Dear Bernhard,
> we are trying to specify a bloc design fMRI analysis with variable
> durations within trials. When we enter variable durations:yes, SPM asks
> to specify the number of scans. Later on this number (vector) has to
> be specified again. The resulting design matrix seems to make no sense
> to us. Entering "no", SPM allows to specify the length of the
> different epochs of each trial by entering a vector. The resulting
> design matrix seems to be ok. Can anyone tell us what is the sense -
> and how to use - the function "variable durations" within SPM99? Any
> help would be highly appreciated.
This issue has been addressed before on the help-line. In brief the
critical thing to remember is that variable-length epochs are modeled
using event-related basis functions as trains of events. If you
specify variable lengths then ensure your subsquent basis function
selection comes from the event-related options (not epoch-related).
The reason variable length-epochs are specified in such a clumsy way is
that fixed-length epochs allow you to use the same (epoch-related)
basis set for each epoch. This allows you to subsequently plot the
epoch-related response in exactly the same way as event-related
repsonses (in fact SPM99 treats event and epoch-related design
identically - the only thing that differs is the basis set).
I hope this helps - Karl
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
|