I am trying to run psychophysiological interaction analyses created
using spm_peb_ppi.m - both within the context of 1st-level SPM analyses
and outside of it. In reading spm_peb_ppi.m, I realized that apparently
the program convolves the critical interaction term with an HRF at the
end. This happens at line 341 (after the interaction term has been
constructed via the deconvolved task and brain signal parameters):
% convolve and resample at each scan for bold signal
%---------------------------------------------------------------------
ppi = conv(PSYxn,hrf);
I found this a bit surprising, as the one demo I know of regarding PPI:
ftp://ftp.fil.ion.ucl.ac.uk/spm/data/attention/README_GLM_PPI.txt
appears to implement a 1st-level PPI GLM that also does a HRF
convolution of the input parameters (see line 140 of that file). I
could be misunderstanding something, but I wanted to be sure... does
this result in the PPI parameters being convolved twice with an HRF?
Thanks - John
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