Hi,
> I would like your opinion about an analysis I have done:
> I use a contrast 1, sensitive to psychological parameters A and B (but not
> C) to define a functional ROI. I thresholded this contrast at .01
> uncorrected. I assume that the regions surviving from contrast 1, encode
> only A and B.
> Then I use contrast 2, sensitive to psychological parameters B and C (but
> not A) and I search for voxels surviving WITHIN the abovementioned fROI. I
> find an area surviving at p=.05, corrected this time, and I assume that this
> area encodes B, since C has been excluded by the fROI.
Sadly, no, you can't conclude these voxels encode B but not C.
Firstly, I wasn't sure if your second contrast was orthogonal to your
first. I guess, if they are both sensitive to B, you might have used
the same regressors in both, and they may not be orthogonal. In that
case, you can't easily interpret a search for voxels sensitive to B in
the ROI, because the ROI has already been selected to be sensitive to
B in the first contrast. If the contrasts are not orthogonal.
Second, the fact that you did not use C in the first contrast does not
imply that there is no effect of C - so you can't conclude that voxels
in your second contrast are not also sensitive to C.
Best,
Matthew
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