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									Stop Signal Task (SST)
SCRIPT INFO

Script Author: David Nitz (dave@millisecond.com) for Millisecond Software, LLC
last updated: 09-27-2016 by K.Borchert (katjab@millisecond.com) for Millisecond Software LLC

Script Copyright © 09-27-2016 Millisecond Software


BACKGROUND INFO

											*Purpose*
	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*
After looking at a fixation cross, participants see arrows pointing 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 cross) 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.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
values.rt:						reaction time in ms (0 = no response)
values.ns_ntotal :				total number of ns trials 
values.ss_ntotal:				total number of ss trials  
values.ssd:						ssd for the current trial
expressions.p_rs:				probability of reacting in ss trials => p(response|signal)
expressions.ssd: 				mean stop signal delays (in ms) in ss trials 
expressions.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)
expressions.sr_rt:              mean reaction time (in ms) in ss trials	in ms
expressions.ns_rt: 				mean reaction time (in ms) 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 subtraction method to calculate SSRT (see expressions.SSRT) cannot be used)

(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 
expressions.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)

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 subtraction method to calculate SSRT (see expressions.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 crosses)
* half the arrows in each trialtype point left, the other right

1 Practice Block: 32 trials (8 signal trials : 24 no signal trials = 3:1)
3 Test Blocks: 64 trials each (16 signal trials : 48 no signal trials = 3:1)	



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 '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'.
	- posttrialpause			Duration of posttrialpause in ms. Default is '0'. (Note: the posttrialpause would add a period of time after a trial during which responses are no longer accepted)
	- 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.

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