Output of 0/1 is exactly what you want, so you can multiply it
(or maybe minus some other 0/1 file) times the seed. The goal
is to restrict the seed stimulus time series to stimulation TRs,
with the TRs multiplied by 0 (no stim), 1 (stim A) or -1 (stim B)
to handle an A-B interaction.
- rick
