A three stimulus visual oddball task was employed with 280 visual stimuli of three different types: targets (rarely occurring letter X to which the subjects responded quickly and accurately with a button press, non-targets (frequently occurring white squares) and novels (rarely occurring random colored geometric figures). Stimuli subtended a visual angle of 2.5° with stimulus durations of 60 ms and inter-stimulus intervals of 1.625 ms. The task comprised 35 target, 35 novel and 210 non-target stimuli with a probability of occurrence of 0.125, 0.125 and 0.750, respectively. The stimuli were presented pseudo-randomly with the only constraint that the targets and novels always followed non-targets.
