Thanks for everyone's replies. However, I believe that many researchers who use fMRI analysis software are not with a firm statistical background, just like me. For practical reasons, we need a "guideline," if any, to control multiple comparisons problem. Concerning cluster-wise thresholding, below is what I would follow according to Woo et al., 2004 and the recent cluster failure paper in PNAS, but I hope some erperts here can comment a bit.
(1). For SPM and AFNI 3dClustSim users, the first arbitrary cluster-forming threshold (CFT) is suggested to be not too lenient. 0.001 is good, but 0.01 is definitely poor (I have no idea if 0.005 is ok or not?). Then you can report clusters that survive a FWE-corrected p<.05 at the cluster-wise level (but can I report FDR-corrected p<0.05?). The commonly used "P = 0.001 uncorrected with a k of 10 voxels" should be abandoned.
(2). The commonly used "P = 0.001 uncorrected with a k of 10 voxels" should be abandoned (but it seems that many people still use it...).
Besides, I have a naive question: since cluster-extent based thresholding might be more problematic, why don't we just stick on voxel-wise thresholding?
Mike
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