I'm conducting longitudinal VBM analysis with 5 time points, each
roughly 3 months apart.
Since some sort of TIV / total GM volume, sex and age are typical
covariates in structural analysis I'm wondering if it's the same for
longitudinal analysis. I think entering age in months as a covariate may
regress out changes happening during our training between time points.
But I still want to control between subjects differences in age as they
may affect GMV.
I was wondering if entering only "initial" age (in first time point) as
a value for each scan would be a correct solution to regress "global"
differences between subjects and still keep temporal dynamics of GMV
within our study?