I am saying two things:
(1) You are contrasting the positive weight conditions against the
negative weighted conditions in such a manner that the connectivity of
the positive weights (1s) are assumed to be the same (e.g. if you have
3 conditions and a control AND contrast conditions versus control, you
assume that the connectivity during the conditions is the same) and
the same is true for the negatives (-1s). They should sum to 0 unless
there are 2 conditions OR you think the connectivity is different
between the positive and negative conditions, but assumptions like
this are bad.
(1b) Along those same lines, if you have 2 conditions plus fixation,
then you are assuming that connectivity for yoru conditions is less
than fixation in one condition and greater than fixation in the other.
(2) gPPI -- model each condition as a separate interaction. Now if you
have 3 conditions and fixation, you will get three PPI values - one
for each condition. Then, you can compare the three conditions to each
other as you do in fMRI activation analyses. Additionally, if you were
to add the vectors for each condition together with the weights used
in step 1, you would recover the same joint vector. The critical
difference is the joint vector has imposed a relationship on your data
that may or may not be true more complex designs.
Hope that clarifies my comment from earlier. Let me know if it didn't.
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 Fri, Feb 25, 2011 at 10:26 AM, Stephen J. Fromm <[log in to unmask]> wrote:
> You wrote,
> "The issue of summing was fixed in one of the later releases of SPM5,..."
> Are you saying that it's now OK if the weights used sum to zero, when before they shouldn't?