Hello,
I have a question about modeling interaction effects. I believe that I
have it right but I want to double check.
I would like to look at a Group x Modality interaction effect. I have
subjects with measurements from 2 different imaging modalities. I have
the fixed effects analysis (fmri data) from each subject in 2 groups:
Normals and Patients. In addition, I have a PET image from each
subject. The assumption from this point on is that there is a linear
relationship between the PET data and my fmri results on a voxel by
voxel level. In the preprocessing, I also took care that the final
smoothing of the data is the same in both modalities (i.e. the fmri data
was smoothed more than the pet data).
Setting up the RFX analysis (I have about 20 subjects/group - using only
3 here for simplication).
The group membership would NOT indicate Normals and Patients, but would
differentiate between fmri and pet data.
So Group 1 membership would indicate fmri data. Group 2 membership
would indicate pet data.
Under the Group column I also indicate which data comes from normals
(-nor) and which are from patients (-pat)
Group EV1 EV2 EV3 EV4
1(fmri-nor) 1 1 0 0
1(fmri-nor) 1 1 0 0
1 (fmri-nor) 1 1 0 0
1(fmri-pat) -1 1 0 0
1(fmri-pat) -1 1 0 0
1 (fmri-pat) -1 1 0 0
2 (pet-nor) 0 0 1 1
2 (pet-nor) 0 0 1 1
2 (pet-nor) 0 0 1 1
2 (pet-pat) 0 0 1 -1
2 (pet-pat) 0 0 1 -1
2 (pet-pat) 0 0 1 -1
EV1 would indicate a difference between normals and patients (fmri data)
EV2 - the average of the fmri data
EV3 - the average of the pet data
EV4 - would indicate a difference between normals and patients (pet data)
The contrasts would be:
1 0 0 0 = Simple main effect fmri-nor > fmri-pat (that is, a t-test)
-1 0 0 0 = Simple main effect fmri-pat > fmri-nor
0 0 0 1 = Simple main effect pet-nor > pet-pat
0 0 0 1 = Simple main effect pet-pat > pet-nor
1 0 0 -1 = Interaction (greater activation in fmri (nor-pat) = greater
"deact" in pet (nor-pat)
-1 0 0 1 = Interaction (greater activation of fmri (pat-nor) = greater
"deact" in pet (pat-nor)
Does the above sound reasonable ? I would appreciate any advice.
Another questions, is that I am unsure which contrast would show that
difference between normals and patients in the fmri set is linearly
related to the difference between normals and patients in the pet data set.
Thanks in advance.
Cheers, Arun
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