The application relates to a hearing assistance system comprising first and second spatially separated parts adapted for being mounted fully or partially at a first ear and on the head of a user, respectively, the first and second parts comprising a number of first and second electrodes located at a surface of first and second housings, respectively, to allow said first and second electrodes to contact the skin of the user when said parts are operationally mounted on the user's head. The first and second electrodes are adapted to pick up low-voltage electric signals from the user's brain and a reference voltage, respectively, to provide voltage difference signals. An improved binaural hearing assistance system further comprising a comparison unit for determining the voltage difference signal, and an electric interface allowing the reference voltage or a measure representative thereof to be transferred from said second part to said comparison unit is provided.
