Hi Janina,
> I would like to compare fMRI data of two groups (14+14) in a 2nd
> level analysis in spm. Is it OK to do a cluster-size correction with
> http://www.sph.umich.edu/~nichols/JG5/CorrClusTh.m by Nichols and
> Wilke (or other tools compatible with spm) INSTEAD of FWE or FDR?
Obviously, the complicated stuff in that script was conceived and
written by Tom :) but in any case, the idea is to generate a cluster
size that "ensures" (at the specified level of significance) that these
clusters did not arise by chance. As you can see, you first have to
specify a voxel-level threshold, which is then used to derive the
cluster size. The question as to whether you can use this approach
instead of FWE or FDR is not an easy one to answer as these approaches
control different things: the "regular" FWE/FDR approaches control the
error on the voxel level, while the script determines the cluster size
that, in combination with a (likely less-stringent) voxel-level
threshold, results in a cluster-size corrected result.
> What does the value "Expected voxels per cluster" in the spm results
> table mean (as this is compleatly different from the value
> calculated with the tool).
This refers to the estimated smoothness of the images that is needed to
calculate the correction for multiple comparisons on the voxel level,
based on the Gaussian random field approach as used by FWE (see Matthew
Brett's excellent tutorial on this one, at
http://imaging.mrc-cbu.cam.ac.uk/imaging/PrinciplesRandomFields).
Hope this helps,
Marko
--
____________________________________________________
PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
Leiter, Experimentelle Pädiatrische Neurobildgebung
Universitäts-Kinderklinik
Abt. III (Neuropädiatrie)
Marko Wilke, MD, PhD
Pediatrician
Head, Experimental Pediatric Neuroimaging
University Children's Hospital
Dept. III (Pediatric Neurology)
Hoppe-Seyler-Str. 1
D - 72076 Tübingen, Germany
Tel. +49 7071 29-83416
Fax +49 7071 29-5473
[log in to unmask]
http://www.medizin.uni-tuebingen.de/kinder/epn
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