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Dear all, A couple of related questions concerning the modeling/control for reaction time differences between conditions in an event-related design. If I want to include RT as a covariate to control for common RT effects across 4 conditions (rather than use condition-specific parametric modulations), but not turn my analysis into a parametric analysis (such as in Will Penny's example: http://www.fil.ion.ucl.ac.uk/~wpenny/datasets/face-rep/SPM2/README-SPM2.txt) , could I do the following: a) create a model where I only have one even-type (all events), and employ RTs as a parametric modulation of that regressor; then extract the RT regressor from the design matrix and enter it as a user-specified regressor in my original model? b) How about directly modeling each event as having the duration of its associated RT (i.e. event durations would vary between ca. 500-1000ms, rather than being modeled as stick functions)? Is there a strong a priori reason not to analyze data in this way? And finally, would these 2 analyses possibly produce the same results? Many thanks, Tobias. Tobias Egner, PhD fMRI Research Center Columbia University Neurological Institute, Box 108 710 West 168th Street New York, NY 10032 Tel: (+1) 212 342 0121 Fax: (+1) 212 342 0855 Email: [log in to unmask]