[Freesurfer] refeventdur in mkanalysis-sess

Hi,

I am making an analysis in freesurfer 5.1 using mkanalysis-sess and I am a bit confused about the flag refeventdur. In the help it is said that it should be set to the event duration if the events have similar durations. I have four conditions that last for 16 second each in a block design paradigm, but my fixation condition lasts for 8 second. What should I set the refeventdur to? Also I have an event related design with variable fixation durations. what should it be in that condition? How about if I have an event related design with variable stimulus durations? How does this value influence the design matrix?

Thanks, Maryam