Vanessa,

In the typical experiment, if you are comparing PM, then the PMs have the
same trailing name for each condition. In that case, you can just list the
single contrail. The contrail can be determined from SPM.Sess and/or
SPM.Sess([sesssion]).U(condition).P.name.

If the contrails are different names, then you'll need to specify
contrail.L={};
and contrail.R={}. This should work, although has not be thoroughly tested
across systems as most pmods have a common name across conditions.

Hope this helps.

Donald