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Linda. 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 > -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. I don't the issue is an unequal number of events. I am more concerned about this last step in which you orthogonalize the interaction regressor vs. its main effects (Y and P). This is not usually done in a PPI analysis and perhaps leads to the local negative correlations. Darren > > Thanks a lot! > > Kind regards, > Linda > -- Darren Gitelman, MD Northwestern University 710 N. Lake Shore Dr. Abbott 11th Floor Chicago, IL 60611 Ph: (312) 908-8614 Fax: (312) 908-5073