LLN, le 21/04/06
Dear SPM users,
I posted a related question some time ago and though I had understood
the answer (see
<http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind04&L=SPM&D=0&I=-3&P=370551>,
but I'm again confused. So, allow me to give it a second try.
I have an fMRI study with several pairs of conditions (say A and B, C
and D, etc.) + fixation, each pair+fixation repeated in 2 runs. I
want to run a RFX analysis to look at, say, [A-B] and I tried this:
(a) compute [A-B] for each subject, then use the con* file at the
second level in a 1-sample t-test with a contrast value set at [1],
(b) compute [A-fix], [B-fix], [C-fix], etc. for each subject, then
use the con* files at the second level to get something like
([A-fix]-[B-fix]) in a 1-way within-subject anova, non-sphericity
correction, with a contrast set at [1 -1 0 ...]. NB. The reason for
porting several contrasts in a big Anova is that I also want to use
masks, calculate conjunctions, etc.
Both should give roughly the same results (leaving aside any
"deactivation" issue). Yet, solution (b) gave much more significant
foci than (a). Note that this is the case only if there are more than
2 pairs of con* files in the Anova.
What am I missing here? Does this have something to do with df? Which
of these two solutions is more appropriate?
Any comment appreciated. Thanks in advance,
Mauro.
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Mauro PESENTI
Research Associate, National Fund for Scientific Research (Belgium)
Unite de Neurosciences Cognitives
Departement de Psychologie
Universite Catholique de Louvain
Place Cardinal Mercier, 10
B-1348 Louvain-la-Neuve
tel.: +32 (0)10 47 88 22
fax: +32 (0)10 47 37 74
E-mail: [log in to unmask]
http://www.nesc.ucl.ac.be
http://www.nesc.ucl.ac.be/mp/pesentiHomepage.htm
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