Allahverdloo, E.; Chiu, L. K.; O'Farrell, A.; Harroum, N.; Dancause, N.; Neva, J. L.

Dorsal (PMd) and ventral (PMv) premotor cortices can modulate contralateral primary motor cortex (M1) excitability, but their distinct interhemispheric influence via transcranial magnetic stimulation (TMS) remains unclear. Single-pulse TMS over PMd, PMv and M1 assessed transcallosal inhibition via the ipsilateral silent period (iSP). Dual-site TMS examined short-(10 ms inter-stimulus interval [ISI]), long-(50 ms ISI) and non-callosal-(0 ms ISI) interhemispheric inhibition (IHI). An iSP was elicited from PMd, PMv, and M1, with distinctly evoked iSP parameters. The iSP magnitude was greatest from M1, followed by PMd and then PMv, while iSP duration was greatest for M1 and showed no differences between PMd and PMv. Dual-site TMS revealed that PMd and M1 inhibited contralateral M1 excitability across all ISIs, while PMv showed inhibition at 0- and 50-ms ISIs. PMd and M1 demonstrated greater short-IHI compared to PMv, all demonstrating similar long-IHI, and PMd demonstrating greater non-callosal-IHI than M1. PMv displayed distinct IHI across ISIs, PMd showed differences across most ISIs and M1 demonstrated the fewest differences across ISIs. Longer iSP duration related to greater long-IHI magnitude elicited from PMd and PMv. Our findings demonstrate differential IHI from PMd and PMv on contralateral M1, which may inform neuromodulation strategies in rehabilitation contexts.