Dear SPM-list,
I am currently in the process of analysing the data from an experiment for
which I would expect considerably small, but widespread activation in
several regions of the brain. An exploratory analysis using the
voxel-level statistics and a threshhold of p<0.05 corrected for multiple
comparisons across the whole brain as well as in my prespecified ROIs
revealed only some significant SINGLE voxels. However, a look at the
results threshholded with p>0.001 uncorrected revealed that these voxels
were indeed peaks of large clusters with smaller Z-values, while not many
other clusters emerged. I hence feel justified to analyze the data with
respect to the spatial extent of the activations.
I have two questions in this context:
First, is the calculation of the (un)corrected p-values for the clusters
in SPM99 based on the size of the clusters only, or on a combination of
their spatial extent and peak activation? I have already come across both
explanations on the help list, so a clarification would be greatly
appreciated.
My second question in this context is which INITIAL height threshhold for
the SPM(Z) should be used for this analysis (in other words: which p-value
should be entered when prompted for the height threshhold of the
statistical map, before having a look at the p-values for the clusters
present in the map)? I understand that this choice considerably modifies
the cluster-level inference (since it alters the size of the clusters),
and that the initial height-threshhold should be theoretically
pre-specified instead of "fishing around" at different height
tresholds. As a default I would assume p<0.001 uncorrected is OK, but I
wonder if there are actually papers dealing with the issue, or if anybody
has any experience/suggestions.
Thanks a lot,
Christian
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Christian Carl Ruff
Center for Cognitive Science
Albert-Ludwigs-University Freiburg
Friedrichstrasse 50
D-79098 Freiburg
Germany
MAIL : [log in to unmask]
PHONE: +49 761 2034937
FAX : +49 761 2034938
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