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Claus - I then used an F-contrast (1 0 0; 0 1 0; 0 0 1) testing for whether any of the three regressors explains some variance in my data. In addition, I calculated a T-contrast (1 0 0) for the canonical hrf only. In general, the pattern of results looks very similar for the two contrasts. However, what I do not understand is why the t-contrast shows some voxels and clusters which fail to be significant at all in the F-contrast. The reason is that, if the majority of variance is captured by the canonical HRF, then the T-test is a more sensitive test of this (it tests only one dimension of the subspace tested by the F-test, including a specific direction - ie one-tailed). As you say, if variance loaded on the derivatives, you could find the opposite pattern of significance in the F but not T map. In many conditions, subjects and brain regions, the canonical HRF does seem the capture the majority of variance. However, there are some regions/subjects/conditions where the derivatives capture additional variance, see, eg, section 3.1 of: ftp://ftp.fil.ion.ucl.ac.uk/spm/data/rfx-multiple/rfx-multiple.htm Rik ---------------------------------------- Dr Richard Henson MRC Cognition & Brain Sciences Unit 15 Chaucer Road Cambridge CB2 2EF, UK Tel: +44 (0)1223 355 294 x522 Fax: +44 (0)1223 359 062 http://www.mrc-cbu.cam.ac.uk/~rik.henson ---------------------------------------- ----- Original Message ----- From: Claus Lamm To: [log in to unmask] Sent: Monday, October 10, 2005 1:39 PM Subject: [SPM] t-test on hrf for model with derivatives Dear SPM community, I recently reanalyzed data I had modeled before using the hrf only, this time using a model with hrf+temporal and dispersion derivative. According to the suggestions by Rick Henson (e.g. ftp://ftp.fil.ion.ucl.ac.uk/spm/data/rfx-multiple/rfx-multiple.htm) I set up a rfx 2nd level analysis for this. I then used an F-contrast (1 0 0; 0 1 0; 0 0 1) testing for whether any of the three regressors explains some variance in my data. In addition, I calculated a T-contrast (1 0 0) for the canonical hrf only. In general, the pattern of results looks very similar for the two contrasts. However, what I do not understand is why the t-contrast shows some voxels and clusters which fail to be significant at all in the F-contrast. Shouldn't it be the other way round (if at all)? I.e. that I see activation in the F contrast which I do not see in the T contrast (with the reason for this being that the F test also uses variance explained by the derivatives)? Thx a lot for helping me with this claus