THETA-BURST STIMULATION OVER DORSAL AND VENTRAL PREMOTOR CORTICES DIFFERENTIALLY MODULATE MOTOR CORTEX EXCITABILITY AND INTERHEMISPHERIC INHIBITION

Distinct regions of premotor cortex impact primary motor cortex (M1) excitability. Our previous work showed that dorsal (PMd) and ventral (PMv) premotor cortices exhibit distinct influences on contralateral M1 excitability. Yet, it remains unclear whether the distinct impact of PMd and PMv on contralateral M1 can be modulated with non-invasive brain stimulation. Here, we examined the ability of theta-burst stimulation (TBS) over PMd and PMv to bidirectionally modulate M1 excitability, and the potential interhemispheric inhibition (IHI) mechanisms underlying these effects. Twenty-two right-handed adults (12F; aged 18-40) participated in five TBS conditions delivered to the right hemisphere: 1) continuous TBS (cTBS) over PMd, 2) cTBS over PMv, 3) intermittent TBS (iTBS) over PMd 4) iTBS over PMv and 5) sham cTBS. Single-pulse TMS measured M1 excitability bilaterally. Dual-site TMS assessed IHI via conditioning stimuli over right PMd/PMv and test stimuli over left M1 at interstimulus intervals (ISI) of 0, 10, and 50 ms. M1 excitability and IHI were recorded pre- and post-TBS. Our results demonstrate two main findings: 1) cTBS over PMd modulated bilateral M1 excitability, and 2) iTBS over PMv modulated bilateral M1 excitability. Specifically, cTBS over PMd increased left and right M1 excitability. Whereas iTBS over PMv decreased left M1 excitability and increased right M1 excitability. Finally, cTBS over PMd decreased IHI at the 10 and 50 ms ISIs. Our findings demonstrate that TBS over PMd and PMv can bidirectionally modulate M1 excitability, and the potential role of intra- and inter-hemispheric mechanisms underpinning such modulation.