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Dear SPM experts, I am relatively new to fMRI data analysis in SPM and have a short question on how to deal with missing/unspecified values of a regressor: In my design I use a continuous subject-specific intensity rating related to some auditory stimuli as a regressor in the GLM. In the analysis I'm mainly interested in brain activity related to these intensity ratings. However, between presentation of the auditory stimuli participants also perform some additional ratings. For these rating periods I obviously don't have any intensity ratings that I could put into the intensity regressor. I could just code these periods as zero (or assign some average intensity value to them), however, I was wondering if there is a way to leave them unspecified (i.e. not making any implicit assumption on the intensity values during the rating periods). Another thing I was wondering about is whether it is possible to just delete the respective scans from the session or whether this would mess up my autoregressive model. I'd greatly appreciate any help! Thanks, Moritz