Effects of online motor cortex transcranial ultrasound stimulation on interhemispheric inhibition

Transcranial ultrasound stimulation (TUS) of the motor cortex (M1) has been found to modulate interhemispheric inhibition (IHI) after sonication (offline effect). However, the effects of M1 TUS on IHI during sonication (online effect) remain unknown. The current study examined the effects of online M1 TUS on IHI from the sonicated M1 to the contralateral M1 and on the contralateral M1 excitability in 15 healthy subjects. IHI was measured with paired-pulse transcranial magnetic stimulation (TMS) at 10 ms (short-latency IHI, SIHI) or 40 ms (long-latency IHI, LIHI) interstimulus interval (ISI), and the contralateral M1 excitability was measured with single-pulse TMS. The results showed that online TUS decreased the sonicated M1 to the contralateral M1 at LIHI but not at SIHI, while the contralateral TUS M1 excitability was unchanged. TUS may modulate complex circuits that mediate LIHI. These findings showed that TUS can be used to investigate the functional interactions between the two hemispheres. KEY POINTS: Offline non-invasive brain stimulations have been shown to modulate interhemispheric inhibition. The current study examined the effects of online primary motor cortex (M1) transcranial ultrasound stimulation (TUS) on interhemispheric inhibition from the sonicated M1 to the contralateral M1, and the contralateral M1 excitability. TUS decreased long- but not short-latency interhemispheric inhibition from the sonicated M1 to the contralateral M1. TUS could be a tool for investigating functional interactions between the two hemispheres.