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dear listmembers, in a task I have 2 types of stimuli A and B both of which vary in intensity. I am interested in the neural response to A and B independent of intensity. One way I can do this is to include intensity as a parametric modulator for A and B. However, there are 2 reasons why I might not want to do this: 1) it doubles the number of regressors per session and eats up degrees of freedom and 2) the euclidian normalization performed on the modulator (spm_en) scales the parameters independently for each regressor, so that variance in intensity is not preserved across conditions/sessions. Is there a way to include intensity as a single regressor across all conditions and/or sessions? there is of course the option for 'user-specified regressors' but this option requires a vector equal in length to the number of scans (not the number of trials). Should I resample my intensity such that values correspond to the number of scans? thanks - chris Christopher Summerfield Psychology Dept Columbia University 1190 Amsterdam Ave New York NY 10027 [log in to unmask]