Dear FreeSurfer experts,

I am trying to use the LME toolbox to analyse the hippocampal change over time in a group and also among 2 groups (2 scans per subject).

When I estimate the parameters with lme_fit_FS I become a warning saying that the matrix is singular and my results are NaN. I guess that my design matrix X is not correct, can you please help me?

I have followed the example from the wiki. My model for the two groups study is the following:

Y_ij = ß₁ + ß₂*t_ij +ß₃*Group1 + ß₄*Group1 *t_ij + ß₅*Group2 + ß₆*Group2 *t_ij + ß₇*Gender_i + ß₈*Age_i + ß₉*ICV_i + b_1i + b_2i*t_ij + e_ij

1) intercept (all ones)

2) time (t_ij) . Here I have 0s for the first scan of each subject and a number in years for the second scans. Is it correct?

3) one for Group1 and zero otherwise

4) colum 3) .* time

5) one for Group2 and zero otherwise

6) colum 5) .* time

7) Gender

8) Age of the subject at each scan moment.

9) ICV (converted to liters).

Thank you in advance,

Yolanda