Dear SPM experts
I have done a VBM analysis and compared two groups with a t-test. If I
corrected both the height threshold (FWE = 0.05) and the extent threshold
(FWE = 0.05) then the resulting extent threshold is one voxel. If I
corrected only the height threshold (FWE = 0.05) and leave the extent
threshold uncorrected (p = 0.05 uncorr.) then the resulting extent threshold
is 71 voxels. This also occurs even when the height threshold is not
corrected, e.g. p = 0.00001 uncorr. (66 voxels vs. 116 voxels).
Intuitively, I expected that the number of voxels should increase when
correcting stronger, but the contrary is true.
Should I be worried about that, or is it a normal behaviour of extent
thresholding?
I used SPM5 (upd. 1782) and the extent correction tool from the VBM5 toolbox
(by Christian Gaser)
Thanks in advance
Best regards
Juergen
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Juergen Haenggi
Ph.D. (Dr. des.)
Division of Neuropsychology
Institute of Psychology
University of Zurich
Binzmuehlestrasse 14, PO Box 25
8050 Zurich, Switzerland
0041 44 635 73 97 (phone office)
0041 76 445 86 84 (phone mobile)
0041 44 635 74 09 (fax office)
BIN 4.D.04 (office room number)
[log in to unmask] (email)
http://www.psychologie.uzh.ch/neuropsy/ (website)
http://www.juergenhaenggi.ch (private website)
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