Dear Manja,
It is statistically valid for the threshold suggested by FDR to vary in
the different hemispheres. The reason is that FDR defines a proportion,
not an absolute. The FDR procedure suggests a p-value threshold with the
property that of all positive results at that threshold, one can expect
that no more than X percent are false positives (where X is the desired
FDR rate). It is very possible that the threshold with this property would
be different in the right compared to the left hemisphere.
More informally, one is claiming that of all results declared
"significant", X% are probably B.S. Since X is presumably small and the
number of results large enough to be interesting, one can safely conclude
that the experimental condition explains cortical variation at (most of)
the locations indicated. Now divide the brain into subregions (perhaps
right and left hemisphere, or perhaps something else) in each of which X%
of the significant results are probably false. Then the claim that "X% of
the overall results are probably false" is valid.
+glenn
------------------------------------
Here miracles become marvels, and
marvels recurring wonders.
-- William Beebe
Dr. Glenn Lawyer
+352 661 967 244
Instituttgruppe for psykiatri
Seksjon Vinderen
PB 85 Vinderen
0319 Oslo
http://folk.uio.no/davidgl
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