Plasticity induced by pairing brain stimulation with motor-related states only targets a subset of cortical neurones

Movement-related brain stimulation (MRBS) interventions associate endogenously generated movement-related brain states with external brain stimuli to induce targeted plastic changes in the motor cortex (M1) [1e4]. These studies have emphasised the importance of the timing of stimulation relative to movement onset. However, none has examinedwhether the effects are specific to the cortical circuits activated by the stimuli. The question arises because previous work has shown that different sets of inputs to corticospinal neurones can be activated using TMS. Stimulation with a posterior-anterior (PA) direction activates a set of neurones that have a shorter latency connection to corticospinal neurones than those activated with an anteriorposterior (AP) current [5]. Previous MRBS studies have paired movement onset with PA pulses [1]. The present work tests whether the after-effects of MRBS are specific to PA-sensitive neurones, or whether those activated by AP pulses are also affected. Here we applied AP or PA TMS pulses applied just prior to the onset of volitional index finger movements in two experiments conducted on separate days in the same group of individuals [3]. Corticospinal excitability changes induced by these interventions were assessed using AP and PA TMS pulses in the effector muscle and in a control muscle. Twenty right-handed volunteers (12 males; ages 20e40) participated in two sessions on different days. Participants filled awritten consent form approved by our local ethics committee and following the Declaration of Helsinki. None of the participants had contraindications to TMS. Participants sat in front of a computer screen with their righthand index finger resting on a keypad. EMG was acquired from the right hand first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles. Signals were amplified, band-pass filtered (20e2000Hz, Digitimer-D360, Digitimer Ltd, UK), digitised at 5kHz and stored using a Power1401 DAQ controlled with Signal6.2 (CED, UK). TMS was delivered with a 70mm coil (MagPro-TMS, MagVenture, Inc., GA, USA) over the M1-FDI “hot-spot”, which was defined