Behavioral Task

Participants performed a two-condition task-switching experiment. The task was generated using PsychoPy 1.80.06 (Peirce, 2007). On each trial, a cue (square or diamond) was presented for 300 msec, after which a number between 1 and 9 was presented for 2000 msec. For square cues, participants determined whether the number was odd (right button press) or even (left button press). For diamond cues, participants determined whether the number was greater than 5 (right button press) or less than 5 (left button press). The number 5 was never presented. Participants were given 2000 msec to respond and were instructed to respond as quickly as possible without sacrificing accuracy. Fixation periods of duration 2000, 4000, or 6000 msec separated each trial. Responses were limited to two buttons, rather than four buttons, which introduced response overlap.

Participants performed six functional runs composed of 48 trials each. Four runs contained both conditions, whereas two runs contained just a single condition. The single-condition runs are not included in the present report. For the two-condition runs, stimuli were pseudorandomized to ensure the same number stimulus was not presented twice in a row, the same cue was not presented more than four times in a row, consecutive right versus left hand responses did not repeat more than three times in a row, and there were no more than three consecutive rule repeat trials or rule switch trials. Participants performed out-of-scanner practice with and without trial-to-trial feedback, until they reached 60% accuracy. In the scanner, participants performed a brief practice to ensure they could see the stimuli.