Hi Jeanette - yes, issues of demeaning are crucial to the
interpretation - see some of Jill's comments on these specifics at:
http://www.fmrib.ox.ac.uk/Members/joreilly/what-is-ppi
Cheers.
On 20 Mar 2009, at 17:16, Jeanette Mumford wrote:
> Hi,
>
> I had a question about how the PPI term is created in
> spm_peb_ppi.m. Specifically I was wondering why the psychological
> variable and neural signals are demeaned prior to taking their
> product and convolving with the HRF to get the final interaction
> term. I'm new to PPI, so I could be thinking about this wrong, but
> it seems like if the psychological variable had a value of 0 over a
> certain span of time the PPI term should also have a value of 0 over
> this time, but since the psychological variable is demeaned this
> isn't exactly the case.
>
> So, is the demeaning necessary?
>
> Thanks for your help!
>
> Jeanette
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Stephen M. Smith, Professor of Biomedical Engineering
Associate Director, Oxford University FMRIB Centre
FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
+44 (0) 1865 222726 (fax 222717)
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