Hi,
I was just trying to recreate the varcope image from a first level
analysis using the sigmasquareds and threshac1 images as well as the
design.mat file. My result is close, but I'm wondering why it isn't
closer. Here's what I've done
1. Create the correlation matrix V using the first element of
threshac1 as the main diagonal, 2nd element as the first diagonal
above and below the main diagonal and so forth.
2. Demean the design matrix that's stored in design.mat (call it X)
3. Calculate inv(X'*inv(V)*X)*sigma^2 Where sigma^2 is the
sigmasquareds value for that voxel and take the appropriate contrast
of this matrix to correspond with my varcope of interest.
My method seems to produce a variance that is consistently smaller
than the varcope, but within 20% or so. Did I miss something?
Highpass filtering was used in the analysis (both the data and the
design), but I figured that would already be incorporated in all the
pieces that I used. The analysis was originally carried out in Feat
version 5.43.
Thanks!
Jeanette
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