Dear SPMers,
I have a data set of 12 subjects who had 12 run water PET scans. The
subjects split into 2 groups (n=8 & 4 for the groups). I would like to
see which areas of activation are common to the subjects in each group.
I therefore want to do conjunction analyses for each group of subjects.
My question concerns appropriate thresholding (p values) for the results
from these conjunctions.
I have entered the data using the subject x condition interaction design
for PET studies with each subject having 6 scans under each condition.
I have then done a separate contrast for each subject (-1 1). I then
obviously do a conjunction of these individual contrasts including the 4
or 8 subjects I am interested in. I have no a priori hypotheses of
particular areas that I am expecting differences so suspect that I
should be using corrected p values.
I have specified low thresholds for the contrast conjunctions (eg.
p<0.5) which for 8 subjects give fairly discrete activations in sensible
places, but for 4 subjects most of the brain is pale gray. Intuitively
I can understand that the fewer subjects I include in a conjunction the
more likely activations will be common in the conjunction. What I do
not understand is some way to quantify this so that I do not make type I
or type II errors interpreting the conjunctions.
Help on this would be appreciated.
Mark
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Dr. Mark Daglish
Clinical Research Fellow
Psychopharmacology Unit
University of Bristol
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