hi johan,
have a look at spm_fmri_spm_ui.m (spm8) l. 430ff:
%-Masking structure automatically set to 80% of mean
%=======================================================================
try
TH = g.*gSF*defaults.mask.thresh;
catch
TH = g.*gSF*0.8;
end
SPM.xM = struct( 'T', ones(q,1),...
'TH', TH,...
'I', 0,...
'VM', {[]},...
'xs', struct('Masking','analysis threshold'));
so, I suppose the signal value in your voxels are below the 80% threshold.
hth
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