model increases and decrease in activity over time


I am analysing an event related fMRI paradigm and would like to model a delay period of 18s- specifically I would like to model any increases and decreases in activity over this time period, as well as activity that remains constant. I have been looking the the 3ddeconvolve command and am not sure which option would be best. Could you advise on the best option to model this?

I look forward to hearing from you,