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Hi John

The PPI parameters are not convolved twice. The demo does mention an HRF
basis set when setting up the PPI, but that's only because it is a default
value. The PPI vectors are entered as user defined regressors, and as such
are not further convolved by SPM.

Darren 

> -----Original Message-----
> From: SPM (Statistical Parametric Mapping) 
> [mailto:[log in to unmask]] On Behalf Of John Herrington
> Sent: Tuesday, July 15, 2008 8:46 AM
> To: [log in to unmask]
> Subject: [SPM] PPI, spm_peb_ppi.m and HRF convolution
> 
> 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
>