Thank you very much Alfonso, this is very helpful!! Can I ask a clarification question? What is the difference between the contrast you specified [1 -1 0 0 0 0 0; 0 1 -1 0 0 0 0], and [1 -1 0 0 0 0 0; 0 1 -1 0 0 0 0; 1 0 -1 0 0 0 0 0]? My guess was that I'll need the later in order to specify any possible group effects, and I see that these two contrasts give slightly different beta values. If you could shed some light on this it would be a great help.

Thank you very much!!

Liron.