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Dear Bruno, The model you propose for all the subjects seems fine, but if you want to test Beta1>0 & Beta1>Beta2 & Beta1>Beta3. then the contrasts you want to define are [ 1 0 0 ] [ 1 -1 0 ] [ 1 0 -1 ] and then take a conjunction of those three t-contrast. *That* procedure would be correct, and as long as you tell SPM that you have repeated measures (i.e. don't just treat it as 3*N independent scans) you'll be fine. -Tom On Sat, May 22, 2010 at 4:58 PM, Bruno L. Giordano < [log in to unmask]> wrote: > Hello, > > I am modeling the extent to which different stimulus-related features > explain how the BOLD-baseline response varies across stimuli. > > For the sake of the argument, let's assume that the baseline and covariates > of no interest (e.g., head motion parameters) have already been filtered out > of the data. Let's also assume that the features are perfectly orthogonal, > i.e., not correlated. > > As such, my first level model would be a multiple regression of the type: > > BOLD = Beta1*Feature1+Beta2*Feature2+Beta3*Feature3+constant. > > At the group level one thing I am interested in is where Beta is positive. > To this purpose, I run one simple T-test for each of the features using the > Beta images from each subject as dependent data. > > My problem now concerns how to setup a more complex 2nd level model where I > want to find those regions where, e.g.,: > > Beta1>0 & Beta1>Beta2 & Beta1>Beta3. > > Again for the sake of the argument, let's assume that the Betas for the > different features are comparable (e.g., regressors have been standardized > prior to entering the model). > > So, what I am doing for the moment is: > > > [1] setup the following design matrix (N = n subjects): > > Beta1 Beta2 Beta3 > 1 0 0 subj1 > 1 0 0 subj2 > 1 0 0 subj... > 1 0 0 subjN > 0 1 0 subj1 > 0 1 0 subj2 > 0 1 0 subj... > 0 1 0 subjN > 0 0 1 subj1 > 0 0 1 subj2 > 0 0 1 subj... > 0 0 1 subjN > > [2] set up these two contrasts: > > c1:[1 0 0] > c2:[1 -1/2 -1/2] > > [3] find those regions where both c1 and c2 are significant (or test the c1 > & c2 conjunction). > > Is this procedure correct? I am afraid, for example, of inflating the > degrees of freedom. > > Thank you for any feedback, > > Bruno > > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > Bruno L. Giordano, PhD > Postdoctoral Research Fellow > CIRMMT – Schulich School of Music > 555 Sherbrooke Street West > Montréal, QC H3A1E3 > Canada > +1 514 398 4535, Ext. 00900 (voice) > +1 514 398 2962 (fax) > http://www.music.mcgill.ca/~bruno > -- ____________________________________________ Thomas Nichols, PhD Principal Research Fellow, Head of Neuroimaging Statistics Department of Statistics & Warwick Manufacturing Group University of Warwick Coventry CV4 7AL United Kingdom Email: [log in to unmask] Phone, Stats: +44 24761 51086, WMG: +44 24761 50752 Fax: +44 24 7652 4532