Variable interstimulus interval (ISI) for stop trials in a stop signal task

[Kelsey A Brown]

Hello Everyone, 

I received incredibly helpful answers for my previous question that I 
posted here, so I thought I would seek your valuable input on a second part 
of a stop signal task I have encountered some difficultly building. One of 
the defining parts of a stop signal task is having a variable ISI before 
the presentation of a stop signal. I would like to write a script that 
tells E-prime to make the ISI=250 ms for all go trials. For stop trials, 
the ISI for the first stop signal would be 250 ms, but the ISI for all 
future stop signals would be dependent on previous performance on stop 
trials. If the participant was successful at inhibiting pressing the button 
on the previous stop trial and their percent correct (accuracy on stop 
trials) is greater that 30% then the program would increase the previous 
trials stop signal delay (the ISI) for the next stop signal by 50 ms. If 
the participant was successful in inhibiting in pressing the button on the 
previous stop trial and their percent correct is less than 30% then the 
program would keep the stop signal delay for the next trial the same as it 
was in the previous trial. If the participant was not successful in 
inhibiting their button pressing on the previous stop trial and their 
percent correct is greater then 30% the program will keep the stop signal 
delay (ISI) equal to the ISI in the previous stop trial. If the participant 
was not successful in inhibiting their button pressing on the previous stop 
trial and their percent correct is less than 30% then the program would 
subtract 50 ms from the ISI of the previous stop signal trial. 

stop signal delay and ISI are used interchangeably, but they mean the same 
thing. 

To begin, in the experiment object properties, I made initDelay(initial 
delay)= 250. I also set DurationChange=50. 

In the script I wrote: 

Dim nDelay As Double 'actual delay for each trial

Dim min, max, nerr, ncor, ff, ncc As Integer
Dim pctcor, pcterr, pct, fpct, cpct As Single
Dim ferr As String
Dim pp As Single

'set guidelines for what to present when the person was correct too often 
and you want to increase the delay
Sub SetSlowState(c As nDelay)
   nDelay = nDelay + CDbl(c.GetAttrib("DurationChange"))
   c.SetAttrib "ISIduration", c.GetAttrib("nDelay")
End Sub
Sub SetFastState (c As nDelay)
   nDelay = nDelay - CDbl(c.GetAttrib("DurationChange"))
   c.SetAttrib "ISIduration", c.GetAttrib("nDelay")


Dim Delay As Integer
If fileexists(c.GetAttrib("Subject") & "-ISI.txt") Then
   Open c.GetAttrib("Subject") & "-ISI.txt" For Input As #1
   Input #1, Delay
   Close #1
   nDelay = CDbl(Delay)
Else
   nDelay = CDbl(c.GetAttrib(initDelay))
End If

'I'm still confused how to set variable ISI in a text file that changes 

nerr=0 'num forced errors 
ncor=0 ' num correct
pcterr=0.0 ' percent error
ncc=0 ' num correct??


'ISI.duration= nDelay If c.GetAtrib("TrialType")="go" 


If c.GetAttrib ("TrialType")="stop" Then
If (pctcor>.30) Then 
SetSlowState c,nDelay
 'this routine should increase the ITI making it harder
If (pctcor<.30) Then 
SetFastState c,nDelay
 'this routine should decrease the ITI making it easier
If c.GetAttrib ("TrialType")="stop" And If (pctcor<.30) Then ISI.duration= 
x - 50 

 ISI.duration = 250 If c.GetAtrib("TrialType")="go" 
Then If c.GetAttrib ("TrialType")="stop" 
Then ISI.duration = If c.GetAttrib("CorrectAnswer")= "" And 
If (pctcor>.30) 
     debug.Print "Forced Error"
     nerr = nerr + 1' Forced Errors
 Then ISI.duration= 'previous  stoptrial ISI + 50 
If c.GetAttrib("CorrectAnswer")="" And If (pctcor<.30) Then ISI.duration= 
'previous stop trial ISI - 50. 

'i'm trying to set it up so that the isi for the first stop trial is equal 
to 250ms but increases by 50ms if pctcor>.30 but decreases by 50 ms if 
pctcor<.30. 
I'm assuming it is some sort of if then statement structured something like 
what I wrote below. 
 Then ISI.duration = 250 Else ISI.duration = 250+50 


The script above is what I came up with my professor, but I'm not sure the 
script is accomplishing exactly what it is meant to by tracking the stop 
signal ISI on all trials and either adding to that ISI or subtracting 50 ms 
from that ISI for the following trial depending on the participants stop 
trial accuracy percentage. I know that some of the above script is writing 
the same thing in multiple ways, but I figured it would be helpful to post 
everything that I have come up with so far. The script is basically a 
jumble of things we think might potentially work to write the program in 
the way we want it to run, but I would certainly appreciate any input. 
Hopefully this all makes sense. Perhaps there is an easier way to keep 
track of the ISI on all stop trials and program the variable ISI that I 
have not previously considered? 

Thanks for any help in advance. 
-Kelsey 

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