Hi - I have a couple of questions about statistics and contrasts in
spm99b.
1) I used spm99b to analyze an fmri dataset with 4 conditions. I tried it
in two ways: specifying all four conditions explicitly, or specifying
three conditions and leaving one unspecified (as an implicit baseline
condition).
I then did the analysis either by specifying a contrast (1 0 0 -1) in the
first model or (1 0 0) in the second model (where the fourth condition was
the implicit baseline). The glass brains look about the same but the
statistics are appreciably different, e.g., one cluster changed in size
from 240 to 202 voxels; I'm guessing that this has to do with the slight
different in dF between the two models.
It seems to me that the model with all four conditions specified is more
appropriate than the implicit-baseline model, particularly if one wants to
go on and do comparisons of the non-baseline conditions - this is based
upon the intuition that these comparisons will necessarily include the
implicit baseline in that model in a way that could lead to misleading
results. Is that correct, or are there other reasons to use an implicit
baseline beyond the 2-condition situation?
2) Is there a simple way to create contrast images for all contrasts in a
particular model? It would be nice if this could be done in one step,
rather than having to click through for each contrast in the results step.
I am thinking along the lines of what spm96 does when it finishes an
analysis and prints all of the contrasts - with added creation of the
contrast images for use in randFX analysis.
thanks again for all of the hard work that you folks have put into SPM!
best,
russ
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Russ Poldrack Phone: (650) 725-0797
Dept. of Psychology Fax: (650) 725-5699
Stanford University
Jordan Hall Email: [log in to unmask]
Stanford, CA 94305 Web Page: http://www-psych.stanford.edu/~poldrack/
After August 1, 1999:
MGH-NMR Center
Building 149, 13th St.
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Email: [log in to unmask]
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