It's probably because the # of voxels being tested increases so much in your third case. Go to one of those bright spots in the con and spmT images, and see if the value is the same in all 3 images. The con images should be the same, I would think. The T images will obviously have different values, but as you can see the same pattern is there.
I think it still would affect a 2nd level analysis, as well.
________________________________________
From: SPM (Statistical Parametric Mapping) [[log in to unmask]] on behalf of Tobias Hauser [[log in to unmask]]
Sent: Friday, July 20, 2012 4:01 AM
To: [log in to unmask]
Subject: [SPM] mask threshold and significance
Dear SPM experts
I am currently playing around with the mask threshold
("defaults.mask.thresh") of the "spm_defaults.m"-File.
I used a simple visual stimulation and looked at the first-level GLM,
which revealed puzzling effects I cannot explain. Could you please
explain me these effects?
When I analyze the data with the default threshold (0.8), I receive a
very strong occipital activation - as expected. The same happens when I
lower the threshold to 0.0 and use an explicit mask. However, when I
analyze the data with threshold 0.0 and without explicit mask, the
activation suddenly disappears. The only activation found is at the
border of the recorded EPI (see attachments).
If I would have used a multiple comparison correction, this would be
understandable due to the increased voxel space. However, these effects
occur when using p<0.001, uncorrected. As far as I know, the 1st-level
GLM is a *mass-univariate analysis*, which means that each voxel is
analyzed independently. Why then are there differences when altering the
mask threshold? Am I right that this effect does not affect the
second-level analysis because it uses the con-files which are roughly
the same across all 3 conditions?
Thank you very much for your help!
Best,
Tobias Hauser
--
Tobias U. Hauser, lic. phil.
Department of Child and Adolescent Psychiatry
University of Zürich
Neumünsterallee 9
CH-8032 Zürich
Switzerland
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