EEG was recorded using a BioSemi ActiveTwo Recording System with a 32-channel electrode cap conforming to the standard International 10-20 System of electrode location, sampled at a rate of 1,024 Hz. Subjects were instructed to minimize jaw and muscle tension, eye movements, and blinking. Electrooculogram (EOG) was monitored in the horizontal direction and vertical direction, and these data were used to eliminate trials contaminated by blink, eye movement, or other artifacts.