Dear SPM ers:
I recently used SPM96 to compare 10 subjects, 2 conditions (bas vs act)
using multisubject different conditions design.
contrast 1 -1 Bas > Act showed one cluster in the frontal cortex at p <
0.005, size = 64 voxels.
I plotted the data and I got the following values
at voxel -56, 10, 28 for
bas 128.87 +/- 2.4684
act 114.56 +/- 2.4684
and for another voxel at -52, 10, 26
bas 161.09 +/- 4.9552
act 149.65 +/- 4.9552.
Interestingly plot of act along the y axis ( . . . . . . ) is
identical but inverted compared to bas ( . . . . . . ) and shifted
down.
Obviously variance of bas versus act is the same for same voxel location
and is heterogeneous between voxels.
Can someone explain why the variance between bas and act for a given voxel
location
is absolutely identical ? Is it a constraint inherent to the math of SPM
or possibly a simple statistical relationship that I disregarded?
Thanks for your input.
Badreddine Bencherif, MD
Division of Nuclear Medicine
Johns Hopkins University School of Medicine
601 N. Caroline St. / JHOC 4230
Baltimore, MD 21287-0855
Phone: (410) 614-2787
Pager in hospital: 3-2050 out hospital: (410) 283-2050
Fax: (410) 614-1977
email: [log in to unmask]
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