Hi Freesurfer experts,
I'm hoping you can help me understand how to interpret interactions in clusters identified in whole brain analysis using glmfit and glmfit-sim. Below I describe what I've done and what I'd like to be able to do. Any suggestions would be most appreciated!
- I have two groups (patients, controls) and a behavioral variable of interest (social functioning). I am interested in cortical thickness differences between groups (main effect of group), whether cortical thickness relate to social functioning across the group (main effect of social functioning), and whether this relationship differs by group (group x social functioning interaction). - I ran whole brain analysis using mri_glmfit with group and functioning as variables of interest whilst controlling for/regressing out gender, age, and mean thickness. i.e. 4 classes (conmale,confemale, ptmale, ptfemale) and 3 continuous variables (age, AvgThickness, Functioning) = 16 regressors. - I tested the group x functioning interaction with the following contrast - is it correct?
0 0 0 0 0 0 0 0 0 0 0 0 0.5 0.5 -0.5 -0.5
- I then ran mri_glmfit-sim to identify clusters that survive multiple comparisons. This revealed 4 clusters (3 in LH; 1 in RH) that represent regions showing significant group x functioning interaction. - I visualized the clusters in tksurfer, and by loading the y.fsgd file was able to visualize the plotted data to get a sense of the interaction, but this is as much as I know in terms of how to examine interactions in the cluster data......
My specifc questions include:
- I understand that the values in xxx.sig.cluster.mgh overlay reflect log10 p values, the signs of which indicate the direction of the relationship (i.e. -3 = negative correlation between thickness & variable) but I'm not sure how to interpret this in the context of an interaction with group? - I understand that the values in xxx.y.ocn.dat contain the average thickness value for each subject in that cluster and that in a simple between groups test this data could be used to conduct post hoc t-tests to show the direction of the difference, but again I'm not sure how to use this data in the context of the interaction. What do the values represent in a group x variable interaction?
Ideally, I'd like to extract the contrast estimates for each subject in the group x functioning contrast and plot it in another program and conduct pairwise comparisons (t-tests) in order to get a better understanding of the interaction). I'm not sure how to do this - is it possible? My thinking is that I do something similar in fMRI analysis in spm where I can plot the contrasts in a significant cluster and then extract both the average contrast estimates for each group and the contrast estimates for each individual subject.
Thanks in advance!
Laura.