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.