Hi all,
My lab is interested in performing a PPI analysis on an event-related
design (345 scans; TR=2s; 6 conditions; 96 trials; each modeled as
3s). I have been reading through the relevant posts on this listserv,
as well as the Gitelman Neuroimage paper. I understand that I need to
deconvolve the hrf before forming the PPI, then reconvolve, but am not
clear on how to actually implement these concepts in SPM2.
After setting up first-level analysis and grabbing my VOI, I tried a
few different things. First I did a "simple deconvolution" using my
VOI, which gave me a ppi file with a PPI.Y regressor. Then I tried
doing a "psychophysiologic interaction", using my first-level SPM.mat
and my VOI. The PPI.Y regressor from the psychophysiologic
interaction and the PPI.Y regressor from the simple deconvolution
contain the exact same values, which leads me to believe I'm missing
something important.
Does anything need to be changed during the first-level analysis or
does that get modelled as usual? It seems what I'm missing are the
steps in SPM2 to deconvolve the hrf and use that to form the PPI. I
assume the reconvolution takes place when setting up the PPI analysis
in "fMRI design", and selecting "hrf" as basis set under Hemodynamic
Basis functions.
I've tried to study up on this so as not to waste anybody's time, but
I think I've hit a wall. Thanks for any help you can provide!
Patrick Pruitt
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