afni.nimh.nih.gov

That is expected. Basically, those Gaussian FWHM parameters were considered obsolete, and so now 3dFWHMx outputs them as zeros (also to allow for some script backwards compatibility). The mixed-model auto-correlation function actually models the ACF of the data very well, while a Gaussian curve does not. So we highly recommend use of those mixed-model ACF parameters instead.

- rick