Hi all. I'm new to FSL, so I've got a rookie question that I haven't quite seen through based on the answers posted here. (Quick pre-digression: I've noticed that Dan Keeser has posted to this list a couple of times -- Dan, I think I'm doing something very similar to what you did in your 2011 J. Neurosci paper.)
I need to set up a design matrix for dual-regression on concatenated ICA of resting-state data. The experimental design incorporates a between-subjects factor (Group: true vs. sham brain stimulation) and a within-subjects factor (Time: pre- and post-treatment). My effect of interest is a Group x Time interaction.
My initial impulse is that the design matrix should look like the following:
Grp TRPR TRPO SHPR SHPO
1 1 0 0 0
1 0 1 0 0
2 0 0 1 0
2 0 0 0 1
... with the first row being a truly stimulated subject pre-stimulation, the second a truly stimulated subject post-stimulation, and the next two rows pre- and post-stimulation for a sham subject. Then the contrast I want looks like this:
Title TRPR TRPO SHPR SHPO
C1 IXN -1 1 1 -1
... or, phrased conceptually, (TRPO-TRPR)-(SHPO-SHPR).
Two problems. First, this matrix doesn't incorporate the repeated measures nature of the design. It seems like the usual solution is to add one EV per subject. That can be done, but does it then compromise the between-groups part of the interaction? Do I need to add an EV for each group mean as well? (That seems redundant with the "Group" designation, but I don't really understand what groups do in this context.) And second... it seems like no one else's solutions look like this. I can't tell if this is because interactions like these with within- and between-subjects components are actually not really handled in FEAT yet (some emails on the list indicate they aren't), or if it's just because I don't really understand how FEAT works yet.
It seems like a common approach people around here use for problems like these is to compute pre-post differences offline and then do basically an unpaired t-test on the differences, putting it on a totally between-subject basis. But I think the fact that I'm using dual_regression takes that off the table, because dual_regression needs time course inputs. I would really, really rather not try to modify dual_regression, though I'll try if that's what's required.
I hope this all makes a bit of sense. Happy to expand on any of it. Thanks in advance!
Matt
