> Yes I can easily obtain the ISC between the two levels, so I would just do that. I'm still curious what a model
> with such a factor incorporated would look like.

Maybe something like:

Y ~ group*socialness + (group*socialness | ROI) + (socialness |Subj1:Subj2) + (socialness | mm(Subj1, Subj2, weights = cbind(1, 1), scale = FALSE))

> the paper you attached is precisely what I am trying to accomplish, so I was wondering if the adaptation i
> mentioned to MBA would the correct one (i.e. remove the subject:subject interaction).

Try this:

Y ~ group*age + (group*age | ROI) + (1 | Subj1:Subj2) + (1 | mm(Subj1, Subj2, weights = cbind(1, 1), scale = FALSE))

Let me know if this makes sense to you.

Gang