Dear all,
Concluding the answers (many thanks to Steven, Alex, Matthew and Dr. Perry)
to my previous question
[
> At equal CORRECTED thresholds (P=0.05), there were
> (more and especially) larger clusters with spm96 (identical smoothing!).
> Altogether, activation appeared to be more robust with spm96.
]
and taking into account that the spatial preprocessing and thus smoothness
of the data was really identical, it seems to me, that the smaller
clustersize can only be due to a more stringent correction algorithm in
SPM99. However, Matthew pointed out in his tutorial that excess false
positives in spm96 especially occur at low degrees of freedom (<40). In my
special case this was 56.6 (spm96) and 59.4 (spm99) respectively, and this
should not be to small?
Shall I conclude from this that spm96 is not more sensitive to real
activation (even if it looks reasonable) but more susceptible to false
positive activation and should thus not be used any longer? Is there a
general recommendation what to do if I find significant activation at a
corrected level with spm96 (which seems reasonable), which does not appear
in spm99 with equal significance?
I would appreciate any comment.
Christine Preibisch
Klinikum der Universitdt Frankfurt
ZRAD - Institut f|r Neuroradiologie
Schleusenweg 2-16
60528 Frankfurt
Tel: ++49 69 6301 4651
Fax: ++49 69 6301 5989
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