Hi Freesurfer experts,

I am trying to look at differences in cortical thickness between the following three groups : (1) MCI, (2) AD, and (3) control group using mri_glmfit (DODS), and want to control for age (continuous) and gender (categorical). 

I have read some other posts on the topic of demeaning age (i.e. subtracting each participant's age from the entire sample's mean age) but I am not sure if I will need it/am on the right track so would like to seek some guidance from you.

1) In order to decide whether I should demean age when including age as a covariate in the fsgd file (i.e. using demeaned age instead of participants' 'real' age) and control for gender, I should include gender as a categorical factor and check if there are any interactions between groups (MCI/AD/Control) x gender x age by running the following:

Research question: 

Are the three groups different in terms of cortical thickness, when controlling for age and gender

Groups:

FemaleMCI

MaleMCI

FemaleAD

MaleAD

FemaleControl

MaleControl

Contrast: 

0 0 0 0 0 0 0.5 -0.5 -0.5 0.5 0 0 for group (i.e. MCI versus AD) x gender x age interaction

0 0 0 0 0 0 0.5 -0.5 0 0 -0.5 0.5 for group (i.e. MCI versus control) x gender x age interaction, and 

0 0 0 0 0 0 0 0 0.5 -0.5 -0.5 0.5 for group (i.e. AD versus control) x gender x age interaction

If none of these is significant, then re-run the analyses using DODS/fsgd file with contrasts of interest e.g.:

0.5 0.5 -0.5 -0.5 0 0 0 0 0 0 0 0 for differences between MCI and AD. However, if there's there're any significant interactions, I will have to demean age? Is this correct? Or do i have to check for interactions in the first place at all given my research question? Also, say if there's a gender effect, how should I control for it?

2) On a separate question, if I want to correlate disease groups (MCI/AD) with behavioural data such as depression - i.e. to see if cortical thickness in MCI/AD is correlated with depression, do i just have to create fsgd files with only one group (MCI OR AD) in each file and correlate with depression by running contrast [0 1]? Sorry about this silly question, just wanted to double check as I am new to freesurfer!

Thanks for your help in advance!

Regards,

Jen