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Hi Anderson, I'm not sure I'm convinced by your argument. All data analyses make some model assumptions and we do statistical tests given those assumptions. In this particular case, as long as the assumptions of the statistical test are independent of the assumptions of the propagation model used to identify the tracts, there should not be any problem. It is certainly reasonable for you to question the underlying assumptions of the tractography, but I don't see how that invalidates the statistics. Moreover, it sounds to me that you are arguing that it's not possible to make group comparisons of tractography data and that randomise cannot be used for tractography statistics. This seems contrary to the recommendations in the TBSS user guide (http://fsl.fmrib.ox.ac.uk/fsl/fslwiki/TBSS/UserGuide). Perhaps Steve has an opinion on this? jack On Wed, 2 Sep 2015 09:19:14 +0100, Anderson M. Winkler <[log in to unmask]> wrote: >Hi Zachary, > >Exactly, I am suggesting that this voxelwise comparison isn't meaningful. >First, there is the issue of the tracts not being in perfect register, but >even if we put aside the registration aspect, what would an eventual >voxelwise significant result mean? It would depend on the model used for >the propagation, but then the comparison wouldn't be about underlying >biology, but about the model used, and what the values in each voxel >represent for that model. > >Please note that I am not in any way suggesting that tractography wouldn't >be good or appropriate. What I am saying is that a comparison across >subjects isn't quite trivial as running a voxelwise GLM (be it parametric >or non-parametric, as in randomise). > >All the best, > >Anderson >