Dear Axel
Re:
>I have a question regarding conjunction analyses in SPM96. The question is
>quite simple, but I haven't been able to find an answer in the archives.
>I have two task-control comparisons and would like to identify voxels
>activating conjointly during both. To be able to do a conjunction analysis,
>I understand, I first identify task effects for each paradigm individually.
>Since I use a custom 'hemodynamic' model, there's only one weight per
>contrast:
>
>Session 1 (= Paradigm A) +1
>Session 2 (= Paradigm B) 0
>to identify task effects for Paradigm A
>
>Session 1 0
>Session 2 +1
>to identify task effects for Paradigm B.
>
>This, I believe, corresponds to Cathy Price's advice to Andre Guillemin
>(12-18-99) in a similar context. What troubles me, though, is that the
>results are dissimilar from those I get when I analyze each session
>separately. I.e., my Z scores are substantially lower and the Z peaks occur
>in different loci. All data are intensity normalized to a common mean.
>I would be very grateful for suggestions on what might be the problem.
>Thanks!
>Axel Mueller
>
It isn't possible for you to have higher Z scores for each of the
individual contrasts than you do in the conjunction if you are taking the
values from the same voxel. This is because in SPM96, the conjunction is
the sum of the two contrasts (with voxels excluded where there is a
difference between them). It would be more likely that the peaks in the
individual contrasts differ from one another and are therefore not included
in the conjunction. If the Z scores you are referring to in the
conjunction and individual contrasts all come from the same voxel, let me
know and I will think more deeply about it.
Just to check, I presume you had four different conditions. Two for
paradigm A and Two for paradigm B?
Cathy Price
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