Difference between revisions of "STOPSIGNAL"

Revision as of 15:13, 15 March 2012

STOP-SIGNAL TASK (SST)

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 We’re going to review the task with the arrows and tones. During this task you will see an arrow in the middle of the screen pointing either to the left or right. As soon as you see the arrow, respond as quickly and accurately as possible, by pressing the LEFT (blue) button if it’s pointing left and the RIGHT (yellow) button if its pointing right.

When you hear a beep, do not respond to that particular arrow. Both going fast and stopping are equally important. This task is designed to be difficult and for subjects to make mistakes, so don’t get frustrated it it’s hard. Just make sure NOT to slow down your responses to wait for the beep so that you are no longer going when you are supposed to. You won’t always be able to stop when you hear a beep, so just try your best.

Behavioral Instructions Now we’re going to run through the whole task with the arrows and tones. You want to respond as quickly and accurately as possible to the arrows, but to not respond when you hear a beep.

Remember that both going fast and stopping are equally important. Make sure NOT to slow down your responses to wait for the beep so that you are no longer going when you are supposed to. You won’t always be able to stop when you hear a beep, so just try your best.

Scan Instructions The next task is the one where you have to press the button that corresponds to the direction the arrow is pointing. So press the first button if it’s pointing left, and the second button if it’s pointing right. You want to push the button fast, but sometimes, you’ll hear a beep, and when you hear the beep you should try really hard to not push any button at all. It’s equally important to try to go fast and to stop when you hear the beep, so if you’re trying to stop every time it means you’re going too slow. As long as you’re stopping some of the time, though, it means you’re doing a good job.

Participants saw on the screen Press the left button if you see the arrow pointing left Press the right button if you see the arrow pointing right

Press the button as FAST as you can when you see the arrow.

But if you hear a beep, try very hard to STOP yourself from pressing the button.

Stopping and going fast are equally important.

Scoring Behavioral Data

Creating Onset Files (EVs)

Running First Level Analyses

List of Models

Behavioral Variables

Link back to LA5C page.