The fMRI response to transcranial photobiomodulation: the effect of wavelength, irradiance, frequency and skin tone on the BOLD and CBF response in healthy young adults

Transcranial photobiomodulation (tPBM) is an emerging technology that utilizes light to penetrate the skull to stimulate neural tissue. However, the physiological impact and the in vivo dose dependence of tPBM on the human brain have yet to be fully understood. In this study, we utilize functional magnetic resonance imaging (fMRI) to evaluate the effect of tPBM on the blood-oxygenation-level-dependant (BOLD) and cerebral blood flow (CBF), while varying specific stimulation parameters such as wavelength, irradiance, and frequency. We further examine the influence of skin tone and sex. Our results show spatially varying fMRI responses to tPBM, with the site of irradiation and more distal sites (including the default-mode network) responding. Certain regions display a sustained fMRI response after tPBM cessation. Furthermore, the responses are dependent on stimulation parameters, sex and skin-tone. Biophysical modeling revealed a temporal dependence of potential neurovascular coupling underlying the tPBM response, one that declined during the course of the stimulation but became elevated immediately after stimulation cessation. This study helps to establish some physiological fundamentals of tPBM and demonstrate the importance of parameter optimization for therapeutic interventions.