n-back

[Nick Coupland]

Hi, I'm trying to get up a two back task with a randomized letter list,
which takes from one of two trial lists :-
no match        TwoNoMTrialList
match           TwoMTrialList



The no match procedure has an inline script to modify the letter if it is
an accidental match to the two back letter:


'Add contents of TwoList to the "context" in which we are working.
c.Add TwoList
c.Update


'Declare a variable, i, for use as a counter in a while-loop
Dim i As Integer
i = 0


'Make sure that the letter to be presented is not accidentally a
'two-back match.  If it is, select a different letter to present.
While c.GetAttrib("Letters") = TwoLettersBack
         c.SetAttrib "Letters", TwoNoMTrialList.GetAttrib(i, "Letters")
         i = i + 1
Wend


'Set TwoLettersBack to what was previously OneLetterBack.
'Set OneLetterBack to the current letter.
TwoLettersBack = OneLetterBack
OneLetterBack = TwoNoMTrialList.GetCurrentAttrib("Letters")


However, this only works when the two back letter is from a no match trial.
If it is from a match trial, it crashes. How do I modify the script so it
can recognize accidental matches from both match and no match trials?
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