Transcranial Photobiomodulation: Red Light Therapy for Brain Health

Light Through the Skull

Transcranial photobiomodulation (tPBM) involves applying near-infrared light to the scalp to deliver photons through the skull and into cortical brain tissue. While the skull attenuates most light, approximately 2-3% of 810nm NIR light penetrates through to the brain surface — enough to produce measurable biological effects in cortical neurons.

This is one of the most exciting emerging applications of photobiomodulation, with active research in traumatic brain injury, depression, cognitive enhancement, Alzheimer's disease, and Parkinson's disease. The evidence ranges from emerging to preliminary depending on the specific application, but the trajectory of research is compelling.

How Light Reaches the Brain

The skull presents a significant barrier to light transmission. Bone absorbs and scatters photons, with transmission efficiency depending on wavelength, skull thickness, and application location. Key factors:

810nm is optimal: Among commonly available wavelengths, 810nm has the best skull penetration. It sits in a "sweet spot" between hemoglobin absorption (which peaks at shorter wavelengths) and water absorption (which increases at longer wavelengths).

Frontal and temporal regions: The frontal bone and temporal bones are thinner than the parietal and occipital bones, allowing better light transmission to the underlying cortex.

Cumulative dose matters: While only 2-3% of surface irradiance reaches the brain, higher surface doses and longer treatment times can deliver therapeutically relevant fluence to cortical tissue.

The Applications

Traumatic Brain Injury (TBI) and Concussion

Naeser et al. (2014) published groundbreaking work using transcranial LED therapy in patients with chronic TBI. Patients who had suffered TBI years earlier and had persistent cognitive symptoms showed significant improvements in executive function, verbal memory, and inhibition after a course of tPBM treatment.

Hamblin (2016) published a comprehensive review of tPBM for brain disorders, detailing the mechanisms by which NIR light improves neuronal mitochondrial function, reduces neuroinflammation, promotes neuroplasticity, and enhances cerebral blood flow in injured brain tissue.

The proposed mechanisms for TBI include:

• Improved mitochondrial function in stunned but surviving neurons
• Reduced chronic neuroinflammation that persists after the initial injury
• Enhanced synaptogenesis and neuroplasticity
• Increased brain-derived neurotrophic factor (BDNF) expression
• Improved cerebral blood flow in hypoperfused regions

Depression

Schiffer et al. (2009) conducted a pilot study applying 810nm light to the forehead of patients with depression and anxiety. Significant improvements were observed in both conditions two weeks after a single treatment session.

Cassano et al. (2016, 2018) at Massachusetts General Hospital published both a pilot study and a randomized controlled trial of tPBM for major depressive disorder. The treatment targeted the left dorsolateral prefrontal cortex (the same region targeted by transcranial magnetic stimulation, an FDA-cleared depression treatment). Results showed significant improvements in depression scores compared to sham treatment.

The rationale: neuroimaging studies consistently show hypoactivation of the left DLPFC in depression. By enhancing mitochondrial function and blood flow in this region, tPBM may help normalize prefrontal cortex activity.

Cognitive Enhancement

Preliminary studies in healthy adults have shown that tPBM can improve cognitive performance on tasks involving sustained attention, working memory, and executive function. Barrett and Bhatt (2014) demonstrated improved reaction time and positive emotional states after a single tPBM session in healthy volunteers.

While cognitive enhancement in healthy individuals is the least clinically urgent application, it suggests that tPBM has measurable effects on cortical function even in non-pathological conditions.

Neurodegenerative Diseases

The most preliminary but perhaps most important area of tPBM research involves neurodegenerative diseases. Animal models of Alzheimer's disease have shown that tPBM can reduce amyloid-beta plaque burden, improve mitochondrial function in hippocampal neurons, and preserve cognitive performance.

Human studies are in early stages. Saltmarche et al. (2017) published a case series of five patients with mild to moderate dementia who showed improvements in cognitive function, sleep, and behavioral symptoms after tPBM treatment. While case series are low-level evidence, they provide justification for larger trials.

The Protocol

Wavelength

810nm is the primary wavelength for tPBM research. It has the best skull penetration and the most evidence base. 850nm is an alternative with slightly less evidence for transcranial applications.

Application Sites

• Forehead (bilateral): Targets the dorsolateral prefrontal cortex — the primary site for depression and executive function studies
• Temporal regions: Targets temporal cortex and underlying hippocampus
• Posterior skull: Targets occipital cortex and posterior parietal regions

Dosing

• Surface fluence: 10-30 J/cm² at the scalp
• Irradiance: 50-100 mW/cm² at the scalp
• Treatment time: 10-20 minutes per session
• Frequency: 2-3x per week for 6+ weeks

Important Caveats

tPBM is experimental for all neurological applications. It is not an established treatment for TBI, depression, or neurodegeneration. The evidence is emerging and promising but not yet sufficient for clinical recommendation.

Anyone considering tPBM for neurological conditions should:

• Coordinate with their neurologist or psychiatrist
• Not discontinue any existing medications or therapies
• Track symptoms with validated assessment tools
• Understand that results are not guaranteed

The Bottom Line

Transcranial photobiomodulation represents one of the most scientifically fascinating frontiers in photobiomodulation research. The ability to non-invasively deliver therapeutic light to the brain opens possibilities for treating conditions that have limited treatment options. While the evidence is still emerging, the combination of plausible mechanisms, positive preliminary results, and an excellent safety profile makes tPBM an area to watch closely as the research matures.