Delay- and Pressure-Dependent Neuromodulatory Effects of Transcranial Ultrasound Stimulation

Objective: Despite the growing interest in transcranial focused ultrasound stimulation (TUS), our understanding of its underlying mechanisms remains limited. In this study, we aimed to investigate the effects of TUS on several functional magnetic resonance imaging metrics by considering their latency, duration, and relationship with applied acoustic pressure. Materials and methods: We recruited 22 healthy volunteers and used a pre- vs post-TUS protocol. Half of the volunteers were stimulated in the right inferior frontal cortex and the other half in the right thalamus. The fractional amplitudes of low-frequency fluctuations, regional homogeneity, degree centrality, local functional connectivity density, and eigenvector centrality were considered. These metrics were compared before TUS and at three different time points in the first hour after TUS. Results: Our results showed that 1) TUS primarily alters functional connectivity metrics at both the local and global levels; 2) stronger alterations are observed when the delay after TUS increases and 3) when the applied acoustic pressure is close to the maximum. Conclusion: These results suggest that some consequences of TUS might not be immediate, inviting us to revise the premise that TUS consequences are immediate and will progressively disappear.