Thank you.
Very helpful!
A.
Hi everybody,
in the context of a shape analysis using FIRST, I want to observe the effect of shape, SNPs and their interaction on a cognitive variable.
Anderson Winkler and Mark Jenkinson suggested that the correct way of doing this would be to test a model in randomise using cognition as a dependent variable.
In particular, the model will look like this
Cognition = SNP + Shape + SNPxShape + a bunch of covariables like sex etc
To do so, i need to use some voxelwise EV in my models, and being the first time, I am getting confused.
The first thing that I did is multiplying the structure of interest mask for the cognitive measure of intereset for each subject, and then create a 4D volume. This volume will be the dependent variable in the randomise model (i.e. the input volume).
Then, for the interaction between shape and SNP, I would need to multiply the shape volume for the SNP, again for each subject.
Now, in this analysis SNP is coded as group (3 levels), rather than as a continuous variable.
This make unclear to me what I should do. I am thinking of coding the three groups with a dummy coding (i.e. two columns, 0 0 for group 1, 1 0 for group 2, 0 1 for group 3) and then multiplying the shape volume for these two columns.
In this way, the model in randomise would look like this:
Cognition(4D volume) = SNPDummy1 + SNPDummy2 + shapeXSNPDummy1 (4D volume) + shapeXSNPDummy2 (4D volume) + covariates
Does this make sense ?
I have more doubts:
1) Should I demean the two dummy columns that code for the group ?
2) Is correct to leave SNPDummy1, SNPDummy2 and all the other covariables as usual EV in the .mat file ?
3) Since I am mixing EV in the .mat file and voxelwise EV, how can I construct the contrasts of interest ? In which order the voxelwise EV are entered in the model ?
Thank for your time
Alain