n-back test

[Ben Robinson]

I've been working on an n-back test where letters are briefly presented to screen, one letter at a time.  There can either be a match or no match, and subjects press one of two buttons in response.  In the one-back test, a match occurs when the letter currently on screen is the same as the letter that was presented immediately prior to it.  In the two- and three-back tests, a match occurs when the current letter is the same as the letter that appeared two or three letters back, respectively.

I've been trying to guarantee that 1 in 3 stimuli presented is a match.  To do this, I've created two Procedures: a MatchProc and a NoMatchProc, and I run the NoMatchProc twice as often as I run the MatchProc.  Well, that's the idea, anyway.

Some of the problems I'm having are:

   How to allow for random selection of the two procedures, MatchProc and NoMatchProc, with the only stipulation being that MatchProc can never be called in the first trial (or second or third trial in the two- and three- back conditions, respectively), since a match cannot occur until at least one stimulus has already been presented?  I've set the List.Order to PermutationOrder(1), where NoMatchProc occupies Level 1, but since I don't fully understand what PermutationOrder is meant to accomplish I don't know if this method will work, and since my code currently won't compile (!) I can't yet test this.

   In the one-back condition I need to test to make sure one trial has already been run before saving the first letter to an attribute column I've titled, OneLetterBack.  The code I'm trying to use for this task isn't working, and all I can guess is that it has a problem when I ask it to GetAttrib from one list and SetAttrib to another, higher-level list.  It's in this little block of code that at least the compile error occurs ― SetAttrib: Missing parameter(s).
        If c.GetAttrib(c.GetAttrib("Running") & ".Sample") > 1 Then
        OneList.setAttrib "OneLetterBack", OneNoMTrialList.**GetPrevAttrib("Letters")
**Here's where the compiler stops.  If I change "OneNoMTrialList.GetPrevAttrib" to "c.GetPrevAttrib" I'm told that GetPrevAttrib is not a method of the object.

Anyway, I know this may be a lot of information at once, but if anyone has any suggestions I'd surely appreciate it.

Thanks for any help.

Ben Robinson
brobinso at mprc.umaryland.edu
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