Eric,

Given that you have at least 3 conditions and probably an implicit baseline, you should use the gPPI toolbox. gPPI has been shown to be more accurate than the traditional PPI for experiments with more than 1 condition (plus baseline).

The difference is that the PPI regressor is split by condition. If you have 3 tasks, you will have 3 PPI regressors. The concept behind this is that if you would use 3 task regressors to explore the task effects, then you should use 3 regressors to explore the PPI effects. Attempts at using a single regressor assume a parametric structure to the connectivity relative to baseline.

In my opinion, studies of multiple conditions using PPI might be reporting false positives and might have also false negatives in them.

The toolbox can be found here: www.nitrc.org/projects/gppi

The two papers showing the difference between gPPI and PPI are:

Bush et al. 2013: http://www.ncbi.nlm.nih.gov/pubmed/24055504

McLaren et al. 2012: http://www.ncbi.nlm.nih.gov/pubmed/22484411

Let me know if you have any questions.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Postdoctoral Research Fellow, GRECC, Bedford VA
Website: http://www.martinos.org/~mclaren
Office: (773) 406-2464
=====================
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 (773)
406-2464 or email.

On Thu, Dec 5, 2013 at 8:13 AM, Eric Allard <[log in to unmask]> wrote:

Hi all:

I have a quick PPI question. I am creating PPIs from a contrast where Condition 1 > Condition 2 + Condition 3. For this, I used contrast weights of 1, -.5, -.5 when creating the PPI from the VOI. When the PPI is created, and the three graphs are printed (hemodynamic and neuronal response, convolved psych variable, and PPI) I notice that up at the top, next to the "Factors" prompt, I see that the listed factors have weights of 1, -0, and -0, respectively. Does this mean that SPM defaulted those -.5 weights to 0? I wonder because when I look at the convolved psych variable graph, it appears that the contrast weights conform to a 1, -.5 -.5 structure. Any insight on this would be appreciated. Thanks.