Background: Interest in low-intensity transcranial focused ultrasound (tFUS) has led to discovery of its potential neuromodulatory properties, which hold impact for both neurotherapeutics and brain mapping efforts. Compared to existing non-invasive brain stimulation techniques, tFUS offers higher spatial resolution and deeper tissue penetration. In recent years, tFUS has been used to target non-specific activity in human somatosensory and visual cortices, but has not been tested on the motor cortex.
Objective: In this pilot study, we aim to better understand the neuromodulatory capabilities of transcranial focused ultrasound (tFUS) by comparing its effects on the human motor cortex to transcranial magnetic stimulation (TMS).
Methods: 6 healthy subjects received stimulation with single-pulse TMS and tFUS targeted to the left motor cortex hand representation areas. Surface electromyogram (EMG) was recorded from the right first dorsal interosseous (FDI) during resting state for motor evoked potential (MEP) trials and during 20% maximal voluntary contraction for cortical silent period (CSP) trials. TMS-induced MEPs were recorded before and after tFUS to evaluate plasticity changes.
Results: At the low-frequency pulse protocol used in these experiments, there were no observable MEPs or CSPs with tFUS. TMS-induced MEP amplitude was significantly reduced after tFUS in one subject, but there were no significant differences in other subjects.
Conclusions: While we did not observe an effect of tFUS on excitatory MEPs and inhibitory CSPs in this study, we have only tested this technique on 6 individuals. Additionally, the ultrasound parameters used for this study, while safe, may not have transmitted enough acoustic energy to stimulate the motor cortex. It may also be possible that tFUS is capable of modulating somatosensory and visual neural activity in humans, but is not suited for motor applications.