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Dear Darren, > > Are there iother ways to show how much difference there is between subjects- > that is, a measure of the heterogeneity in the activations? I am referring > not just to whether one group activated more or less in a particular area > but whether the overall pattern of activations is significantly similar or > dissimilar. If you mean testing the hypothesis that the variability differs between groups (voxel-wise), then this could be implemented by a voxel-wise F-test. The numerator of the F would have the residual ss of the young (or old) effects divided by their df (= number of young ss -1) while the denominator would be the analogous value for the other group. This test does not regard central tendency of the effects, but rather their relative dispersions. I don't know if such a test can be performed easily within Andrew's plug-in. It is relevant to note that if the dispersions of the groups are not the same, then one of assumptions of the GLM is violated, though I have heard that it is robust to such violations for equal sample sizes. Your question about "heterogeneity in the activations" could also mean other things, such as different patterns of activations, which would not imply differences in variability of activations between groups. Sincerely, Eric %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%