Dear Dr Weber,
> >2.) in relation to question 1.) a fourier set as basis function would to
> >our knowledge eliminate the need for correcting for different slice
> >acquisition times (at least if no interpolation across slices is
> >performed before). Unfortunately, we encountered problems in
> >understanding the design matrix and setting the contrasts.
> >Furthermore, how many functions would be appropriate and how long
> >should the window length be?
>
> With my relative inexperience in using more than one function to model the
> events, I share your problems with setting appropriate contrasts. However,
> my understanding is that people in the department who have used only the
> primary basis function as the effect of interest, and treated any
> derivatives as effects of no interest, have obtained plausible results.
Generally when using mulitple basis functions one makes inferences
using the SPM{F}. The disadvantage is that refined characterizations
of the evoked response (i.e. activation or deactivation) are not
possible in terms of inference (one has the plot the fitted responses
at every significant point of interest). This is why some poeple like
to use basis functions that have a physical interpretation (e.g. an
hrf [amplitude], its derivative w.r.t. time {latency] and dispersion
[duration]). In this instance contrasts (and the ensuing SPM{T}) have
some physical meaning.
All the best - Karl
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