You may be able to use stim_times_IM with a fixed-shape HRF (e.g. GAM) and get the amplitude measure for each trial. One previous post that may be helpful [
afni.nimh.nih.gov]
[-stim_times_IM k tname Rmodel]
Similar, but each separate time in 'tname' will get a separate
regressor; 'IM' means 'Individually Modulated' -- that is, each
event will get its own amplitude estimated. Presumably you will
collect these many amplitudes afterwards and do some sort of
statistics or analysis on them.