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Hi, I'm confused about some concepts and hope someone could clarify my thought. Please point out any errors in this post. In SPM8, I intend to get an averaged gray matter (GM) masks from 10 subjects. Each subject's T1 images are used as input in "Segment" and then I can get unmodulated normalized GM image "wc1xxx" as well as modulated normalized GM image mwc1xxx". I understand that wc1xxx is a tissue probability map (for example, if the intensity of a voxel is 0.45, it means that it is 45% probability that this voxel belongs to GM) and mwc1xxx represents the amount of GM (for example, if the intensity of a voxel is 0.45, it means that 45% of the voxel consists of GM). Right? If I want to obtain an average of thresnolded and binarized GM mask fro these 10 subjects, should I use wc1xxx [in ImCalc, use (i1+i2+...+i10)/10 and then i1>0.2, then I can get the average GM mask with each voxel showing at least 20% probability of being GM] or mwc1xxx [in ImCalc, use (i1+i2+...+i10)/10 and then i1>0.2, then I can get the average GM mask with each voxel containing at least 20% GM]? I guess both methods are acceptable. Finally, I want to use this average GM mask to serve as -mask input in 3dClustsim. Before that, should I smooth it? I guess no. Thanks in advance. Mark