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Stop Signal Task (SST) // 2019 VERSION
This script incorporates the recent recommendations in:
Verbruggen F, et al. (2019) A consensus guide to capturing the ability to inhibit actions and impulsive behaviors in the stop-signal task.
eLife, 8, e46323.
- Increased number of trials.
- Additional on-screen reminders to NOT wait for the stop signal before responding.
- Warnings in summary output if race model assumption has been violated or p(respond|signal) deviates drastically from 0.5.
- Integration method estimate of SSRT (recommended) in addition to traditional mean method estimate (no longer recommended).
Script Author: David Nitz (firstname.lastname@example.org) for Millisecond Software, LLC
last updated: 09-05-2019 by Katja Borchert (email@example.com) for Millisecond Software LLC
Script Copyright © 09-05-2019 Millisecond Software
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.
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.
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.
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: Inquisit build
computer.platform: the platform the script was run on
date, time, subject, group: date and time script was run with the current subject/groupnumber
blockcode, blocknum: the name and number of the current block
values.blocknumber: Number of Test Blocks
expressions.blocknumber: adjusted number of test blocks
trialcode/trialnum: Name/Number of the current trial (built in variables)
values.trialnumber: Number of trials run in a block
expressions.trialnumber: adjusted Number of trials run in a block (does not count the first one)
values.ssd: ssd for the current trial
values.stimulus: 1 = left arrow, 2 = right arrow
values.signal: 1 = presence of an auditory signal, 0 otherwise
values.correct: 0 = incorrect; 2 = correct
response: key pressed by subject (scancode of response button pressed)
32 = D (left)
38 = K (right)
0 = no response
/response: 1 = left; 2 = right; 0 = no response
values.rt: reaction time in ms (0 = no response)
(2) Summary data file: 'StopSignalTask_summary*.iqdat' (a separate file for each participant)
script.startdate: date script was run
script.starttime: time script was started
script.subjectid: subject id number
script.groupid: group id number
script.elapsedtime: time it took to run script (in ms)
computer.platform: the platform the script was run on
/completed: 0 = script was not completed (prematurely aborted); 1 = script was completed (all conditions run)
expressions.p_rs: probability of reacting in ss trials => p(response|signal)
expressions.ssd: mean stop signal delays (in ms) in ss trials
Mean Method SSRT: as of 2019 not recommended anymore (see Verbruggen et al, 2019)
expressions.ssrt_meanMethdod: estimation of the covert stop signal reaction time in ms (SSRT) using the Mean Method (as of 2019: not recommended)
(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)
expressions.sr_rt: mean reaction time in ss trials in ms
expressions.ns_rt: mean reaction time in correct ns trials, across blocks => 'choice reaction time'
expressions.ns_hit : percentage of mean number of hits in ns trials
expressions.miss: percentage of mean number of misses in ns trials
expressions.z_score: z-score of p(response|signal) with expected mean = n*p; standard deviation = sqrt(n*p*q)
expressions.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 mean method to calculate SSRT cannot be used)
expressions.p_rs_deviation: returns true (1) if p(r|s) deviates substantially from 0.5,
returns false (0) otherwise,
i.e. p(r|s) does NOT deviate substantially from 0.5.
The cut-offs are somewhat arbitrary.
expressions. race_model_violation: returns true (1) if race model assumption for calculation of SSRT using the MEAN method is violated,
returns false (0) otherwise,
i.e. race model NOT violated.
Integration Method SSRT (as of 2019): recommended (see Verbruggen et al, 2019)
values.nth: the index in list of ns trial RTs that corresponds to the probability of reacting in ss trials
values.nth_rt: the RT in the ordered list of ns trial RTs that corresponds to the probability of reacting in ss trials
values.max_ns_rt: the max. RT of actual responding in ns trials
values.ns_rts_total: number of RTs in ns trials
values.SSRT_integration: estimation of the covert stop signal reaction time in ms (SSRT) using the INTEGRATION method;
the recommended method as of 2019
(the time required to stop the initiated go-process; measure of inhibition ability)
* 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: 72 trials each (18 signal trials : 54 no signal trials = 1:3)
see section Editable Stimuli
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 '32' ("D").
- rightkey Keyboard scan code for the right response key. Default is '37' ("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.