On the relation between the interstimulus intervals and multi-muscle nTMS motor mapping

Although the interstimulus interval (ISI) is one of the crucial parameters in the transcranial magnetic stimulation (TMS), the ISI effect on the results of the TMS motor mapping is usually overlooked. This study explored the influence of ISI, ranging from 1.5 to 41 s, on multi-muscle navigated TMS (nTMS) motor mapping results. Twenty-six healthy male volunteers underwent four nTMS motor mapping sessions on two separate days. We mapped the muscles' cortical representations (MCRs) of the five upper limb muscles: abductor pollicis brevis (APB), abductor digiti minimi (ADM), first dorsal interosseous (FDI), extensor digitorum communis (EDC), and biceps brachii (BB). We estimated the relationship between ISIs and trial-to-trial motor evoked potentials (MEPs) amplitudes and MCR areas. In addition, we accounted for the association between the ISI and TMS mapping procedure parameters such as the distance between the successive stimulation points, the number of stimuli in a TMS session, and the stimulus counting number. A weak positive association was observed between: (1) trial-to-trial ISI and MEP amplitude and (2) median ISI and MCR areas. We recommend reporting ISI values in TMS motor mapping studies and monitoring the impact of ISI on MEP amplitudes.