 |
 |
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