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Re: first level ancova

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Tue, 30 Jun 2009 15:40:53 +0100

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 ```Dear Vitaly, > i have an analysis approach question: > > event -related design with behavior measure (e.g. reaction time) > > what do you think about analyzing the data on the single subject > level as an > ancova, where 1 EV has 1's for all events, zeros elsewhere, and > another EV > has the demeaned behavior measure at each event, zeros elsewhere > > versus > > binning events into fast events or slow events, separate EVs, and then > performing a contrast at the first level to pass up. > > so option A is the discrete approach, option B is a binary approach. I would suggest a third approach. If you look at http://www.fmrib.ox.ac.uk/fsl/feat5/detail.html#stats   you'll find a description of the "3 column format" for specifying event-related designs. The third column can be used to put in a parametric modulatory effect. You would then make one EV that model the average effect of these events and another one that models the modulatory effect. Let us say you have 3 events that ocurred a 5 30 and 50 seconds. Let us further say reaction times were 2 1 and 3 seconds respectively. For your modulatory EV you would then specify 5 0.1 0 30 0.1 -1 50 0.1 1 where the second column gives you "eventy events" and the third column is the mean-corrected reaction times. You can then use the contrast [0 1] (assuming that the first column is the vanilla events) to take the dependence on reaction time to the 2nd level. Note that this assumes that the reaction times have a effect on the amplitude of the response, and not on the duration. In principle one could equally well assume the reverse, and unfortunately it is very hard to test since a changes in amplitude and duration will result in very similar models for event related designs. Good luck Jesper```