Would it be feasible to wear something on a wrist that could detect when the wearer's thumb is touching another finger on that same hand? Through changes in capacitance and/or resistance maybe – or something? If so, would it be able to tell which finger the thumb is touching? Where on the finger? If not something worn on the wrist, perhaps a ring worn on the thumb (or both – as long as they're not hard-wired together? Perhaps transmitting an RF signal and measuring the interference – kind of like the way a metal detector works? Something else (other than visual)?