EEG activity was recorded with a Biosemi ActiveTwo measurement system (BioSemi, Amsterdam, Netherlands) using 64 Ag-AgCl scalp electrodes attached in an elastic cap, arranged according to the standard international 10–20 system. Four external electrodes were additionally attached to the head: left and right mastoids, which were used for later offline re-referencing to the average of these two channels, and a bilateral electro-oculogram (EOG) electrode pair next to the outer canthi of the eyes referenced to each other to measure horizontal eye movements. Signals were amplified and digitized with a sampling rate of 1024 Hz.
