I think you can measure GABA via MRI… at least I think that's what one of the articles I read said… there is a relatively recent study about GABA being involved with regard to Tourettes (where common comorbidities are anxiety, adhd, OCD) so I think GABA may be a major player in this game… http://www.medicaldaily.com/controling-tourette-syndrome-tics-controling-levels-gaba-brain-305084