Details of the visual oddball paradigm for eliciting event-related potentials employed in the present study have been previously described (Porjesz and others 1998). It consists of presentation of three types of visual stimuli (N = 280), 60 ms duration, subtending a visual angle of 2.5°, with an inter-stimulus interval of 1.625 second. The rare target stimulus (n = 35) was the letter X, to which the subject was required to press a button as quickly as possible; the responding hand was alternated across subjects to counterbalance any laterality effects due to responding. Speed was emphasized, but not at the cost of accuracy. The frequently occurring non-target stimuli (n = 210) were squares and the novel stimuli (n = 35) consisted of colored geometric polygons that were different on each trial; the subject was not required to respond to the non-target and novel stimuli.
