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No real explanation. The only thing I can think of is that there is less smoothing in the warped tissue class images with the high-res priors. Maybe you could try comparing the native-space tissue class images. Best regards, -John -----Original Message----- From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]] On Behalf Of Eric Pierre Sent: Tuesday, May 01, 2007 11:15 PM To: [log in to unmask] Subject: [SPM] Influence of priors resolution over segmentation Dear SPMers, We've been trying to undrestand to what extent the resolution of the prior probability maps affects the outcome of the segmentation. We performed 2 segmentations on the same brain using the VBM5 toolbox (by Christian Gaser) and a pediatric prior probability maps (courtesy of Cincinnati Children's Hospital Medical Center) at 1x1x1mm resolution, then with the same prior maps at 2x2x2mm (created out of the first one using spm_reslice). We got rather significant differences in the segmentation results, a sample of those results is attached to this post. What we don't understand is the following : Since the prior maps are very smooth, how can changing the resolution of the prior maps influence that much? We couldn't even tell the difference between the 2 prior maps. Any explaination? Thanks in advance, Eric Pierre Case Western Reserve University, Case Center for Imaging Research, Cleveland, Ohio.