Dear Karl and SPM:
Following up on the issue of conjunctions, in looking at a paper by
Keith Worsley and yourself (A test for a conjunction), it suggested
[to me] the possibility that a conjunction could be taken over a
region of interest as well as at the voxel level. Would this be
possible, that is- designating a region of interest, and performing a
conjunction analysis which would test the null hypothesis that all
subjects did not have an activation in a certain area? Then the
localization would then pertain to the area and not to a particular
voxel.
Unfortunately the maths of the paper escape me, so if this is
possible I'm not sure how to implement this over an area as opposed
to a voxel. Any help appreciated.
Thanks,
Darren.
>Dear Paul,
>
> > I have a few questions concerning conjunction analyses. I have read the
> > mail archives and actually am now more confused. I would really
> > appreciate any input that you may have.
> >
> > I have a study with 9 subjects. I would like to do a conjunction
> > analysis but have several questions.
> >
> > 1. Should corrected or uncorreted p values be used for the analysis.
> > The mailbase continually refers to uncorrected p values.
>
>In SPM99 a conjunction SPM comprises the minimium t values of the
>component SPMs. These minimum t values have their own distributional
>approximation which allows one to compute both corrected and
>uncorrected p values, just like ordinary SPMs. The criteria for using
>corrected or uncorrected inference is exactly the same as for any other
>SPM.
>
> > 2. In relation to question 1, should a height threshold be used. I have
> > worked with SPM and realize that extent thresholds cannot be used with
> > conjunction analyses.
>
>The distributional approximations for the spatial extent of a
>conjunction SPM are not known (at present) and therefor inference based
>on spatial extent is precluded. Consequently height is currently used
>to specify thresholding. This can be corrected or uncorrected and both
>pertin to the final significance of the conjunction SPM (Pconj) (not the
>components - in SPM99b the uncorrected height threshold refered to the
>components).
>
> >
> > 3. What p value is most appropriate? A recent mail from Karl stated
> > that between .5 and .05 is most approptiate. At what level (i.e. simple
> > threshold or height threshold) is this entered? Also, could you explain
> > why such a p value is appropriate.
>
>Thresholds are entered in the results section after specifying which
>contrasts are to enter into the conjunction (by holding down the
>control key). The recommendations above probably refered to uncorrected
>p values for the component SPMs (Pcomp) (in SPM99b). In SPM99 a
>corrected p value of 0.05 or an uncorrected p value of 0.001 would be
>sensible. These might correspond to 0.5 or even more from the
>perspective of the component SPMs. Note that for uncorrected p values
>Pconj = Pcomp^n, where n = number of [orthogonalized] contrasts.
>
>I hope this helps - Karl
Darren R. Gitelman, M.D.
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