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Hi Helmut, Figure 4 in this paper might also be interesting: http://gallantlab.org/_downloads/2008b.Kay.etal.pdf The paper suggest that modeling polynomians is more accurate than high-pass filtering. However, they are not explicit about the type of filter they use and I don't see why there should be a huge difference between e.g. DCT filtering vs. adding DCT regressors to the model (which used to be the default in SPM). Best, Martin 2015-06-17 19:21 GMT+02:00 H. Nebl <[log in to unmask]>: > Dear Torben, > > Yes, I've checked the implementation in AFNI (Legendre polynomials), the > order is based on 1 + floor((duration of the session in s)/150 s), with the > 150 s probably based on the same early event-related fMRI studies that > resulted in the default 128 s in SPM. Thus in case of block designs with > long intervals, long block lengths one would run into the same issues > (adjust to HPF to keep slow frequency signal within the model, remove the > noise but also remove possibly major parts of the signal) with polynomial > regressors. I hadn't really thought about that, as the linked papers > dealing with polynomials usually didn't go beyond 4th or 5th order. > > Best > > Helmut >