The application relates to a hearing assistance system (a method and its use) for processing an input signal representative of sound according to a user's needs, the hearing assistance system comprising an implanted part adapted for being at least partially implanted in a user's head and comprising a sensing unit capable of measuring an endocochlear potential at one or more positions along the length of the cochlear partition. The hearing assistance system further comprises a decoder configured to receive said endocochlear potentials or signals derived therefrom and to transform the received signals into signals appropriately conditioned for use as control inputs to the signal processing unit, and wherein the signal processing unit is configured to process the electric input signal in dependence of said control inputs from said decoder. Thereby the processing of the audio signal of a hearing assistance device can be automatically adapted over time.