The subject was then instructed that he/she should press a response key to designate the passage of a 2-s interval following the onset of each of 50 cue stimuli. The cue stimulus was the letter X presented in the middle of a computer display for 50 ms. A small fixation spot was presented in the center of the monitor at all other times. The computer was programmed to present either a 500 or 2000-Hz tone 3 s after response execution. The 500-Hz tone was presented if response latency was within a ±250-ms window of the designated 2-s target. A 2000-Hz tone was presented if the response latency was outside of this range. The next trial commenced 5–10 s later. Subjects were allowed to practice the task for 5 to 10 trials to verify their comprehension of the instructions.
