Three types of visual stimuli were presented during the task: the letter “X”, a square, and colored geometric figures that changed with each presentation. Their respective probabilities of occurrence were 0.125, 0.75, and 0.125. The subject was instructed to press a response key when she detected the “X” (target stimulus), and ignore the square (frequent nontarget) and geometric figures (novel nontarget). Response speed was emphasized, but not at the cost of accuracy. The task terminated automatically after a minimum of 25 target, 150 frequent nontarget, and 25 novel nontarget artifact-free trials had been acquired. Trials with response times greater than 1000 ms were rejected.
