Dear SPMers
Below is a post of Paul Fletcher concerning the adequate interpretation of a
PPI.
To sort out the ambiguity that can arise from a + / - PPI, I would like to
go to the data and plot the regression slopes separately for psychological
contexts A and B.
I was wondering how to sort my ROI values depending of psychological
contexts A and B given that the psychological variable is convolved ?
Correlating deconvolved responses would be easier but I suppose not
extremely valid.
A third solution could be to select arbitrarily the value of the nth volume
which follows the onset of an event and corresponds approximately to the
peak of the HRF, but this solution seems a bit crude.
Any suggestion welcomed,
Thanks a lot,
Swann Pichon
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
>
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