I am currently applying the WLS method for motion correction proposed by
Jorn Diedrichsen and Reza Shadmehr (Neuroimage,2005,27:3;624-634).
One of the requirements for the application of WLS method is to derive a
variance estimate on unsmoothed images. This means doing the SPM(2)
analysis on the unsmoothed images then smoothing the beta images prior to
deriving contrasts.
However, I have noticed that, when testing this in a standard analysis to
evaluate whether a smoothed image analysis is the same as an unsmoothed
image/smoothed beta file analysis, the results are different. I applied
the same FWHM of filter to the smoothed beta image analysis as for the
analysis of the smoothed images (images were spatally normalized in both
cases and hi-pass filtered at 128 seconds)
Is there a particular reason why the results should be different? Should I
use less smoothing when applied to beta images?
Regards - MFG
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