Can Shining Light on Your Head Make You Think Better?
Possibly, though the evidence is not yet settled. This systematic review of 35 human studies found that 29 of them, 82.9 percent, reported positive improvement in cognitive function after transcranial photobiomodulation. All nine studies on participants with subjective memory complaints, mild cognitive impairment, or dementia showed positive outcomes.
Red light therapy, also known as low-level laser therapy (LLLT) or photobiomodulation (PBM), uses red and near-infrared wavelengths of light delivered to the body.
Transcranial photobiomodulation (tPBM) has been studied for over a decade as a possible cognitive intervention. This is a systematic review of the published human literature, not a new trial, and it was assembled to answer a straightforward question: does shining near-infrared light through the skull improve how the brain works?
What the Research Shows
The researchers searched PubMed, Scopus, and Web of Science from 1987 to May 2022. They identified 35 studies evaluating tPBM’s effects on learning and memory, attention, executive function, language, and global cognitive function. Of those 35 studies, 29 (82.9 percent) reported positive improvement in cognitive function after tPBM.
The clinical populations were the most consistent. All nine studies in participants with subjective memory complaints, mild cognitive impairment, and dementia showed positive outcomes. Seven of the traumatic brain injury studies, 87.5 percent of them, also showed positive results.
Stroke tells a different story. A series of clinical trials in stroke patients showed positive trends on improved neurological deficit at first, then was terminated early at phase III for lack of statistical significance.
One of the most common protocols used in clinical populations delivered near-infrared light at 810 nm, an irradiance of 20 to 25 mW/cm2, and a fluence of 1 to 10 J/cm2. Other protocols in the review used wavelengths from visible red (630 to 635 nm) up to near-infrared maximums of 1060 to 1068 nm.
The authors’ own conclusion is deliberately hedged: tPBM seems to improve cognitive function, but only half of the reviewed clinical trials were randomized controlled trials, and further investigation is warranted.
Dr. Kumar’s Take
I read this as a promising signal, not a settled answer. A large majority of these studies pointed in the same direction, and every study in the memory-complaint, mild cognitive impairment, and dementia group came out positive. That is worth paying attention to.
But only half of the reviewed clinical trials were randomized. In a literature where non-randomized designs are that common, a high positive rate is partly a measure of how the studies were built, not only of how well the treatment works. The stroke program makes the point better than any argument I could construct: positive early trends, then a phase III trial stopped for lack of statistical significance. I have watched that arc play out in neurosurgery more than once.
So my position is that tPBM is a reasonable thing to follow closely and a premature thing to build a protocol around. The authors who reviewed all 35 studies said further investigation is warranted, and I am not going to be more confident than they are about their own data.
Who Benefits Most
Every one of the nine studies in participants with subjective memory complaints, mild cognitive impairment, or dementia reported positive outcomes, and 87.5 percent of the traumatic brain injury studies did as well. The stroke trials are the exception, and a serious one: early positive trends did not survive a phase III trial.
Practical Takeaways
- 29 of 35 human studies (82.9 percent) reported positive improvement in cognitive function after tPBM.
- All nine studies in subjective memory complaints, mild cognitive impairment, and dementia reported positive outcomes, as did 87.5 percent of traumatic brain injury studies.
- Trials in stroke patients showed early positive trends but were terminated at phase III for lack of statistical significance.
- Only half of the reviewed clinical trials were randomized controlled trials, which is why the authors call for further investigation.
FAQs
What cognitive abilities were evaluated in these studies?
The review assessed learning and memory, attention, executive function, language, and global cognitive function.
What light settings were most commonly used?
One of the most common protocols in clinical populations used near-infrared light at 810 nm, an irradiance of 20 to 25 mW/cm2, and a fluence of 1 to 10 J/cm2. The reviewed studies also used other wavelengths, ranging from visible red at 630 to 635 nm up to near-infrared maximums of 1060 to 1068 nm.
How strong is this evidence?
Mixed. Most of the 35 studies were positive, but only half of the reviewed clinical trials were randomized controlled trials, and the authors conclude that further investigation is warranted. The stroke trials, which looked promising early and then failed to reach statistical significance at phase III, show why that caution matters.
Bottom Line
This systematic review of 35 human studies found that 82.9 percent reported positive improvement in cognitive function after transcranial photobiomodulation, with the most consistent results in people who already had cognitive impairment. The authors’ conclusion is that tPBM seems to improve cognitive function, tempered by the fact that only half of the reviewed clinical trials were randomized and that a stroke trial program was terminated at phase III for lack of statistical significance. Promising, and not yet proven.
