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Hi, I have a question relating to practical usage of F-contrast. It's very clear how I employ it with movements (manual example). However, consider the case that I have a simple functional localizer with blocks of faces and objects. I build a model with HRF derivatives (time and dispersion). So, at the end, I have 6 regressors (3 for faces and 3 for objects). Now,by using F-contrast for my derivative regressors I found some significant activations. How should I compute a contrast faces > objects? When I had a model without derivatives, I just made a t-contrast for two regressors. Now, the derrivatives are part of the HRF (in cotrnast to movemenets), so I have to take them in the contrast. But how build t-contrast with more than two regressors? Thanks a lot, John On Tue, May 19, 2009 at 10:00 PM, Guillaume Flandin < [log in to unmask]> wrote: > Dear Michael, > > Yes, an F-test allows you to compare nested models and you can easily do > that in SPM. See slide 20: > http://www.fil.ion.ucl.ac.uk/spm/course/slides08/contrast08_fil.ppt > and Section 5.1, page 12: > http://www.fil.ion.ucl.ac.uk/spm/doc/books/hbf2/pdfs/Ch8.pdf > for a bit of theory and the SPM manual p.233 for a practical example: > http://www.fil.ion.ucl.ac.uk/spm/doc/manual.pdf > > Note that SPM also allows to compare non-nested models with Bayesian > inference by comparing model evidence maps, see: > http://dx.doi.org/doi:10.1002/hbm.20327 > This is available in SPM8 under: > SPM > Stats > Bayesian Model Selection > BMS: Maps > > Best regards, > Guillaume. > > > Michael Froelich wrote: > > Hello, > > > > does anyone have an idea how I would go about comparing the model of a > > block-design fMRI dataset with a regressor to the same without regressor. > In > > simple statistics one could do this with an F-test. > > > > Can SPM produce a contrast image comparing these two models? > > > > Michael > > > -- > Guillaume Flandin, PhD > Wellcome Trust Centre for Neuroimaging > University College London > 12 Queen Square > London WC1N 3BG >