Hello Philipp
thanks for your comments. May some of my questions were not clear:
>Hello,
>
>SPM uses the mask you give it in the prompt (or the box or sphere) as
>search volume
>and indeed corrects on the voxel and cluster level using the volume of the
>mask instead
>of the "whole search volume", usually defined by the "mask.img" in your
>analysis folder.
>
>However, the procedure is designed to focus on one cluster (or clusters)
>and the regions around it
>assuming that you are indeed interested in clusters in your "ROI".
>
>So, as fas as I can see it from other analyses here, the clusters reported
>AFTER s.v.c. show overlap with your ROI (although
>they - but please check) are not necessarily completely within the ROI.
That may correct.
>
>If your cluster is 'cut' by the ROI the cluster size k is of course
>reduced, and you cannot expect that the ROI svc works
>on cluster parts that are not in your ROI.
I dont expect that. I am looking for a way to get the clusters (their
spatial extend) which are significant e.g. at p.svc-FDR = 0.05.
Eventually my spm2 version is corrupt - but I cant observe any changes of
the clusters in the glassbrain after applying svc.
Actually the user is not asked to specify a signifcance level p.svc - so
its not supprising that the clusters do not change. However, I dont
understand which sense makes such a s.v.c. in VBM? Can someone tell me the
whole story please ?
>
>So, statistically the answer on your last question is "no" - only voxels
>within the are interpreted, and so the cluster
>p-value coming out from this does not refer to the "original whole cluster"
>of your whole brain analysis.
The problem is, that so far as I understood, the cluster-levels can not be
used in VBM, because of the assuption of a gaussian distribution of the
respective voxel values which is strongly violated by the propability maps
used in VBM.
>
>You also should check if your ROI definition makes sense if you eventually
>report cluster outside the ROI...
No - we will not report clusters outside the ROIs - they are inverse and
represent in total the whole brain.
best regards
Sebastian
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Dr.med.Dipl.Phys. Sebastian Thees
Klinik und Poliklinik für Diagnostische und Interventionelle Radiologie
Universitätsklinikum Leipzig
Liebigstraße 20
D-04103 Leipzig
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