Visual stimuli were presented on a Dell 18″ flat panel monitor, driven by an nVidia, GeForce FX5200 DVI graphics card. Subjects were presented with an “oddball” Bernoulli series of letters delivered at 1 s intervals. The letters were presented in the center of the screen and subtended 5° of visual angle. The frequently occurring letter “X” occurred with a probability of 88%, with the infrequent letter K making up the remaining 12% of stimuli. In the first presentation (GO), the infrequent letter “K” was designated as the target to which subjects were instructed to press a reaction time (RT) key with the index finger of the preferred hand. In the second presentation (NOGO), subjects were asked to press the RT button to the frequent (non-target) stimuli (Letter X) and not to respond to the letter K. Stimulus presentation was controlled using Neuroscan (Charlotte, NC) STIM software.
