Hi Michal,
If I had to guess it probably is due to using cluster stats with a fairly
conservative threshold (Z>2.3, p<0.01). Basically this requires that you
have fairly extensive clusters -- typically on the order of 1000 or more
voxels for a whole brain analysis with resels sizes of approx 50
voxels. So you may find that in each group there is a certain amount of
activation -- possibly even with very high Z-scores -- but it does quite
reach your cluster threshold. But if you combine them, then the clusters
merges and voila -- instant significant results.
Why not put the p value to 1.0 which would effectively mean you were doing
voxel stats? If you're use to SPM99, then a z threshold of 3.1 corresponds
to an uncorrected p=0.001 which is the default in SPM. That way you'll see
any voxels that pass that threshold and get a better feel for what's going
on in your data.
GOod luck!
Joe
--------------------
Joseph T. Devlin, Ph. D.
FMRIB Centre, Dept. of Clinical Neurology
University of Oxford
John Radcliffe Hospital
Headley Way, Headington
Oxford OX3 9DU
Phone: 01865 222 738
Email: [log in to unmask]
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