Dear SPMers,
I would like to correlate two activation maps to test the hypothesis
that the spatial distribution of activation over a predefined region-
of-interest is very similar under two conditions. In other words, I
would like to test whether voxels which are more activated in
contrast A are also more activated in contrast B, i.e., whether voxel
values in contrast A predict voxel values in contrast B.
What I have done is a simple correlation (outside SPM) between voxel
values taken from contrast images A and B, using voxels as if they
were subjects. Now, the problem is that using N-2 (where N is the
number of voxels) as degrees of freedom in testing the significance
of the correlation coefficient is clearly uncorrect, because nearby
voxels are not independent (spatial autocorrelation). My intuition is
that degrees of freedom should be adjusted, e.g. using the gaussian
field theory... but how?
Note that a related problem has been posted some times ago, but in
that case the aim was to correlate values derived from two contrasts
over a set of subjects/scans, voxel by voxel, ending up with a
correlation map. The question was: is the activation of each voxel in
contrast A correlated to the activation of the same voxel in contrast
B (over a set of subjects/scans)? My question is instead: are the two
contrast images similar as to the spatial distribution of the
activation?
Many thanks in advance.
Gina Pelle
University of Chieti "G. D'Annunzio"
Department of Clinical Science and Bioimaging
Institute of Advanced Biomedical Tecniques
Via dei Vestini 31
66013 Chieti Scalo, Italy.
e-mail: [log in to unmask]
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