Effects of Very Low-Intensity Transcranial Ultrasound Stimulation on Corticospinal Excitability and Motor Performance in Healthy Individuals

Objective: Transcranial ultrasound stimulation (TUS) is an emerging noninvasive brain-stimulation tool that is used to modulate neural circuits in humans. We investigated the effects of the application of very low-intensity TUS to the primary motor cortex on corticospinal excitability and motor performance in healthy individuals. Methods: This crossover, randomized, single-blind study included 24 healthy volunteers. In Experiment 1, participants underwent three interventional conditions designed to evaluate the effects of different TUS intensities (16.0 mW/cm², 8.0 mW/cm², and sham) on corticospinal excitability, which was assessed by measuring the motor-evoked potential (MEP) amplitudes elicited by transcranial magnetic stimulation (TMS). In Experiments 2 and 3, participants underwent two interventional conditions to assess the effects of very low-intensity TUS (16.0 mW/cm² and sham) on spinal excitability (Experiment 2) and motor performance (Experiment 3). Spinal excitability was evaluated by measuring the F/M wave amplitude ratios and F-wave persistence using electrical stimulation of the ulnar nerve whereas motor performance was evaluated by asking the participants to complete a simple reaction-time (RT) task to measure RTs. Results: MEP amplitudes significantly decreased immediately after 16.0 mW/cm² TUS, and 15 min after both 16.0 and 8.0 mW/cm² TUS. F/M wave amplitude ratios and F-wave persistence showed no significant changes. RTs significantly increased during both 16.0 mW/cm² and sham TUS, although no significant differences were found between the conditions. Conclusions: Very low-intensity TUS decreased corticospinal excitability without altering spinal excitability or motor performance in healthy individuals.