Dear SPMers
I am puzzled by a change in both the areas of activation, and the
strength of these activations when I look at subjects individually
compared to as part of a larger contrast.
When I look at the same subject’s activation to one condition I
get different areas of activation (with some common) and
different levels of significance depending on whether it is
separate or part of a larger model. All design matrices and
contrasts have been conducted on the same ‘snra’ files so there
is no chance that different parameters for slice-timing,
reorientation, normalisation etc have been used.
I also get this with group activations. Ten controls analysed
together give strong activation of left middle temporal gyrus (176
voxel cluster; z=6.26) and left medial prefrontal gyrus (40 voxel
cluster; z=5.55) at p<0.05 corrected.
When I look at the same 10 controls as part of a 20 subject
analysis (10 controls, 10 patients) by putting zeros for patients in
the contrast, at p<0.05 corrected there is one 4-voxel cluster in
the left middle temporal gyrus (z=4.94)! When I reduce the
statistical significance to p<0.001 uncorrected I get a larger
activation of left middle temporal gyrus (156 voxel cluster,
z=4.94) but absolutely no left medial prefrontal gyrus activation.
I also get activation of right and left parietal cortex (z=4.14 and
z=3.79) which doesn’t appear on the 10 control SPM ‘on their
own’. The left medial prefrontal gyrus activation appears around
P<0.05 uncorrected! But not surprisingly by that stage so do an
awful lot of other areas!
Why am I getting such hugely different significance levels and
varying areas of activation? I want to do a comparison of
patients minus controls on the tasks, but am not sure how to
interpret the results until I understand which areas and
significance levels are included in the calculation.
I look forward to hearing from you and thank you in advance for
your time.
Best wishes
Tom
Tom F.D. Farrow, PhD
Research Associate,
University of Sheffield,
Academic Department of Psychiatry,
The Longley Centre,
Northern General Hospital,
Norwood Grange Drive,
Sheffield.
S5 7JT.
UK
Tel: (0114) 226 1511 (Direct Line)
(0114) 226 1501 (Departmental Office)
Fax: (0114) 226 1522
E-mail: [log in to unmask]
http://www.shef.ac.uk/psychiatry/
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