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Dear SPMers, I came across several oddities in my m0wc1*.nii (modulated, normalized, gray matter) images yesterday, and I am hoping someone here can shed some light on the situation. Apologies for the length of this correspondence, but I wanted to be precise in explaining the problem observed. Anyways, looking at a typical m0wc1*.nii image, the voxel values have the following strange properties: 1) There are no voxels with a value greater than 1 2) There are ~580,000 voxels with a value of exactly 1. Actually, they all have a value of exactly 1.000000059138983, which in itself is kind of strange. 3) Of the non-zero valued voxels in the m0wc1*.nii image, 42.4% of them are exactly (to double precision) the same as in the wc1*.nii images. In a previous thread, Dr. Ashburner said that > The contents of a modulated image are a voxel compression map multiplied by tissue belonging > probabilities (which range between zero and one) > ... > The total volume of grey matter in the original image can be > determined by summing the voxels in the modulated, spatially > normalised image and multiplying by the voxel volume (product of voxel > size). That the total volume of gray matter in the original image can be determined by integration implies conservation of probability of gray matter. It follows that the voxel compression map would have values >1 in areas where there has been positive compression (shrinking) and values <1 in areas where there has been negative compression (expansion). With this in mind, the properties described above lead me to the following conclusions: A) There are no voxels with high probability (p~1.0) of being gray matter that were positively compressed (shrunk) in normalizing, else there would exist modulated voxels with value > 1. B) There exist several voxels that either i) had a gray matter probability of exactly 1 and were not compressed even one iota or ii) were compressed in exact (to double precision!) proportion to their uncertainty of being gray matter. Else there would not exist modulated voxels with value = 1 exactly C) 42.4% of probable gray matter voxels neither shrunk nor expanded in the process of morphing to standard space. I just can't wrap my head around any of those conclusions. I feel like either I'm totally misunderstanding what happens with modulation or something is very very wrong with my images. I understand that the non-linear only modulation (m0 instead of m) changes things, but substituting "non-linear compression" for "compression" above does not make the observations any less strange. If any guru out there can make sense of all this, it would be much appreciated. Regards, Neil Neil Chatterjee Research Assistant Stanford Systems Neuroscience and Pain Lab 650-724-0522 [log in to unmask]