No group-level changes in corticospinal excitability following low-intensity theta-burst ultrasound stimulation of the primary motor hand area

Abstract Low-intensity transcranial ultrasound stimulation (TUS) is a novel non-invasive brain stimulation technique offering high spatial precision and depth penetrance. Theta burst TUS (tbTUS), featuring a temporal theta burst stimulation pattern, has been reported to facilitate corticomotor excitability, when applied to the human primary motor hand area (M1-HAND). However, replication outcomes have varied, pointing to unresolved dependencies in execution and protocol design. Fifteen healthy human participants underwent neuronavigated tbTUS targeting the left M1- HAND in the anterior wall of the precentral sulcus, as well as anterior and posterior active control sites. The three tbTUS conditions were applied in counterbalanced order on separate days. Corticospinal excitability was assessed via motor evoked potentials (MEPs) recorded before and 5, 15, 30, and 60 min after tbTUS, and analyzed using an rmANOVA and a linear mixed model. None of the tbTUS conditions produced consistent group-level changes in MEP amplitude at any time point. Both intra- and inter-subject variability were high, and individual MEP changes following precentral tbTUS did not meaningfully correlate with changes after control-site stimulation. We found no evidence that tbTUS targeting the anterior wall of the central sulcus reliably facilitated corticospinal excitability under the specific experimental conditions employed in the present study. Methodological and hardware differences may account for discrepancies across studies. To reliably modulate cortical excitability, future tbTUS research requires individualized acoustic modeling, harmonized protocols, and adequately powered designs.