At the beginning of the experiment, the EMG electrode location was determined during contraction to isolate one motor unit in the recorded activity. Surface EMG recordings are non-invasive, and do not cause any damage to muscle tissue, which in turn influences the motor unit potential (De Luca et al., 2006). Nevertheless, only the largest motor units with the lowest firing thresholds could be investigated. Visual feedback (EMG activity was displayed on an oscilloscope) and auditory feedback (a sound was triggered each time the motor unit

potential occurred in the EMG) helped the subjects to maintain a constant motor unit discharge of ∼10 Hz for studying the effects of TMS on the motor unit firing rate (Bawa & Lemon, 1993). TMS was delivered through a figure-of-eight coil (70 mm), generating postero-anterior learn more (PA) currents in the primary motor cortex (with the handle orientated in an anterior, antero-medial or antero-lateral axis depending on the subject), at the optimal site GSK3235025 in vivo (hot spot) for evoking an MEP in the contra-lateral

FDI EMG. The coil was connected to a Bistim module combining two stimulators (Magstim 200; Magstim Company Ltd, Whitland, UK), to provide paired pulses at a 2-ms interval through the same coil. The 2-ms interval, known to evoke strong SICI (Fisher et al., 2002; Roshan et al., 2003), was kept constant throughout the experiment. The optimal coil position was marked on the scalp and, for protocol 2, TMS was assisted by the navigated brain stimulation (NBS) system (Nexstim, Helsinki, Finland), using a standard magnetic resonance imaging brain scan of each individual (http://www.nexstim.com). The NBS system uses a sophisticated algorithm to predict the actual location of the stimulating electric fields in the cortex, and to keep the coil location constant throughout the experiment. TMS intensity was adjusted in relation to the resting motor threshold (RMT), which was the lowest intensity for evoking an MEP of ∼50 μV in at least 50% of trials. The method

is explained fully in Pierrot-Deseilligny & Burke (2005). Briefly, EMG activity was displayed on an oscilloscope to monitor the shape of the investigated motor unit during the experiment. In parallel, the EMG signal was conveyed to a window discriminator with variable see more trigger levels, which converted the motor unit potential into a standard pulse (3-ms duration, 5-V amplitude); the trigger level position was constant throughout the experiment (Kirkwood & Sears, 1978). Each time the motor unit potential occurred in the EMG activity, the window discriminator delivered a pulse. The pulses were conveyed to a computer, which generated a histogram of the discharge (0.5-ms bin width), according to the latency after a delay R1 relative to the previous pulse; 0 ms in the PSTH thus corresponds to the delay R1.