Interactive effect between transcranial focused ultrasound and transcranial magnetic stimulation on human motor cortex

Objective: Transcranial focused ultrasound (TUS) can suppress human motor cortical excitability. However, it is unclear whether the TUS may interact with transcranial magnetic stimulation (TMS) when they co-delivered in multiple trials.

Methods: Nineteen subjects received three different TUS-TMS co-stimulation protocols to the motor cortex including concurrent stimulation (TUS-TMS-C), separated stimulation (TUS-TMS-S), and TMS only. In each condition, two runs of 30 stimulation trials were conducted with a five-minute rest between runs. Motor-evoked potentials (MEP) were recorded during stimulation and at 0, 10, 20, and 30 min after stimulation. The MEP amplitudes after intervention were normalized to the mean pre-intervention MEP amplitude and expressed as MEP ratios. An additional test with TUS alone was applied to all participants to assess whether TUS itself can elicit after-effects.

Results: There were no significant after-effects of all three interventions on MEP ratios. However, 11 subjects who showed online inhibition (OI + ) during the TUS-TMS-C protocol, defined as having MEP ratio less than 1 during TUS-TMS-C, showed significant MEP suppression at 10, 20 and 30 min after TUS-TMS-C. In 8 subjects did not show online inhibition (OI-), defined as having MEP ratios greater than 1 during TUS-TMS-C, showed no significant inhibitory after-effects. OI + and OI- status did not change in a follow-up repeat TUS-TMS-C test. TUS alone did not generate inhibitory after-effects in either OI + or OI- participants.

Conclusions: Our results showed that co-delivery of TUS and TMS can elicit inhibitory after-effect in subjects who showed online inhibition, suggesting that TUS and TMS may interact with each other to produce plasticity effects.

Significance: TUS and TMS may interact with each other to modulate cortical excitability.