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If you set up the weights as proposed by Gitelman et al. the mean of the psychological term (before computing the interaction) is zero and thus you need not worry about it. However, if the mean of the weights times the number of trials is not zero, then demeaning will shift the vectors used in PPI.

However, I believe that (1) having weights that are proportional to trials in not optimal as it assumes a relationship between trial types is bad; and (2) if you have separate vectors for each task, then they should not be demeaned in order to keep the interaction term 0s during the task and 1s elsewhere. 

I am currently writing a PPI script that allows the creation of a vector and interactions for each task separately.


On Fri, Mar 20, 2009 at 12:16 PM, Jeanette Mumford <[log in to unmask]> 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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Best Regards, Donald McLaren
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D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
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--
Best Regards, Donald McLaren
=====================
D.G. McLaren
University of Wisconsin - Madison
Neuroscience Training Program
Office: (608) 265-9672
Lab: (608) 256-1901 ext 12914
=====================
This e-mail contains CONFIDENTIAL INFORMATION which may contain PROTECTED HEALTHCARE INFORMATION and may also be LEGALLY PRIVILEGED and which is intended only for the use of the individual or entity named above. If the reader of the e-mail is not the intended recipient or the employee or agent responsible for delivering it to the intended recipient, you are hereby notified that you are in possession of confidential and privileged information. Any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited and may be unlawful. If you have received this e-mail unintentionally, please immediately notify the sender via telephone at (608) 265-9672 or email.