What happens when you use gPPI or PPI in SPM?
The way it seems that you did your PPI is non-standard, so its hard to
tell if its the method or something about the underlying activity that
is making the results look the way they look.
Do you have condition A, condition B, and fixation in your task?
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 Fri, Jul 1, 2011 at 11:06 AM, L. Geerligs <[log in to unmask]> wrote:
> Hi SPM users,
> I have a problem with the interpretation of the findings in a
> psychophysiological interaction (PPI) analysis.
> Recently I did a PPI analysis on event related fMRI data in which one event
> was presented more frequent than the other (oddball task).
> I selected a seed region by using the first eigenvariate of the time courses
> of all voxels in a 6 mm radius around a peak voxel.
> Then I looked at the difference between the two stimulus types (contrast 1
> The results of this analysis showed decreased connectivity from the seed
> region to a large scale network, in the less frequent condition
> compared to the more frequent condition. The network we identified made
> sense according to existing literature.
> The curious thing about the result is that we also observed a decrease in
> connectivity within the brain area which I chose as the seed.
> Moreover, when I repeated this analysis with different seed regions, I found
> different networks, but similar decreases of connectivity
> with the brain area from which the seed time course was constructed.
> Therefore, I started to wonder about the validity of the findings. Is it
> possible that a PPI analysis with conditions with
> unequally frequent stimuli gives spurious results? And if not, how can it be
> that I find a decreased regression from the seed region to
> itself in one condition compared to another?
> The PPI model was constructed in SPM8, in such a way that the ppi variable
> was orthogonal to the Y and P variables (using spm_orth)
> and the data was filtered with the first eigenvariate from the signals of
> white matter and csf voxels.
> Thanks a lot!
> Kind regards,