PPI is fundamentally different the task activity.
PPI explains the covariation in signal that is in addition to the
task. One way to think about PPI is to think of the BOLD response as
smooth and then think of another region cause fluctuations up and down
from that smooth response and that these fluctuations are the result
of the activity in the seed region.
Thus, the task activity is separate from the PPI activity.
Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General Hospital and
Harvard Medical School
Office: (773) 406-2464
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On Sun, Jun 26, 2011 at 1:48 PM, Phil Yoss <[log in to unmask]> wrote:
> Hello all,
> is it possible to obtain highly significant PPI results (i.e. areas that
> significantly correlate with a psycho-physiological interaction regressor)
> which show a very different response profile than the seed region going into
> the PPI regressor? I.e. if my PPI regressor shows high activity on task A
> and low activity on task B, why is it that I am getting regions correlating
> with this PPI regressor, but which show e.g. negative activity on task A and
> B, or negative activity on task A but positive on task B, or positive on
> both task A and B? (I am plotting the % signal change for the areas pulled
> out by the PPI).
> I don't understand why the BOLD response profile varies so much in areas
> correlating with the PPI regressor, and why the response profile (% signal
> change by condition) doesn't look like the PPI regressor (i.e. high on task
> A, low on task B). The areas in question survive thresholding at p < 0.005
> or lower, corrected. Am I doing something wrong?
> Thanks much,
> Phil
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