Dear SPM-experts!
We are running an event related fMRI experiment with a single event type
comparing a group of 8 healthy volunteers to 8 patients. (TR
506ms, 6 slices). Each subjects undergoes 3 fMRI scans (590 scans in each
run) patients get 3 additional scans after application of a specific drug.
We want to compare the activation between both groups and within the group
of patients before and after medication applying a second level analysis.
First we built a seperate design matrix with the simple hemodynamic
response function and its temporal derivative as basis functions including
3 scans per subject resulting in a design matrix with 9 columns. (2
seperate design matrices for patients before and after medication) Then we
calculated the contrast images for the contrast 1 1 1 1 1 1 0 0 0. Those
contrast images were entered into the second level applying a one sample
t-Test (patients only, healthy only), a two sample t-test (patients vs
normals) and a paired t-test (patients after vs before medication).
The results for the within group analysis show little statistical
significance whereas single subject analyses show consistent activation in
very similar regions)
Our questions are:
1.) Is the choice of the basis functions and the contrast definition
appropriate. Is it possible to enter a contrast image based on two basis
functions into the rfx-models or have the two functions to be considered
seperately (contrasts 1 0 1 0 1 0 or 0 1 0 1 0 1 respectively). Do we get
different results if only the hrf is enterd into the design matrix
(contrast 1 1 1) (in comparison to a contrast 1 0 1 0 1 0).
2.) Based on an a priori hypothesis what threshold has to be applied in
the second level analysis?
3.) Since addition of more subjects results in more degrees of freedom,
statistical significance should increase. Nevertheless in our analysis,
significance seems to decrease.
Thanks in advance for any help
Bernhard Haslinger
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Dr. Bernhard Haslinger
Neurologische Klinik und Poliklinik
Klinikum Rechts der Isar
Technische Universitaet Muenchen
Moehlstrasse 28
D-81675 Muenchen
E-mail: [log in to unmask]
phone: ++49/89/4140/4699
fax: ++49/89/4140/4966
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