Hello Gang,
thanks for your thoughts, those are good points to ponder. In fact, when I want to get more spatially-specific information about the activations, I tend to raise the statistical threshold until I get reasonably spatially-restricted clusters. However, it just happens sometimes that, in the same study, you have some contrasts that look good at a reasonable threshold of, say, p=0.001 and k=20 voxels, while others contrasts give you such large clusters that you are forced to pick a really conservative first-level threshold to split them up (eg, p=1e-07), obviously setting yourself up for lots of false negatives. Furthermore, it is probably not correct to pick a t-threshold for a contrast and a different one for another... or is it?
I guess one alternative is to forsake cluster-thresholding altogether and use FDR although, in my experience, the FDR thresholding scheme produces results that are quite a bit more erratic compared to cluster-extent thresholding.
best
giuseppe
