___________________________________________________________________________________________________________________
Stop Signal Task (SST)
___________________________________________________________________________________________________________________
Script Author: David Nitz (dave@millisecond.com) for Millisecond Software, LLC
last updated: 06-30-2020 by K. Borchert (katjab@millisecond.com) for Millisecond Software, LLC
Script Copyright © 06-30-2020 Millisecond Software
___________________________________________________________________________________________________________________
BACKGROUND INFO
___________________________________________________________________________________________________________________
This script implements the 'Stop Signal Task' as described in:
Verbruggen, F., Logan, G. D., & Stevens, M. A. (2008). STOP-IT: Windows executable software for the stop-signal paradigm.
Behavior Research Methods, 40(2), 479-483.
Additional References:
Logan, G. D. (1994). On the ability to inhibit thought and action: A user’s guide to the stop signal paradigm. In D. Dagenbach
& T. H. Carr, Inhibitory processes in attention, memory, and language (pp. 189-239). San Diego: Academic Press
___________________________________________________________________________________________________________________
TASK DESCRIPTION
___________________________________________________________________________________________________________________
After looking at a fixation circle, participants are presented an arrow withing this circle that either points right or left.
The task is to press the left response key if the arrow points to the left and press the right response key if the arrow
points to the right UNLESS a signal beep is played AFTER the presentation of the arrow. In this case the response should be
stopped before execution.
The delay between presentation of arrow and signal beep (starting at 250ms) is adjusted up or down (by 50ms) depending on performance.
The delay gets longer if the previous signal stop was successful (up to 1150ms) and it gets smaller if the previous signal stop was not
successful (down to 50ms). The stimlus onset asynchrony between the start of each trial (onset of fixation circles) is 2000ms.
Participants can respond up until the next trial starts.
___________________________________________________________________________________________________________________
DURATION
___________________________________________________________________________________________________________________
the default set-up of the script takes appr. 12 minutes to complete
___________________________________________________________________________________________________________________
DATA FILE INFORMATION
___________________________________________________________________________________________________________________
The default data stored in the data files are:
(1) Raw data file: 'stopsignalTask_raw*.iqdat' (a separate file for each participant)
build: The specific Inquisit version used (the 'build') that was run
computer.platform: the platform the script was run on (win/mac/ios/android)
date, time: date and time script was run
subject, group: with the current subject/groupnumber
session: with the current session id
blockcode, blocknum: the name and number of the current block (built-in Inquisit variable)
blocknumber: Number of Test Blocks (custom variable)
expressions.blocknumber_adj: adjusted number of test blocks
trialcode, trialnum: the name and number of the currently recorded trial (built-in Inquisit variable)
Note: trialnum is a built-in Inquisit variable; it counts all trials run; even those
that do not store data to the data file such as feedback trials. Thus, trialnum
may not reflect the number of main trials run per block.
trialnumber: Number of trials run in a block (custom variable)
trialnumber_adj: adjusted Number of trials run in a block (does not count the first one)
stimulus: 1 = left arrow, 2 = right arrow
signal: 1 = presence of an auditory signal, 0 otherwise
ACC: 0 = incorrect; 2 = correct
response: key pressed by subject (scancode of response button pressed; 32 = D; 37 = K)
responseCategory: 1 = left; 2 = right; 0 = no response
rt: reaction time in ms (0 = no response)
ns_ntotal : total number of ns trials
ss_ntotal: total number of ss trials
ssd: ssd for the current trial
p_rs: probability of reacting in ss trials => p(response|signal)
ssd: mean stop signal delays (in ms) in ss trials
ssrt: estimation of the covert stop signal reaction time in ms (SSRT)
(the time required to stop the initiated go-process)
=> the slower the SSRT the more difficult to stop the go-process
=> the faster the SSRT the easier to stop the go-process
(measure of inhibition ability)
sr_rt: mean reaction time (in ms) in ss trials in ms
ns_rt: mean reaction time (in ms) in correct ns trials, across blocks => 'choice reaction time'
ns_hit : percentage of mean number of hits in ns trials
miss: percentage of mean number of misses in ns trials
z_score: z-score of p(response|signal) with
expected mean = n*p; standard deviation = sqrt(n*p*q);
with n= number of stop signal trials; p = parameters.p; q = 1-p
p_value: p-value that corresponds to the calculated z-value
NOTE: z-score and corresponding p-value indicate whether a participant inhibited significantly more or less than
50% of the time (see parameters.p)
(Verbruggen et al: for participants who inhibited significantly more or less than 50% of the time
the subtraction method to calculate SSRT (see SSRT) cannot be used)
(2) Summary data file: 'stopsignaltask_summary*.iqdat' (a separate file for each participant)
inquisit.version: Inquisit version run
computer.platform: the platform the script was run on (win/mac/ios/android)
startDate: date script was run
startTime: time script was started
subjectid: assigned subject id number
groupid: assigned group id number
sessionid: assigned session id number
elapsedTime: time it took to run script (in ms); measured from onset to offset of script
completed: 0 = script was not completed (prematurely aborted);
1 = script was completed (all conditions run)
p_rs: probability of reacting in ss trials => p(response|signal)
ssd: mean stop signal delays (in ms) in ss trials
ssrt: estimation of the covert stop signal reaction time in ms (SSRT)
(the time required to stop the initiated go-process)
=> the slower the SSRT the more difficult to stop the go-process
=> the faster the SSRT the easier to stop the go-process
(measure of inhibition ability)
sr_rt: mean reaction time in ss trials in ms
ns_rt: mean reaction time in correct ns trials, across blocks => 'choice reaction time'
ns_hit : percentage of mean number of hits in ns trials
miss: percentage of mean number of misses in ns trials
z_score: z-score of p(response|signal) with expected mean = n*p; standard deviation = sqrt(n*p*q)
p_value: p-value that corresponds to the calculated z-value
NOTE: z-score and corresponding p-value indicate whether a participant inhibited significantly more or less than
50% of the time (see parameters.p)
(Verbruggen et al: for participants who inhibited significantly more or less than 50% of the time
the subtraction method to calculate SSRT (see SSRT) cannot be used)
___________________________________________________________________________________________________________________
EXPERIMENTAL SET-UP
___________________________________________________________________________________________________________________
* tests Stop Signal Delays between: 50ms (min) - 1150ms (max) with adjustment steps = 50ms
* The initial Stop Signal Delay is set to 250ms
* The total trialduration of each signal and no signal trial is 2000ms (= SOA between onsets of fixation stims)
* half the arrows in each trialtype point left, the other right
1 Practice Block: 32 trials (8 signal trials : 24 no signal trials = 1:3)
3 Test Blocks: 64 trials each (16 signal trials : 48 no signal trials = 1:3)
___________________________________________________________________________________________________________________
INSTRUCTIONS
___________________________________________________________________________________________________________________
see section Editable Stimuli
___________________________________________________________________________________________________________________
EDITABLE CODE
___________________________________________________________________________________________________________________
check below for (relatively) easily editable parameters, stimuli, instructions etc.
Keep in mind that you can use this script as a template and therefore always "mess" with the entire code
to further customize your experiment.
The parameters you can change are:
Adjustable Task Parameters:
- resetpracticessd Boolean switch to determine whether SSD should be reset to
start value (parameters.ssd_start) after practice block. Default is 'false'.
- leftkey Keyboard scan code for the left response key. Default is "D".
- rightkey Keyboard scan code for the right response key. Default is "K".
- minssd Minimum allowable Stop Signal Delay (SSD). Default is '50'.
Given that a '0' delay does not make sense, this value should usually
be set to match parameters.adjustment.
- maxssd Maximum allowable Stop Signal Delay (SSD). Default is '1150'.
Given the default trial/stimulus timing
'/ stimulustimes = [0=fixation; 250=stim; 1500=erasestim]', this value
should be chosen such that 'maxssd + stopsignalduration <= 1250'.
Otherwise the Stop Signal would occur *after* the given trial has
already terminated, which, of course, would not make any sense.
- adjustment Amount in ms by which SSD should be in- or decreased depending on
successful or unsuccessful response inhibition. Default is '50'.
- ssd_start Initial Stop Signal Delay (SSD) in ms. Default is '250'.
- pretrialpause Duration of pretrialpause in ms. Default is '500'.
- trialduration Total duration for a given trial in ms. Default is '2000'.
- waitbetweenblocks Wait period between blocks in seconds. Default is '10'.
- p Target response inhibition probability. Default is '0.5'.
Do *not* change this value unless you are very familiar with
the Stop Signal paradigm.
