Dear SPM experts,
I have a question raised by a reviewer's comments on our fMRI study:
We have conducted an event-related fMRI study on two groups (patients
and controls) with 3 experimental conditions (3 fMRI runs/subject, each
presenting the 3 conditions in ranomized order).
We were interested in activation differences between patients and
controls for each of the three experimental conditions and for the
differential contrast con1-con2).
We therefore calculated con-images for con1, con2, con3 on the first
level and entered them into a second level full factorial model with two
factors ("condition" with 3 levels; "group" with 2 levels) resulting in
a design matrix containig 6 columns (condition1@controls,
condition1@patients, condition2@controls ...), factor condition:
independence="no", factor group: independence="Yes".
We tested for activation differences by applying t-contrasts (e.g. -1 1
0 0 0 0 for patients>controls during con1, ...-1 1 1 -1 0 0 for the
differential contrast patients>controls during con1 vs. con2).
One of the reviewers is concerned that "Second level random-effects
analysis should be based on simple t-tests which directly compare the
contrast images of interest (rather than using a factorial design that
inflates the degrees of freedom)."
We are now asking if our design is correct/valid or if indeed the second
level analysis has to be performed by calculating separate two sample
t-tests contrasting the corresponding first level con.images (con1,
con2, con1-2).
Thank you very much for your help!
Best regards
Bernhard Haslinger
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PD Dr. Bernhard Haslinger
Oberarzt
Neurologische Klinik und Poliklinik
Klinikum Rechts der Isar
TU-Muenchen
Ismaninger Strasse 22
D-81675 München
E-mail: [log in to unmask]
WWW: http://www.neurokopfzentrum.med.tum.de/neurologie/
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