Dear SPMers
Below is a response of Paul Fletcher concerning PPI.
To sourt out the ambiguity that can arise from a + / - PPI, I would like
to go to the data and plot the regression slopes eparately for
psychological contexts A and B.
I was wondering which ROI values are to be associated with psychological
contexts A or B given that the psychological variable in that case is
convolved ?
Alternatively, I suppose that correlating deconvolved responses for both
regions would not be that helpful.
Thanks a lot for your suggestions,
Swann
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&P=R276113
<http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind05&L=SPM&P=R276113>
Hello,
If you imagine that a PPI arises from a significant difference in the two
regression slopes (of Y on X) under conditions A and B, the positive PPI
would suggest that the slope is more positive for A than B (put simply, X
predicts Y to a greater extent under condition A than B, although this is
ambiguous - see below). The negative PPI suggests that the slope is greater
for B than A. Of course (as with a positive PPI) this might arise for a
number of reasons e.g.:
- X predicts Y negatively under condition A and positively or less
negatively or not at all under condition B - or perhaps X predicts Y
positively under condition B and less positively or negatively or not at
all under condition A -or any combination that could produce a more
positive (or less negative) slope for regression X-Y under condition B than
A.
Plotting the regressions slopes for A and B separately helps to clarify the
true state of affairs. Good luck
Paul
On Jun 9 2005, Amit Etkin wrote:
> A simple question...
>
> how do you interpret a negative PPI interaction?
>
> (i.e. if you have conditions A and B, and region X, the interaction of
> A-B with activity in X shows a significant negative interaction with
> activity in region Y)
>
> Amit
>
--
Cognitive neuroscience Ph.D. student
Laboratoire de neurosciences cognitives - UMR 742 INSERM/UPMC
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Swann Pichon
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