Thanks for the reply.
It still seems to me that explicitly orthogonalizing within-trial can lead to more problems than good. Although, if any multiple regression experts want to chime in, I'm all ears.
In the case of FIR models, it seems like orthogonalizing each successive regressor with respect to the first (or is it done with respect to the preceding regressor?) would introduce bias. Moreover, in most of the FIR designs I've looked at, the correlation between each successive time bin is usually negligible (0.02 to 0.1).
In the case of parametric designs, I've yet to see a situation where the event defining regressor (convolved with the HRF) is correlated with the parametric modulator. I think you would have to be very unlucky to chance upon a parametric modulator that is correlated with the regressor defining your events.
If memory serves, the addition of within-trial orthogonalisation occurred between spm2 and spm5 (or perhaps it was spm5 and spm8) do any of the developers remember why? I'm a little antsy about disabling something that may be essential.
Cheers.
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