A three-stimulus visual oddball task was employed with 280 visual stimuli of three different types: 35 targets (rarely occurring letter ‘X’) to which the subjects responded quickly and accurately with a button press, 210 non-targets (frequently occurring white squares) and 35 novels (rarely occurring random colored geometric figures) (probabilities of occurrence of 0.125, 0.750 and 0.125 respectively). Stimuli subtended a visual angle of 2.5 degrees with stimulus durations of 60 ms. and inter-stimulus intervals of 1625 ms. The stimuli were presented pseudo-randomly with the only constraint that a target or novel stimulus never preceded a target or novel stimulus.
