Right. If there was a fairly long interval, there should be no problem.
I assume, however, in most such designs the interval won't be very long (on the order of 1-3 sec). In such situations it's my strong impression that it's hard to get around the collinearity of the cue and response, as a practical matter.
-----Original Message-----
From: S.F.W. Neggers [mailto:[log in to unmask]]
Sent: Wednesday, November 09, 2011 12:21 PM
To: Fromm, Stephen (NIH/NIMH) [C]
Cc: [log in to unmask]
Subject: Re: [SPM] Assessing Multicollinearity from a Review Perspective
Dear Andrew, Stephen,
when one models both factors (cue and response), with some time in
between (several seconds), one should be able with T or F tests to
create a contrast testing the variance uniquely attributable to either
cue or response, right? Provided such unique variance exists of course.
So the question to my opinion really is whether there is enough time
between cue and response, and whether the onsets of both are jittered
substantially. Namely, when there is a lot of time in between cue and
response, say 12 seconds, no one would doubt, even when there is still a
little collinearity, that one can attribute variance to one event or the
other.
How long is the interval in this design, and the jittering?
My 2 cts...
Cheers,
Bas
Op 09-11-11 16:16, Stephen J. Fromm schreef:
> Did they model both cue and response in the subject-level GLM? Or did they just model cue?
>
> From my experience with such models, it's impossible to disambiguate one part of the trial (e.g. cue) from the other parts of the trial (response, etc), at least with fMRI. Because response must always follow cue, you can't get sufficient jittering to separate them. (That is, the usefulness of jittering is limited when you can't randomize the order of the events.) If they model multiple parts of the trial (e.g. cue, response and maybe other phases), they might get something that survives some measure of statistical significance, but it's hard to see how it would be a meaningful measure of anything.
>
> I did see one paper that claimed to separate the trial parts by using incomplete trials. So if you have a trial with cue that's truncated, you could claim that the cue-only trials measure cue; and you could get response by comparing the parameter estimates for complete trials with parameter estimates for the truncated trials. But that has an obvious weakness: at least naively one would expect that the cognitive processes during truncated trials would be different from those in complete trials.
>
> So, on the information you provide, I'd venture to say this is a weakness in the paper you're reviewing.
>
--
--------------------------------------------------
Dr. S.F.W. Neggers
Division of Brain Research
Rudolf Magnus Institute for Neuroscience
Utrecht University Medical Center
Visiting : Heidelberglaan 100, 3584 CX Utrecht
Room B.01.1.03
Mail : Huispost B01.206, P.O. Box 85500
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Web : http://www.neuromri.nl/people/bas-neggers
: http://www.neuralnavigator.com (CEO)
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