Influence of current direction on short-interval intracortical facilitation probed with paired-pulse TMS

Background: Transcranial magnetic stimulation (TMS) of the human primary motor hand area (M1-HAND) can produce multiple descending volleys in fast-conducting corticospinal neurons, especially so-called I waves which result from trans-synaptic excitation. The facilitatory interaction between these I waves can be studied with paired-pulse TMS using a conditioning-test paradigm referred to as short interval intracortical facilitation (SICF). Here, we examined whether SICF depends on the current direction of the TMS pulses. Methods: In ten healthy volunteers aged 22–29, we applied paired-pulse TMS to right M1-HAND a…