|ELECTRONIC SUPPLEMENTARY MATERIAL
Participants were free from any contraindication to TMS (Wasserman, 1998; Rossi et al., 2009). All participants gave their written informed consent prior to their inclusion on the study and were naïve as to its purpose. Specific information concerning the study was provided after the experimental session was terminated. The experimental procedures were approved by the Ethics Committee of the University of Padova and were carried out in accordance with the principles of the 1964 Declaration of Helsinki.
Each participant was tested in a single experimental session lasting approximately 40 minutes. Testing was carried out in a sound-attenuated Faraday room. Participants were seated in a comfortable armchair with their head positioned on a fixed head rest so that the eye–screen distance was 80 cm. The right arm was positioned on a full-arm support, while the left arm remained relaxed with the hand resting on the legs. During the first five seconds of the rest period, a message informing the participants to keep their hand still and fully relaxed was presented. Such a message was replaced by a fixation cross for the remaining five seconds. The task was to pay attention to the visual stimuli presented on a 19” monitor (resolution 1280 x 1024 pixels, refresh frequency 75 Hz, background luminance of 0.5 cd/m2) set at eye level. The animation effect was obtained by presenting series of single frames each lasting 33 ms (resolution 720 x 576 pixels, color depth 24 bits, frame rate 30 fps) plus the first and last frames which lasted 500 and 1000 ms, respectively. The presentation of stimuli and the timing of TMS stimulation were managed by E-Prime V2.0 software (Psychology Software Tools Inc., Pittsburgh, PA, USA) running on a PC.
TMS and MEP recording
The coil was angled 45° relative to the interhemispheric fissure and perpendicularly to the central sulcus with the handle pointing laterally and caudally (Brasil-Neto et al., 1992). This orientation induced a posterior-anterior current in the brain, which tends to activate corticospinal neurons indirectly via excitatory synaptic inputs (Di Lazzaro et al., 1998). Electromyographic (EMG) recording was performed through pairs of 9 mm diameter Ag-AgCl surface electrodes. The active electrodes were placed over the belly of the right ADM and FDI muscles and the reference electrodes over the ipsilateral proximal interphalangeal joint (belly-tendon technique). Electrodes were connected to an isolated portable ExG input box linked to the main EMG amplifier for signal transmission via twin fiber optic cable (Professional BrainAmp ExG MR, Brain Products, Munich, Germany). The ground was placed over the participants’ left wrist and connected to the common input of the ExG input box. The raw myographic signals were bandpass filtered (20 Hz-1 kHz) and amplified prior to being digitized (5 kHz sampling rate), and stored on a computer for off-line analysis. In order to prevent contamination of MEP measurements by background EMG activity, trials in which any EMG activity greater than 100 µV was present in the 100 ms window preceding the TMS pulse were discarded. EMG data were collected for 200 ms after the TMS pulse. Prior to video presentation, baseline corticospinal excitability was assessed by acquiring 5 MEPs while the participants passively watched a white-colored fixation cross on black background on the computer screen. Another series of 5 MEPs was recorded at the end of the experimental session. Comparisons of MEP amplitudes for the two series allowed us to check for any corticospinal excitability change related to TMS per se. The average amplitude of the two series allowed us to set the individual baseline for data normalization procedures. Held by a tripod, the coil was continually checked by the experimenters to maintain consistent positioning. The resting motor threshold (rMT) was determined for each participant as the minimum stimulation intensity producing reliable MEPs ( ≥ 50 μV peak-to-peak amplitude) in a relaxed muscle in five out of ten consecutive trials. The stimulation intensity during the recording session was 110% of the rMT and ranged from 38% to 59% (mean 48.5%) of the maximum stimulator output intensity. MEPs were recorded simultaneously from electrodes placed over the contralateral ADM and FDI muscles.
Kinematical analysis of the human model’s action
To create the stimulus material, we filmed and analysed a model performing a reach-to-grasp action towards either a small or large target object (small = 2 cm in diameter; large = 11 cm in diameter) presented in isolation or flanked by a distractor of a similar (e.g., 2 cm) or different (e.g., 11 cm) size. Trials were recorded with a digital video camera. A digitising technique (VIDEOTRACK, ABACUS srl, Italy) was used to extract the kinematics of the model from the videos. Paired t-tests were performed to compare the no-distractor with the distractor conditions. On the basis of previous literature (for review see Castiello, 1999; Tipper et al., 1998) the dependent variables thought to be specifically relevant were movement duration (MD), deceleration time (DT; the time from the moment of peak velocity to the end of the movement) and the maximum hand aperture calculated as the distance between the thumb and the index finger (AGAI) and the thumb with the little finger (AGAL). These two fingers were considered because they corresponded to those specifically targeted in the TMS study. These variables were chosen because consistent results within the reach-to-grasp literature have shown that the reach-to-grasp movement is dependent upon the size of the stimuli. In particular, MD and DT are shorter, and AGAI and AGAL are greater for larger than for smaller stimuli (Jakobson & Goodale, 1991). Thus, if the results will show shorter MD and DT and larger AGAI and AGAL in the presence of a large distractor, inferences regarding the influence of the distractor on the kinematics of the model for movements towards the smaller target could be advanced and viceversa (Castiello, 1999). The dependent measures that were investigated showed a significant change in this direction. For the condition in which the small target was presented alongside a large distractor, the classic kinematic patterning that characterizes the reach-to-grasp for small objects (target) was modified according to the classic kinematic pattern that characterizes the reach-to-grasp for larger objects (large distractor). MD and DT were shorter for movements directed to the smaller target in the presence of a large distractor than when presented alone (MD: 776 ± 82 vs. 854 ± 87 ms, t9 = 8.32, p < .001; DT: 582 ± 61 vs. 640 ± 70 ms, t9 = 7.13, p < .002). AGAI and AGAL for the small target were greater in the presence of the large distractor than for the no distractor condition (AGAI: 52 ± 2 vs. 45 ± 3 mm; t9 = 9.01, p < .001; AGAL: 74 vs. 70 mm; t9 = 11-03, p < .001). For the condition in which the large target was presented alongside a small distractor, the classic kinematic patterning that characterizes the reach-to-grasp for large objects (target) was modified according to the classic kinematic pattern that characterizes smaller objects (small distractor). MD and DT were longer for movements directed to the larger target in the presence of a small distractor than when presented alone (MD: 832 ±78 vs. 668 ±70 ms, t9 = 10.11, p < .001; DT: 599 ± 64 vs. 480 ± 57 ms, t9 = 9.53, p < .001). AGA and AGAL for the large target were smaller in the presence of the small distractor than for the no distractor condition (AGAI: 126 ± 4 vs. 131 ± 3 mm, t9 = 12.21, p < .001; AGAL: 140 ± 11 vs. 152 ± 14 mm, t9 = 8.67, p < .001). No differences were detected when a distractor of a similar size as the target was presented (ps > 0.05). Therefore the TMS paradigm did not include the condition in which the distractor was of a similar size as the target. Furthermore, preliminary analyses as to ascertain whether the position of the distractor (left or right of the target) determined different results, indicated that this factor played no role. Therefore the factor distractor position was collapsed within the analyses regarding the TMS data.
Mean raw MEP amplitudes during the 2 baseline blocks administered at the beginning and the end of the experimental session were not significantly different for either the ADM (466 vs. 351 μV, respectively; t29 = 1.56, p = 0.13) or the FDI (1018 vs. 895 μV, respectively; t29 = 1.77, p = 0.27) muscles.
Brasil-Neto, J. P., Cohen, L. G., Panizza, M., Nilsson, J., Roth, B. J., & Hallett, M. 1992 Optimal focal transcranial magnetic activation of the human motor cortex: effects of coil orientation, shape of the induced current pulse, and stimulus intensity. J. Clin. Neurophysiol., 9, 132–136.
Castiello, U. 1999 Mechanisms of selection for the control of hand action. Trends Cog. Sci., 7, 264–271.
Di Lazzaro, V., Oliviero, A., Profice, P., Saturno, E., Pilato, F., Insola, A., et al. 1998 Comparison of descending volleys evoked by transcranial magnetic and electric stimulation in conscious humans. Electroencephal. Clin. Neurophysiol., 109, 397–401.
Jakobson and Goodale
Rossi, S., Hallett, M., Rossini, P. M., & Pascual-Leone, A. 2009 Safety, ethical considerations, and application guidelines for the use of transcranial magnetic stimulation in clinical practice and research. Clin. Neurophysiol., 120, 2008–2039.
Tipper, S. P., Howard, L. A. & Houghton, G. 1998 Action-based mechanisms of attention. Philos. Trans. R. Soc. Lond. B. Biol. Sci., 353, 1385–1393.
Wassermann, E. M. 1998 Risk and safety of repetitive transcranial magnetic stimulation: report and suggested guidelines from the International Workshop on the Safety of Repetitive Transcranial Magnetic Stimulation, June 5-7, 1996. Electroencephal. Clin. Neurophysiol., 108, 1–16.