This is fine. You are simply asking whether or not an active region
has differential connectivity with other regions.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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On Tue, Jul 19, 2011 at 6:13 PM, <Firstname> <Lastname>
<[log in to unmask]> wrote:
> Dear PPI methods experts,
>
> Suppose, we have performed a pretty standard GLM mixed effects
> analysis and have identified peak voxels in a contrast of "condition_A
> vs condition_B".
>
> Is it theoretically sound to then proceed by using those voxels in a
> PPI analysis, where the psychological context is again "condition_A vs
> condition_B".
>
> Even though conceptually this seems intuitive, after studying
> Friston's et al (1997) "Psychophysiological and Modulatory
> Interactions in Neuroimaging" paper in detail, it seems like we should
> not be using as source area a seed voxel whose activity is known to
> have the most systematic variation between the levels of the factor
> used in the PPI (i.e. condition_A vs condition_B).
>
> This issue is best summarized in a paragraph from page 227 of the paper:
> Psychophysiological Interactions and Factorial Designs
>     Psychophysiological interactions generally depend on factorial
> experimental designs, wherein one can introduce neurophysiological
> changes in one brain system that are uncorrelated with the stimulus or
> cognitive context one hopes to see an interaction with. We make this
> point explicit, suggesting that this is another example of the
> usefulness of factorial experiments: Although it is possible to test
> for psychophysiological interactions in almost any experimental
> design, the use of factorial designs ensures that any
> psychophysiological interactions will be detected with a fair degree
> of sensitivity. This is because the activities in the source area, the
> psychological context, and the interaction between them, will be
> roughly orthogonal and therefore one can use the first two as
> confounds with impunity. The converse situation, in which only one
> stimulus or cognitive factor has been changed, may render the activity
> in the source area and changes in the factor correlated. If this is
> the case, there is no guarantee that the interaction will be
> independent of either and its effect may be difficult to detect in the
> presence of the ‘‘main effects.’’
>
> To me, it would seem that this paragraph precludes the approach I
> briefly  described, however the phrase "although it is possible to
> test for psychophysiological interactions in almost any experimental
> design" does leave some room for doubt.
>
> Please forgive me if I am misinterpreting something, but to me it
> would seem like it would be more appropriate to take peak voxels from
> a GLM contrast such as "conditions_A&B vs control_condition" and then
> subject those to the PPI with "condition_A vs condition_B" as the
> factor.
>
> I am looking forward to reading your opinion on this matter.
>
> kind regards,
>
> Nu
>