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Hi there. First things first: I've always hated probability calculations and set theory! Say you had superb quality DTI (which I don't) and wanted to track fibers (generate probabilistic streamlines in fsl-speak) from the chiasma to the visual cortex (vc). Easy enough: I'd set up the seed in the chiasma and a waypoint mask in the vc, giving me "waytotals_all". Now I'd like to see those paths going from the chiasma to the vc and! passing through the lateral geniculate nucleus (lgn) (i.e. Meyer's loop; i.e. "waytotals_lgn"). If my reasoning is correct, the proportion of samples running from the chiasma to the vc and passing through the lgn is waytotals_lgn/waytotals_all. Now the hard part: I like to find the proportion of samples running from the chiasma “en route” to the vc, but terminating in the lgn (e.g. “waytotals_stop_in_lgn”). In deterministic tractrography that’s a simple (though theoretically not quite sound) exercise: Visually inspect (or count) those paths reaching your waypoint (i.e. lgn), but not leaving it, in respect to those continuing to the vc. In probtrackx you obviously can’t use a lgn termination mask as that would just give you those samples going from the chiasma to the lgn irrespective of if or if not they would have continued. Is there some way of e.g. using unions/intersections of sets of samples that would give you waytotals_stop_in_lgn? I recall that there was something of a similar discussion on the list some time ago (I can’t find it…). Regards. Stefan Kreisel