Hi Steve,
Let me see if I understand your situation: You have multiple
sessions per subject. In each session, each of the trials is
classified as "correct" (C) or "incorrect" (I); sometimes there are
no I trials in a given session. You want to compute the contrast C
minus I in each subject.
As you already hinted, the issue is not statistical validity (the
p-values for all contrasts will be valid), but correct scientific
interpretation of the contrasts (that is, to make sure that the
contrast weighst chosen estimate the contrast you want). If one
assumes that the expectation of the C and I response amplitudes do
not depend on session, then as long as the contrast weights
(including all sessions) sum to zero, (and in particular the
contrast weights for C sum to 1 and the contrast weights for I sum
to -1), one is still estimating C minus I.
Eric
equal for all ru I think that it is valid to have contrast weights
that do not sum to zero in each session.
Quoting "Stephen J. Fromm" <[log in to unmask]>:
> I'm looking at fMRI data with multiple sessions (or "runs").
> Some of the
> trial types don't occur in every single session for a given
> subject,
> because they're behavior-dependent. In my case, these are
> "incorrect"
> trials, and occasionally a subject will not make an error in a
> given
> session.
>
> Is it crucial that the sum of contrast weights within a given
> session be
> zero, or does it suffice that the sum across all sessions vanish?
>
> The disadvantage of insisting that sums vanish across every
> session is
> that the weights will vary across sessions. Not that I think
> that
> necessarily matters for the validity of the model, though it does
> mean I
> have to be more careful showing others how to choose the weights.
> On the
> other hand, perhaps there's something wrong with allowing nonzero
> sums
> within a session. But given that SPM already takes into account
> the
> effect of run, and furthermore does grand mean scaling, I can't
> see any
> _likely_ scenarios where it would matter much, though obviously
> one can
> think of possibilities.
>
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