The application relates to a hearing assistance device comprising an implanted part and to a method of its operation. The disclosure aims at improving the identification and processing of recorded nerve response data in an implanted part. The implanted part comprises a) A multitude of electrodes; b) Stimulation circuitry electrically coupled to a stimulation electrode during a stimulation time period; c) Measurement circuitry electrically coupled to a recording electrode during a measurement time period; d) A control unit configured to control the timing of the application of the stimulation signal in the stimulation time period and to control the measurement time period relative to the stimulation time period; and e) A processing unit configured to record the measured signal in the measurement time period and to identify a response from the auditory nerve based on said measured signal. The invention may e.g. be used for cochlear implant type hearing aids.