STOPSIGNAL
Contents
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) 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.
All trials were preceded by a 500 ms fixation cross in the center of the screen, then each trial began with the appearance of an arrow and ended after 1000 ms, followed by the null period. Jittered null events separated every trial (with a blank screen), with the duration of null events sampled from an exponential distribution (null events ranged 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
/space/raid2/data/poldrack/CNP/scripts/behav_analyze/STOPSIGNAL
1. All scripts can either be run so that you do all subjects of a given group at once, or do one subject at a time.
There are comments in the code that instruct you to indicate first with group (e.g., CONTROLS) you will be doing. By default, it will run all subjects in the selected group unless you choose to comment in a couple lines and enter in a particular subject's ID (e.g., CNP_10150). This applies to both SST scripts.
2. in matlab, run 'STOPSIGNAL_full_run_ssrtquantile.m'
This will either create behavioral output (in SUBJ/behav/STOPSIGNAL) for each subject in your group or just the subject you specified. This script creates 2 files:
a. It adds to the 'GROUP_stopsig_group_output.txt' file, by adding a line for each subject with their summary scores.
b. It creates a 'SUBJ_STOPSIGNAL_behav_output.txt' file in the subject's behav/STOPSIGNAL directory.
The file created under 2a will be used for analysis of behavioral data collected during the Stop-signal scan. In most cases, this data will be provided after query. For a full description of the task and scoring of variables, see Stop-signal under LA2K CNP_Stop_Signal.
Creating Onset files (EVs)
/space/raid2/data/poldrack/CNP/scripts/behav_analyze/STOPSIGNAL
1. All scripts can either be run so that you do all subjects of a given group at once, or do one subject at a time.
There are comments in the code that instruct you to indicate first with group (e.g., CONTROLS) you will be doing. By default, it will run all subjects in the selected group unless you choose to comment in a couple lines and enter in a particular subject's ID (e.g., CNP_10150). This applies to both SST scripts.
2. In matlab, run 'make_stopsig_onsets.m'
This will either create onset files (in SUBJ/behav/STOPSIGNAL) for each subject in your group or just the subject you specified.
The main issue to consider for Stop-signal onset files is whether the "junk" category is empty: if there were incorrect Go trials, these are classified as "junk" and the default model is appropriate; if there were no incorrect Go trials, then the "junk" category is empty, and we need to run a special (cleverly-titled "no_junk" model for this subject).
There are 2 ways you can check this:
a. Open the 'GROUP_stopsig_log.txt': it will list each subject that has an empty "junk" onset file.
b. Open the subject's 'run1_junk_onsets.txt' file in their behav/STOPSIGNAL directory.
3. running this script will create 4 onset files in each subject's behav/STOPSIGNAL directory:
'run1_go_onsets.txt'
'run1_junk_onsets.txt'
'run1_succ_stop_onsets.txt'
'run1_unsucc_stop_onsets.txt'
4. The onset files will have contents that look something like this:
8.8882 1.5 1
10.8944 1.5 1
13.2653 1.5 1
16.5149 1.5 1
Running First Level Analyses
/space/raid2/data/poldrack/CNP/scripts/run_level1_scripts/STOPSIGNAL
1. As hinted at above, we have 2 Stop-signal scripts, depending on whether the subject has an empty junk file or not, and then we have separate scripts for each group. For CONTROLS, the w scripts are:
'run_level1_STOPSIGNAL_CONTROLS.sh'
'run_level1_nojunk_STOPSIGNAL_CONTROLS.sh'
Each of these scripts creates an individualized .fsf file for each subject (which is then stored in /space/raid2/data/poldrack/CNP/scripts/designs/STOPSIGNAL), then runs it, and the output is stored in the subject's analysis/STOPSIGNAL directory.
2. To run each script, open it in emacs.
a. As is commented into the script, you need to comment in certain sections depending on whether the subject was scanned at BMC vs. CCN. This is in two places, at the top and bottom of the script.
b. Enter in the subject(s) ID(s) on a new line of code (e.g., 'for id in 11131, do').
c. Save and submit to grid to run.
Data Checking
List of Models
level1_STOPSIGNAL_model1
Model description and contrasts
Behavioral variables
STOPSIGNAL model1 behavioral variables
