A specific transcranial near-infrared stimulation parameter combination targeting the motor cortex improves cortical excitability and hemodynamics

Transcranial near-infrared stimulation (tNIRS) is an emerging, light-based, non-invasive neuromodulation technique with great potential to improve functions like cognition and motor performance. Cortical excitability and hemodynamic changes represent key neurophysiological mechanisms of tNIRS effects. The selection of tNIRS parameters is closely associated with their functional effects, and the neurophysiological alterations induced in the human cerebral cortex by different parameter combinations warrant further exploration. Twenty-two healthy participants received four types of active tNIRS and a Sham condition of tNIRS over the left motor cortex in a randomized order, which included five stimulus conditions: Sham, 810-nm continuous wave (CW), 810-nm 40-Hz pulsed wave (PW), 1064-nm CW, and 1064-nm 40-Hz PW. Each session was divided into three phases, namely pre-stimulation, stimulation, and post-stimulation. Changes in cortical excitability were assessed by recording motor evoked potentials (MEPs) before and for up to 30 min after stimulation. Concurrently, behavioral performance and cortical hemodynamic changes induced by tNIRS were evaluated using functional near-infrared spectroscopy (fNIRS) during a finger-opposition task. Compared to the Sham condition, all active tNIRS protocols induced an increase in MEPs recorded from the right abductor pollicis brevis muscle. At 30 min post-stimulation, the 1064-nm 40-Hz PW condition elicited significantly larger MEP amplitudes than both the 810-nm CW and 1064-nm CW conditions. For behavior, the number of completed cycles during the right-hand finger-opposition task was significantly higher in the 1064-nm 40-Hz PW condition compared to the Sham condition. Regarding cortical hemodynamics, the 1064-nm 40-Hz PW condition showed decreased activation in broad regions of the frontal and motor cortices during the left-hand finger-opposition task compared to baseline. Conversely, the 810-nm 40-Hz PW condition exhibited decreased hemodynamic activation only in the motor cortex during the right-hand finger-opposition task. The 40 Hz pulsed tNIRS protocol with a wavelength of 1064 nm, irradiance of 120 mW/cm 2 , duration of 20 min, and stimulation area of 0.24 cm 2 induced more pronounced changes in cortical excitability and hemodynamics. This enhanced effect may be attributed to the more significant neurophysiological cumulative response elicited by this specific parameter combination. This specific parameter set represents a promising candidate for future clinical applications of tNIRS.