Did you mean-center the covariate-condition interaction terms in design2?
There's often more than one way to do mean centering. I don't recall how SPM sets things up here. Obviously if you take the first such term, ie the first such column, the entries are nontrivial for the first (13?) rows, and all zeros after that. I think that means the mean centering was done "over condition", _if_ it was done. (Which would be the right way to do it in this case.)
The model without mean centering is equivalent (but not identical) to the one with mean centering; the one with mean centering is easily to understand, I think.
Stephen J. Fromm, PhD
From: Ryan Herringa [[log in to unmask]]
Sent: Monday, May 09, 2011 4:11 PM
To: [log in to unmask]; Fromm, Stephen (NIH/NIMH) [C]
Subject: Re: Ancova in SPM8
Your first assumption is correct, that the covariates vary across subjects but not condition. (not that I explained it terribly well)
The second level scans represent intra-subject contrasts (emotion vs. shape) so I think you are right that the subject effects should have been subtracted out.
Thanks for helping to clarify the models - sounds like there is some utility in both which I'll have to give some more thought.
For model 2, am I correct in assuming that if I select either a single condition, or a single covariate interaction term in the contrast manager, that I will be looking at the condition x covariate interaction?