Abstract
Transcranial focused ultrasound is an emerging form of non-invasive neuromodulation that uses acoustic energy to affect neuronal excitability. The effect of ultrasound on human motor cortical excitability and behavior is currently unknown. We apply ultrasound to the primary motor cortex in humans using a novel simultaneous transcranial ultrasound and magnetic stimulation paradigm that allows for concurrent and concentric ultrasound stimulation with transcranial magnetic stimulation (TMS). This allows for non-invasive inspection of the effect of ultrasound on motor neuronal excitability using the motor evoked potential (MEP). We test the effect of ultrasound on single pulse MEP recruitment curves and paired pulse protocols including short interval intracortical inhibition (SICI) and intracortical facilitation (ICF). In addition, we test the effect of ultrasound to motor cortex on a stimulus response reaction time task. Results show ultrasound inhibits the amplitude of single-pulse MEPs and attenuates intracortical facilitation but does not affect intracortical inhibition. Ultrasound also reduces reaction time on a simple stimulus response task. This is the first report of the effect of ultrasound on human motor cortical excitability and motor behavior and confirms previous results in the somatosensory cortex that ultrasound results in effective neuronal inhibition that confers a performance advantage.
Highlights
Transcranial focused ultrasound is an innovative approach to noninvasive neuromodulation that uses focused mechanical energy to provide high spatial resolution targeting of distinct cortical areas[1]
To assess the effect of ultrasound to M1 we have developed a novel method of simultaneous transcranial ultrasound and magnetic stimulation that pairs concurrent and concentric delivery of focused ultrasound with transcranial magnetic stimulation (TMS)
Increasing the standoff height of the coil 1.25 cm from the head weakens the strength of the induced electric field to roughly 70% (4.71 vs. 3.31 V/cm2) of the field produced with the TMS coil directly on the scalp and increases the spread of the induced electric field at the level of the cortical surface holding constant the current fed through the TMS coils (Fig. 3A)
Summary
Transcranial focused ultrasound (tFUS) is an innovative approach to noninvasive neuromodulation that uses focused mechanical energy to provide high spatial resolution targeting of distinct cortical areas[1]. TFUS directed at the thalamus resulted in an attenuation of somatosensory evoked potentials generated in the ventroposterior-lateral nucleus and serially connected cortical regions[13] We extend these findings to the human primary motor cortex (M1). To assess the effect of ultrasound to M1 we have developed a novel method of simultaneous transcranial ultrasound and magnetic stimulation that pairs concurrent and concentric delivery of focused ultrasound with transcranial magnetic stimulation (TMS) This pairing is advantageous as it provides for assessment of ultrasound on established and well-understood TMS metrics like the motor evoked potential (MEP). Using different TMS methods such as short interval intracortical inhibition (SICI) and intra-cortical facilitation (ICF) that probe different motor micro-circuitry[15] in addition to assessing a simple motor task we hope to gain a better understanding of the effect of tFUS in human motor cortex and the neuronal circuits that ultrasonic mechanical energy affects
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