This is the conceptual circuit diagram for a ideas detector. It's from The Brain Bulb - I've Got an Idea! by Jeri Ellsworth. She uses an instrumentation amplifier (AD620) with a feedback element which is highlighted below:-
This arrangement is consistent with that chip's datasheet example for an ECG monitor, as:-
I found this question that has a similar circuit, but outputs the common voltage which I can understand if you want to measure it. By why would Jeri's circuit (and the ECG example) feed it back into the person? Isn't the point of an instrumentation amplifier to automatically remove the common signals? And why then do this for Jeri's circuit, and not a Wheatstone bridge? She just says that it's to reduce noise. Lorry weigh bridges have noise too.