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Dear Paul,

It sounds as though you have checked the raw global intensities for outliers
etc, which one could do with something like:

P = spm_get(inf,'img', 'Select all the images in the analysis');
V = spm_vol(P);
for i = 1:length(V)
   g(i) = spm_global(V(i));
end
g

The compression effect you are seeing implies that there is a much greater 
range in the patient's globals than for the controls.

Have you checked whether the globals correlate with the design - for the 
depressed patients for example?  

In PET, higher global counts may be associated with better signal to noise - I 
guess this might explain some of what you are seeing.

See you,

Matthew 




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