Interactions between pairs of transcranial magnetic stimuli over the human left dorsal premotor cortex differ from those seen in primary motor cortex

A single TMS pulse (110% resting motor threshold, RMT) to the left dorsal premotor cortex (PMd) (CS2) suppresses the amplitude of motor evoked potentials (MEPs) from a test pulse (TS) over the right motor cortex (M1), and facilitates MEPs from the left motor cortex. We probed how this interaction was changed by a prior conditioning pulse over PMd (CS1) using a paired pulse TMS design. In the main experiments, the intensity of CS1 was 80% RMT. Basal suppression of right M1 was removed when CS1-CS2 was 1 ms or 5 ms with a similar tendency at 15 ms. Basal facilitation of left M1 was suppressed at CS1-CS2 of 5 ms. A similar time course was seen if CS2 was increased to 100% RMT, but there was no significant effect if CS1 was 70% RMT. Preconditioning PMd with continuous or intermittent theta burst repetitive TMS (cTBS, iTBS) abolished the basal CS2-TS interaction between premotor and motor cortices. Finally, if very short interstimulus intervals between CS1 and CS2 were explored to detect interactions similar to I-wave facilitation in M1, we found that the basal suppression of right M1 was abolished at CS1-CS2 intervals of 1.8 and 2.8 ms. We suggest that paired pulse TMS may be capable of investigating properties of intrinsic circuits in PMd and that their properties differ from those in the nearby M1. Paired TMS may be a useful method of studying the excitability of intrinsic circuits in non-primary areas of the motor system.