Dear SPM Team,
I have a common "button-press-question" to SPM2.
I have 4 contrasts arising from a second level analysis. First, I wanted to
know what is the common regional activation for all of the four contrasts.
For that I defined an ANOVA including all for contrasts. Then I choosed all
four contrasts to calculate a conjunction analysis (using the "conjunction"
button). I got nice results at a p<.001 uncorrected level. O.K. But
afterwards, I tried an "interaction analysis". For the contrasts A, B, C
and D I built a contrast as described by "1 -1 -1 -1" to get the exclusive
part of activation represented only(!) by contrast A. I did the same for
the other contrasts. What is SPM concretely doing in this case? Does it
pool B, C and D? And when doing so, how does it do this? Is there a good
reference to this? Next thing is. When I perform the latter procedure, I
get activation in regions for which contrast A calculated alone does not
show any significant results. So, Iīm a bit confused about that and I hope
that someone of the Team can help me.
It seemīs to be a trivial question but Iīm convinced that many community
members are curious to get more knowledge about this. To the time it seemīs
not possible to publish fMRI data without using these new fashion
procedures. So, the community of users should be better informed about the
realizability of those new methods in a "button press" way. The necessity
for this seems to me sufficiently documented by a recent discussion between
Nichols and Friston in Neuroimage (2005).
Thank you in advance,
T. Fehr (Bremen/Magdeburg)
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