The resting-state infra-slow oscillation (ISO) of the cerebral cortex reflects the neurophysiological state of the human brain. ISO results from distinct vasomotion with endogenic (E), neurogenic (N), and myogenic (M) frequency bands. Quantification of prefrontal ISO in cortical haemodynamics and metabolism in the resting human brain may facilitate the identification of objective features characteristic of certain brain disorders. The goal of this study was to explore and quantify the prefrontal ISO of the cortical concentrations of oxygenated hemoglobin (Δ[HbO]) and redox-state cytochrome-c-oxidase (Δ[CCO]) as hemodynamic and metabolic activity metrics in all three E/N/M bands. Two-channel broadband near-infrared spectroscopy (2-bbNIRS) enabled measurements of the forehead of 26 healthy young participants in a resting state once a week for five weeks. After quantifying the ISO spectral amplitude (SA) and coherence at each E/N/M band, several key and statistically reliable metrics were obtained as key features: (1) SA of Δ[HbO] at all E/N/M bands, (2) SA of Δ[CCO] in the M band, (3) bilateral connectivity of hemodynamics and metabolism across the E and N bands, and (4) unilateral hemodynamic-metabolic coupling at both E and M bands. These features have promising potential to be developed as objective biomarkers for clinical applications in the future.

Our group previously examined 8 min photobiomodulation (PBM) by 1064 nm laser on the human forearm in vivo to determine its significant effects on vascular hemodynamics and cytochrome c oxidase redox activity. Since PBM uses a wide array of wavelengths, in this paper, we investigated (i) whether different wavelengths of lasers induced different PBM effects, and (ii) if a light-emitting diode (LED) at a similar wavelength to a laser could induce similar PBM effects. A broadband near-infrared spectroscopy (bbNIRS) system was utilized to assess concentration changes in oxygenated hemoglobin (Δ[HbO]) and oxidized cytochrome c oxidase (Δ[oxCCO]) during and after PBM with lasers at 800 nm, 850 nm, and 1064 nm, as well as a LED at 810 nm. Two groups of 10 healthy participants were measured before, during, and after active and sham PBM on their forearms. All results were tested for significance using repeated measures ANOVA. Our results showed that (i) lasers at all three wavelengths enabled significant increases in Δ[HbO] and Δ[oxCCO] of the human forearm while the 1064 nm laser sustained the increases longer, and that (ii) the 810-nm LED with a moderate irradiance (≈135 mW/cm2) induced measurable and significant rises in Δ[HbO] and Δ[oxCCO] with respect to the sham stimulation on the human forearm.

Transcranial photobiomodulation (tPBM) is a noninvasive neuromodulation method that facilitates the improvement of human cognition. However, limited information is available in the literature on the wavelength-and site-specific effects of prefrontal tPBM. Moreover, 2-channel broadband near-infrared spectroscopy (2-bbNIRS) is a new approach for quantifying infra-slow oscillations (ISO; 0.005 to 0.2 Hz) of neurophysiological networks in the resting human brain in vivo.Aim: We aim to prove the hypothesis that the hemodynamic and metabolic activities of the resting prefrontal cortex are significantly modulated by tPBM and that the modulation is wavelength-and site-specific in different ISO bands.Approach: Noninvasive 8-min tPBM with an 800-or 850-nm laser or sham was delivered to either side of the forehead of 26 healthy young adults. A 2-bbNIRS unit was used to record prefrontal ISO activity 7 min before and after tPBM/sham. The measured time series were analyzed in the frequency domain to determine the coherence of hemodynamic and metabolic activities at each of the three ISO frequency bands. Sham-controlled coherence values represent tPBM-induced effects on neurophysiological networks.Results: Prefrontal tPBM by either wavelength and on either lateral side of the forehead (1) increased ipsilateral metabolic-hemodynamic coupling in the endogenic band and (2) desynchronized bilateral activity of metabolism in the neurogenic band and vascular smooth-muscle hemodynamics in the myogenic band. Site-specific effects of laser tPBM were also observed with significant enhancement of bilateral hemodynamic and metabolic connectivity by the right prefrontal 800-nm tPBM.Conclusions: Prefrontal tPBM can significantly modulate neurophysiological networks bilaterally and coupling unilaterally in the human prefrontal cortex. Such modulation effects are site-and wavelength-specific for each ISO band.