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Re: SPM.xVol.FWHM - where does it come from?

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Tue, 27 Aug 2013 09:34:02 -0600

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 ```Umi, I don't categorize myself as an expert, but I had the same question a few years ago. The smoothness estimates in SPM.xVol.FWHM are in units of VOXELS, so you need to multiply by the voxel dimensions to get them in mm. Something like this: load SPM; M = SPM.xVol.M; VOX = sqrt(diag(M(1:3,1:3)'*M(1:3,1:3)))'; FWHM = SPM.xVol.FWHM; FWHMmm= FWHM.*VOX; disp(FWHMmm); The calculations of smoothness are carried out by spm_est_smoothness and are based on Gaussian random field theory. They will not match the 3dFWHMx results exactly, but should be close. In the May 2011 spm archives there is a comparison of 3DFWHMx smoothness and the result of SPM.xVol.FWHM, and they got the results below. Very best wishes! Jim *** 3dFWHMx: FWHMx = 9.82782 FWHMy = 10.0744 FWHMz = 9.31861 *** SPM.xVol.FWHM: FWHMx = 10.2832 FWHMy = 10.3741 FWHMz = 9.4135 On 8/27/13, Umi V. <[log in to unmask]> wrote: > Dear Experts, > > I am a relatively new user of SPM and have been unable to find a clear > answer to the question of what is contained in the SPM.xVol.FWHM file. I > would like to obtain the intrinsic smoothness of the data (not the > statistical map) to feed into 3dClustSim of the AFNI package. I know for > AFNI users, this is usually the output of 3dFWHMx, which gives the average > smoothness of the residual time series across all participants. When I look > at the SPM.xVol.FWHM at the group-level for a specific contrast, is this > number the SPM equivalent to AFNI's FWHM output? If not, what exactly do the > numbers in the file represent? I tried running 3dFWHMx on each type of map > (con, spmT, ResMS) and was not able to replicate the values contained in > xVol.FWHM. > > Any help you can provide is highly appreciated! > > Umi >```