Disentangling transcranial and auditory sources of the transcranial ultrasonic stimulation induced suppression of corticospinal excitability

Symposium title Abstract Human studies have shown that online transcranial ultrasonic stimulation (TUS) suppresses motor evoked potentials (MEPs) elicited using transcranial magnetic stimulation over the primary motor cortex (M1). Animal studies suggest that the neuromodulatory effects of TUS may be confounded by a neural response to the auditory co-stimulation, especially when the ultrasound is pulsed. Therefore, the aim of this work is to fill a gap in the literature by systematically examining whether the auditory co-stimulation contributes to any observed MEP suppression in humans. We tested 12 healthy right-handed adults (23 ± 2.9 yrs, 11 F and 1 M). Neuronavigated TUS was applied using a 2-element 250 kHz annular array (CTX-250-2chan, NeuroFUS Pro, Brainbox UK). We used 1000 Hz pulse repetition frequency (PRF), 30% duty cycle, and 400 ms sonication duration, and three extracranial intensities (spatial peak temporal averaged): 3, 6 and 9 W/cm2. The effects of TUS were quantified using TMS-EMG. We stimulated the hand area of M1 and an active control site deeper in the white matter (WM). We examined all conditions with and without a continuous masking sound (pure tone at PRF + white noise) played via bone conduction headphones. Bayesian repeated measures analysis with the factors Masking Sound (on vs. off), TUS Intensity (3 vs. 6 vs. 9 W/cm2) and Target (M1 vs. WM) revealed a Bayes Factor (BF10) of 15.88 for ‘Mask’ suggesting that TUS-induced MEP suppression is considerably lower in the presence of a masking sound. Since the masking procedure was not effective in all participants, we will acquire additional data with improved masking procedures. Our pilot data suggest that the observed effects of TUS on MEPs are, in part, explained by the auditory confound. This finding underscores the need to eliminate or mask the auditory confound in order to study the direct effects of TUS itself. Research Category and Technology and Methods Basic Research: 6. Pulsed Ultrasound (pUS) Keywords: Auditory co-stimulation, TUS, Masking