Transcranial near-infrared stimulation may increase cortical excitability recorded in humans

Abstract Objective Transcranial near-infrared stimulation (tNIRS) can penetrate the scalp and skull and can reach the superficial layers of the cerebral cortex. In this study, we evaluated whether an 820-nm tNIRS can modulate the excitability of the primary motor cortex (M1) as measured by transcranial magnetic stimulation (TMS). Methods tNIRS was applied at a wavelength of 820-nm for 4 min over the representation of the right first dorsal interosseous muscle (FDI) in the left M1, in 12 healthy right-handed participants. Motor evoked potentials (MEPs) from the FDI elicited by single-pulse TMS were measured at baseline and up to 30 min after the tNIRS. Results The ΔMEPs percentage was evaluated, and a significant increase in amplitude was observed 10−20 min post-stimulation compared to that in sham. Conclusion This study provides evidence that an 820-nm tNIRS induces transitory increase in the excitability of the stimulated cortex.