EEG preprocessing

All EEG data were visually controlled for EOG and EMG artifacts and affected epochs were excluded. For each movement, a 10 s-epoch (5 s before, 5 s after movement-onset) was analyzed. Reference-free EEG data were obtained by computing the local average reference [13]. For analysis reported in this paper only data (referential and local average) of one electrode position over the left sensorimotor region (located 2.5 cm posterior and 2.5 cm medial to C3, see Fig. 3) was selected, since it revealed maximal reactivity in the frequency range around 40 Hz.