TMS was implemented using a MagStim Rapid system (The Magstim Company, Dyfed, UK) with a custom-built MR-compatible figure-of-eight stimulation coil (two windings of ten turns each; inner wing diameter 53 mm, distance between outer coil surface and windings of 2-3 mm (variation due to manufacturing tolerance); coil inductance, including cable, of 20 μH; maximal current at 100% stimulator output of ∼5kA). The stimulation unit was housed inside the scanner room in a shielded cabinet, from which the stimulation coil cable was fed through a custom filter box (The Magstim Company). Residual RF transmission along the coil cable was further suppressed using ferrite sleeves. The TMS coil was connected to the stimulator in parallel to a high voltage relay (Magstim ES9486, The Magstim Company). During EPI acquisition, the relay was in closed mode, thereby effectively preventing any residual leakage in current flow from the stimulator. The relay was opened 50 ms prior to TMS pulse discharge, and closed again 8 ms after termination of the last TMS pulse of a trial. The relay and TMS were controlled with a unit developed in-house based on a BASIC Stamp 2 micro-controller (Parallax Inc., Rocklin, California, USA). TMS pulses were applied during the dead time between the EPI navigator echoes and the EPI data readout, and separated from RF slice excitation pulses (Gaussian-like symmetric sync, 2560 ms duration, Bestmann and others 2003a (link)). Throughout scanning, each slice coincided equally often with TMS pulses to avoid any systematic influences on slice-by-slice variance.
The stimulation site over the left dorsal PMd was determined as the scalp point 2 cm anterior and 1 cm medial to the so-called motor ‘hot-spot’ for evoking single muscle twitches in the contralateral first dorsal interosseous (Schluter and others 1999; Johansen-Berg and others 2002 (link); O’Shea and others, 2007 (link)). Inside the scanner, the stimulation coil was placed over the marked location using an MR-compatible custom-built coil holder, allowing stable positioning of the TMS coil with several degrees of freedom. The coil was oriented tangential to the scalp, at approximately 45 degrees from the midline, inducing a biphasic current with an initial antero-posterior induced direction. Foam-padded cushions were used to restrict head-movements.
All visual stimulation, grip-force data acquisition, TMS triggering and intensity regulation, and relay settings were controlled using the toolbox Cogent 2000 (Wellcome Department of Imaging Neuroscience, London, UK;http://www.fil.ion.ucl.ac.uk/cogent ) running under Matlab (The Mathworks, Natick, Massachusetts, USA). Participants wore earplugs (SNR = 36dB) to reduce acoustic noise from the scanner and the TMS discharge sound.
The stimulation site over the left dorsal PMd was determined as the scalp point 2 cm anterior and 1 cm medial to the so-called motor ‘hot-spot’ for evoking single muscle twitches in the contralateral first dorsal interosseous (Schluter and others 1999; Johansen-Berg and others 2002 (link); O’Shea and others, 2007 (link)). Inside the scanner, the stimulation coil was placed over the marked location using an MR-compatible custom-built coil holder, allowing stable positioning of the TMS coil with several degrees of freedom. The coil was oriented tangential to the scalp, at approximately 45 degrees from the midline, inducing a biphasic current with an initial antero-posterior induced direction. Foam-padded cushions were used to restrict head-movements.
All visual stimulation, grip-force data acquisition, TMS triggering and intensity regulation, and relay settings were controlled using the toolbox Cogent 2000 (Wellcome Department of Imaging Neuroscience, London, UK;
