Paired Pulse TMS and the Likelihood of Eliciting Motor Evoked Potentials

Single page posterThe excitability of the corticospinal pathway changes in a dynamic manner. Here, we used paired-pulse transcranial magnetic stimulation (ppTMS) to test the hypothesis that rapid changes within motor cortex determine the likelihood the corticospinal pathway will become active. In six participants, pairs of electromagnetic pulses separated by 3ms were administered over primary motor cortex to induce a twitch, or motor evoked potential (MEP), in a target finger muscle. The first ‘conditioning’ pulse was changed dynamically to a percentage of a subject’s active motor threshold (AMT, the minimum TMS intensity to consistently elicit MEPs during a mild tonic contraction). The second ‘test’ pulse, produced the MEP. Using electromyography to record muscle electrical activity, we calculated MEP sizes of the course of 100 measurements, with 20 measurements at each 5 conditioning pulse intensities. We observed a recurring pattern of an increased likelihood of eliciting an MEP at higher conditioning pulses. Conditioning pulses at 95% of the subject’s AMT produced MEPs 85% of the time, whereas at 50% AMT, MEPs were only detected on less than 40% of attempts. This finding suggests that momentary increases of excitability within motor cortex determine the likelihood of activating the corticospinal pathway and helps shed light on the mechanisms by which motor cortex mediates corticospinal output to muscles