[R-sig-ME] Interaction effects with GAMM

[Louise Baruël Johansen]

I have a question on how to model interaction terms including smooths in a GAMM model (using the mgcv and nlme packages in R).

We have collected longitudinal behavioral and brain imaging data from ~100 subjects across ~6 time points, and I would like to model main effects of age, sex, brain as well as to-way interaction terms (and maybe three-way interaction terms), while correcting for education level and taking random effects into account.

Is using the ti() setup the way to do this:

M = gamm(behav ~ ti(age) + sex + education + ti(age, by = sex) + brain + ti(brain, by = age), random = list(subjectID = ~1+age), data = data)


All help will be appreciated. 

Thanks, Louise