Near-Infrared Wavelength Specificity Yields Phototherapy Insights

Transcranial photobiomodulation (tPBM) is an emerging form of light therapy that uses LEDs or low-intensity lasers that emit near-infrared (NIR) light to stimulate the brain. Although tPBM is in the early stages of development, it shows promise as a potential therapy for enhancing cognitive function and treating neurophysiological disorders.

To deepen scientific understanding of tPBM, researchers at The University of Texas at Arlington (UTA) investigated its effects on the hemodynamic and metabolic activities of the prefrontal cortex in 26 healthy young adults.

Results of the study showed that tPBM has a significant effect on the hemodynamic and metabolic activities of the resting brain.

Further, the study supported the researchers’ hypothesis that hemodynamic and metabolic activities in the infra-slow oscillations (ISO) of the resting human forehead are significantly modulated by tPBM stimulation conditions. The modulation is wavelength- and site-specific, as well as distinct in different ISO bands, the researchers said.

Study participants received an 8-min tPBM treatment with 800- and 850-nm light on the right side of the forehead and with 800-nm light on the left side. They also received two placebo interventions, on the left and right sides of the forehead, for control analysis. Laser-protection goggles were provided to protect the participants’ eyes during the treatments.

The researchers placed a two-channel broadband, near-infrared spectroscopy detecting probe on each side of the participants’ foreheads before and after each 8-min treatment. Two-channel broadband, near-infrared spectroscopy is used to quantify the ISO of neurophysiological networks in the resting human brain in vivo.

Noninvasive transcranial photobiomodulation (tPBM) experimental setup. (a): Placement of two pairs of optical probes on the forehead connected to a two-channel near-infrared spectroscopy (2-bbNIRS) unit to capture brain activity. (b): Test regimen followed for each participant. (c): Light source, two spectrometers, and probe bundles for 2-bbNIRS measurement. (d): Arrows indicating the right and left sites for tPBM and laser-protection goggles. Courtesy of Shahdadian et al., doi 10.1117/1.NPh.10.2.025012.

The two-channel broadband, near-infrared spectroscopy probes enabled the researchers to capture, measure, and compare changes in brain activity before and after stimulation with the NIR light, based on the absorption and scattering properties of the cerebral tissue. The researchers measured time series observations and analyzed them to determine the coherence of the hemodynamic and metabolic ISO over the three frequency bands — for example, endogenic, myogenic, and neurogenic. Four physiological metrics were derived to characterize the connectivity and coupling between each pair of signals in response to the respective tPBM conditions.

Combined with coherence analysis, the before and after dual-channel measurements enabled the researchers to identify consistent, specific alterations induced by prefrontal tPBM at 800 and 850 nm. The researchers additionally observed the wavelength- and site-specificity of the modulation of hemodynamic and metabolic activities in the brain. This was also distinct in the different ISO bands. The site-specific effects of tPBM showed significant enhancement of bilateral hemodynamic and metabolic connectivity by the right prefrontal 800-nm tPBM.

Earlier studies showed that each ISO frequency component is associated with a specific neurophysiological activity in the healthy human brain. Therefore, the detection of impaired or diminished infra-slow activity in cerebral hemodynamic and metabolic functions could serve as a potential indicator of neurological or metabolic disorders. For example, various studies have identified the relation between the impairment of infra-slow cerebral activity and cardiovascular disease, Alzheimer’s disease, hypertension, and stroke.

The researchers said that the metrics of the resting human prefrontal cortex were stable, with relatively high reproducibility among healthy young adults. These resting-state, frequency-specific metrics could serve as potential features for identifying neurophysiological disorders, because these frequency bands are strongly associated with specific neurophysiological activities in the human brain. Furthermore, these metrics could help scientists better understand the mechanism of action behind different stimulation conditions of tPBM.

Still, the researchers said, these metrics will need to be further investigated before tPBM can become an effective intervention tool. One step toward this goal would be to correlate tPBM-induced alterations in brain activity with tPBM-induced improvements in the neurological condition being studied.

The research was published in Neurophotonics (www.doi.org/10.1117/1.NPh.10.2.025012).