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Dear Bo, >It sounds as if there may be a few causes for not finding activation at >the group level, none of which are related to the specific shape of the >HRF being used. > >First, with only 5 subjects you'll rarely find any activation with random >effect analyses. You simply don't have the power. > >Second, if the individuals have activations in different areas, and there >is little-to-no common activation across individuals, then nothing is >going to be significant at the group level regardless of how many subjects >you have. > >Finally, if you're using cluster statistics, then lowering the z-threshold >doesn't change the leniency at all -- in fact, it's only likely to hide >smaller (in space) activation regardless of how strong (in Z--scores) they >are. See previously postings on this list about cluster stats and/or try >looking at Matthew Brett's except pages on gaussian random field theory at >http://www.mrc-cbu.cam.ac.uk/Imaging. Good luck. Joe -------------------- Joseph T. Devlin, Ph. D. FMRIB Centre, Dept. of Clinical Neurology University of Oxford John Radcliffe Hospital Headley Way, Headington Oxford OX3 9DU Phone: 01865 222 738 Email: [log in to unmask]