> Could someone clarify the following for me ?
Not sure, it is actually more a question I have :)
> Following from the recent SVC discussion I was under the impression that
> the SVC when applied only includes those voxels (within whatever volume
> you have defined) that were initially activated at the previously chosen
> uncorrected threshold. In which case the value chosen for this
> threshold should dictate the number of voxels within the region where
> the SVC is applied and therefore also the resulting p value for the
> analysis. However, I find that regardless of the initial threshold
> chosen (ie be it 0.01 uncorrected or 0.99 uncorrected) the resulting p
> value when the SVC is applied (and the same region is used) is the
> same. This would suggest that a small volume correction includes all
> those voxels within the pre-defined area that one is analysing
> regardless of what threshold was originally chosen for the uncorrected
> analysis. Is this is correct, because reading the emails from August on
> this subject, I did not think this was the case.
Without remembering what the details of this exchange were, I would
intuitively expect the behavior you described.
The rationale for a small volume correction, to me, always was that "I
expect activation within a certain region", as defined by the search
volume. Therefore, the correction for multiple comparisons should take
into account the volume of this region, and not of activation seen
within this region. I may be wrong but taking only the activated voxels
of a prior analysis would be post-hoc, which really is not what "having
an a priori hypothesis about the region of activation" would be about.
Also, if you define a search region for a small volume correction, it
really does not matter if there were activated voxels in the first place
as they may only become significant after SVC, which is perfectly fine
(and likely what most people are using it for :) I would see no good
rationale for excluding voxels within the search volume on the basis of
an analysis that you are not interested in anyway (as it corrected for
the whole brain), but there may be factors involved that I do not see
(anyone: are there?). This is just my take on things which may be
perfectly wrong, but perhaps it stimulates somone who actually knows it
to correct me.
Marko Wilke (Dr.med./M.D.)
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Universitäts-Kinderklinik University Children's Hospital
Abt. III (Neuropädiatrie) Dept. III (Pediatric neurology)
Hoppe-Seyler-Str. 1, D - 72076 Tübingen
Tel.: (+49) 07071 29-83416 Fax: (+49) 07071 29-5473