Frequency effects of near-infrared light radiation on the forehead as quantified by EEG measurements

Transcranial photobiomodulation (tPBM) has been shown to be a non-invasive method of brain stimulation that can affect the nervous system. The aim of this study was to determine how the flicker frequency of near-infrared light affects the brain. Methods: An 810nm near infrared array was used as the stimulator. 50 healthy subjects volunteered to participate in the study. 40 subjects were randomly divided into four groups. Each group underwent a 30-minute near infrared array radiation with four different frequencies (i.e., 0Hz,5Hz, 10Hz, and 20Hz) respectively on the forehead. The rest 10 subjects formed the control group, in which the stimulator did not work. Electroencephalogram (EEG) signals of all subjects during each test were recorded. In the simultaneous tPBM-EEG study, EEG source imaging (ESI) was applied for EEGs. The relative energy of the alpha band was analyzed to be the parameter for the default mode network (DMN). Result: The region of the most obvious difference between experimental groups and the control group at the source level was the prefrontal lobe. Higher stimulation frequency would cause a larger difference(P<0.05). Compared with the control group, the DMN network activity and connectivity of the experimental groups were enhanced, and the higher the stimulation frequency, the extent of increase was larger. Conclusions: Larger frequency of the near-infrared light would cause more distinct brain activities in the stimulated areas, and it can enhance the functional connectivity of the brain's DMN network more effectively. It indicates that near-infrared light may improve brain cognitive function and ameliorate neurodegenerative diseases.