The electroencephalogram (EEG) signals were sampled at 512 Hz and continuously recorded using 32 Ag/AgCl electrodes distributed according to the extended International 10–20 system. Two electrodes of the flat type (above and below the left eye) recorded the eye-blinks. Another two electrodes (external canthi of each eye) recorded horizontal eye-movements. The EEG signal was re-referenced to the mastoids (left and right; baseline).