STOPSIGNAL

Task Background Info

Sample Text In the Stop-signal task, participants were presented with a series of Go stimuli (left- and rightwards pointing arrows inside of a circle) to which they were instructed to respond quickly. This speeded reaction time task established a prepotency to respond. On a subset of trials (25%), the Go stimulus was followed, after a variable delay, by a Stop-signal (an auditory signal), to which the participants were instructed to inhibit their response. The onset of the Stop-signal, or stop-signal delay (SSD), was varied and depended on the participant’s performance, such that it was decreased after a previous failure to inhibit and increased after a previous inhibition. The SSD for each stop trial was selected from one of two interleaved staircases of SSD values, with each SSD increasing or decreasing by 50 ms according to whether or not the participant successfully inhibited on the previous stop trial. This one-up/one-down tracking procedure ensures that participants inhibit on approximately half of all trials and controls for difficulty level across participants. Participants were told that correctly responding and inhibiting were equally important.

Trials began with a white circular fixation ring in the center of the screen for 500 ms. Go stimuli were presented for 1 s or until the participant responded, followed by a null period. Stop trials were identical to Go trials, except for the onset of the Stop-signal after a variable SSD. If the participant responded, the arrow and fixation circle disappeared for the remaining time, followed by the null period. Jittered null events were imposed between every trial, with the duration of null events samples from an exponential distribution (null events range from 0.5 to 4 s, with a mean of 1 s). Participants performed the task outside of the scanner; in this behavioral version, the SSD for each of Stop trial was selected from one of two interleaved staircases, each starting with SSD values of 250 and 350 ms. The last SSD values of the two ladders were used as starting values for the subsequent task run performed in the scanner. In each run, participants completed a total of 128 trials (75% Go trials); the task duration was 6 min and 4 s.

Participants first saw a demo version of the Stop-signal task, before completing a full behavioral run (128 trials). The demo consisted of 8 trials (3 Stop trials), and lasted approximately 25 s.

Task Instructions

Demo Instructions

Scan Instructions

Participants saw on the screen

Behavioral Variables

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