Dear Thomas,
>
>thank you for your answer. I see that my problem could be that the
>activated areas have a large magnitude and spatial extent and therefore
>influence the average across all voxel values.
>Maybe the scaling should not use the voxel values of all brain voxels
>for computation of the scaling factor, but only voxels from CSF, where
>we would not expect any useful signal changes. This would imply doing a
>segmentation of CSF before scaling.
>
It would certainly be worth trying. However, when attempting to do that in
the past I have often found that the CSF seems to have a life of its own,
i.e. displays a variance, possibly from pulsatile flow/motion, not found
elsewhere in the brain. Including this as a covariate would certainly
"remove" the ventricles from your SPMs, but I am uncertain whether it would
model "global" changes found in gray matter very well.
Good luck Jesper
Jesper Andersson
Wellcome Dept. of Cognitive Neurology
12 Queen Square
London WC1N 3BG
phone: 44 171 833 7484
fax: 44 171 813 1420
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