Hi Heather,
After pondering for a while, those numbers do not seem out of the realm of reasonableness. Your blur level is pretty low: 3.125 mm voxels, with a final blur (not an added blur) of FWHM = 6 mm.
That would be akin to adding a blur of FWHM = 3 mm, or so, which is very small.
So if the final FWHM is supposed to be 6 mm (and that is estimated with a standard, Gaussian ACF), you are getting a mixed-model ACF FWHM estimate of 6.7 mm, which seems to be in the correct ballpark.
To be sure, note what the 4 parameters are. The first is the fraction of the Gaussian term in the ACF model (so the linear fraction is .43 = 1-.57). The Gaussian term is 2.5 mm (small), and the linear term is 7.35. But that leads to a reasonable FWHM of 6.73 mm.
Also to be sure, are you applying a brain mask in this computation?
- rick
