The visual oddball experiment was administered using the E-prime software system (Psychology Software Tools Inc., Pittsburgh, USA). The task consisted of the presentation of three types of visual stimuli:- (i) standard non-target stimuli – a small hollow white square; (ii) target stimuli – a small white X; and (iii) rare non-target stimuli – different shapes of various colors. The stimuli were displayed for 200ms in the middle of a black screen on a computer monitor, followed by a delay varying between 1000ms and 1100ms before the next stimulus and during which time the screen was blank. Stimuli were presented in a predetermined semi-random order, with standard non-target stimuli appearing 210 times, rare non-target stimuli appearing 35 times and target stimuli appearing 35 times. The total task duration was approximately 6.5 minutes. The subjects were instructed not to respond to the standard and rare non-target stimuli, and to press a response key as quickly as possible to target stimuli. Each subject was shown examples of the various stimuli during a short practice session before proceeding with the task. The paradigm was
