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
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!