 |
 |
Hi Adrian, You may want to compute the F-test below which will tells you whether at least one of your condition differ from all other included in your model. It is a standard 2nd level F-test contrast for 1way within subjects ANOVAs. ncond = 4; nsubj = 11; F = eye(ncond)*(ncond-1)/ncond + (ones(ncond)-eye(ncond))*-1/ncond; F = [zeros(ncond , nsubj) F]; It assumes as in your case that the factor *subject *in your ANOVA precedes the factor *condition*. The first expression calculates the following matrix : * 0.75* -0.25 -0.25 -0.25 -0.25 * 0.75* -0.25 -0.25 -0.25 -0.25 *0.75* -0.25 -0.25 -0.25 -0.25 *0.75* Best wishes Swann 2010/11/25 Jonathan Peelle <[log in to unmask]> > Hi Adrian > > > 1. Are the above contrasts correct. > > Yes, they seem fine. > > > > 2. Why do the contrasts have to sum up to 0? I thought the F-contrast > should be [zeros(4, 11) eye(4)], but SPM says "invalid contrast". > > When you have columns for subject effects, it's not possible to > directly estimate the mean for certain conditions. I.e., if you look > at the indicators along the bottom of the design matrix, they will be > gray. This means you can't do the equivalent of a "[1 0...]" contrast > with these columns, and thus an eye(4) contrast is similarly invalid > (because each row contains an invalid contrast). Comparing conditions > is still fine (as you've discovered). > > > > 3. Can I simply multiply my contrasts by -1 to test for "negative" > effects? > > Yes. > > Hope this helps! > > Jonathan > -- Swann Pichon, PhD Laboratory for Behavioral Neurology and Imaging of Cognition Department of Neuroscience, University Medical Center 1 rue Michel-Servet, 1211 GENEVA 4, Switzerland Tel: +41 (0)22 379 5979 Fax: +41 (0)22 379 5402 Gsm: +33 (0)6 26 43 83 61 http://labnic.unige.ch/