Dear Ritu,
you have to specify all regressors Ai and Bi separately at the first
level, and compute the contrasts explicitly.
Your assumption that this strategy should "provide more power" raises
an interesting issue. If the model were fitted with OLS (ordinary
least squares), I believe it would make no difference, up to rounding
error. First-level EPIs are not modelled with OLS in SPM, however;
your strategy would overfit the errors and -- very likely, given that
you have 80+ regressors now -- have an impact on the autocorrelation
structure of the residuals. This would almost certainly have an impact
on first-level inference, which is said to become more robust, but
this isn't the inference you are interested in. I believe the effect
on second level inference to be an open issue, but it may not be the
increase in power you expect.
My view on this: don't bother, unless you are interested in GLS as a
topic in itself.
Best wishes,
Roberto Viviani
Dept. of Psychiatry III
University of Ulm, Germany
> I have 2 types of conditions in my study: A & B. The A-B contrast
> (for a single subject) does give some expected results. But, I have
> matching pairs of stimuli for the two condition types: e.g. A1
> matches B1, A2 matches B2, etc. (for A1..A40 & B1..B40). They match
> in that Ai-Bi isolates the thing of interest. So, looking at Ai-Bi
> for i=1 to 40 should provide more power. But, where & how would I
> specify this in the SPM Batch GUI?
> Thanks.
>
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