonstrates that standard PMTs used in biophoton research are fundamentally unsuited for extracranial brain imaging because their spectral sensitivity does not overlap with the transmission window of human tissue.
3. Methodological Correction: The study establishes that "darkness" in UPE experiments must be verified by PMT dark count levels, not human visual perception, as the human eye cannot detect the low-level leakage that overwhelms UPE signals.

5. Significance and Conclusion

• Scientific Rigor: The paper emphasizes that attributing UPE signals to brain activity requires eliminating background light to a level far below the expected biological signal and using detectors matched to tissue transmission properties.
• Future Directions: For UPE to become a viable non-invasive tool for neuroscience (e.g., for brain-computer interfaces or clinical monitoring), future research must:
  • Implement strict optical blackouts that reduce background to the PMT dark count level.
  • Develop or utilize detectors sensitive to red/near-infrared wavelengths (>650 nm) where tissue transmission is non-negligible.
• Final Verdict: Under current experimental conditions and using standard PMTs, extracranial UPE signals are overwhelmingly dominated by scalp emission and background leakage, rendering previous claims of direct "brain biophoton" detection invalid.