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Dear FSL masters, I have a design in which 4 tasks are compared 2 x 2 (e.g. TaskA, BaselineA; TaskB, BaselineB). Now, I have a significant difference in RT between TaskA and TaskB (but not between BaselineA and BaselineB), so to prevent the possibility that differential activation between the two tasks is just due to extra difficulty or activation/timing issues, I'd like to use RT as a covariate. I have some questions though, on its usage. - Is the first level analysis the right place for using the covariate? I have RTs for every trial, so I could just insert an extra EV at every 1st level analysis. There is one thing thought that is not-intuitive to me on this point: my EVs of interest are defined using RTs (to select the on-off periods) so I somehow have the feeling that just everything will be caught in the covariate. I suppose this is an incorrect intuition? - Would using the Average RT for each task in the within subject Second-level analysis (I then do a Second-level analysis aggregating all subs with Random Effx) be a more suitable strategy (I'm taking this idea from the example on the feat5 page of the FSL website)? - To Use the covariate, I have to select "Orthogonalize" EVs, correct? thanks, all the best martin