Hearing assistance device comprising an implanted part for measuring and processing electrically evoked nerve responses

1. A hearing assistance device comprising an implanted part adapted for being implanted at a user's ear, wherein the implanted part comprises

A multitude of electrodes adapted for being located in the cochlea in proximity of an auditory nerve of the user;

Stimulation circuitry electrically coupled to a stimulation electrode during a stimulation time period and configured for applying a stimulation signal to the stimulation electrode;

Measurement circuitry electrically coupled to a recording electrode during a measurement time period and configured to measure a signal picked up by the recording electrode in response to said stimulation signal and providing a measured signal;

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;

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,

wherein the measurement circuitry comprises an analogue comparator comprising first and second inputs and an output representing a comparison of the first and second inputs, wherein the first input is operationally coupled to said recording electrode to receive the measured signal during said measurement time period, controlled by the control unit, and said second input is configured to receive an estimated time variant artifact signal,

the processing unit comprises a digital processor and an AD-DA-interface to said analogue comparator, the AD-DA-interface having an analogue input and an analogue output,

an analogue output of the AD-DA-interface is operationally coupled to the second input of the analogue comparator, and the processing unit supplies the estimated time variant artifact signal to the second input via the AD-DA interface,

the output of the analogue comparator, which provides a result of subtracting the estimated time variant artifact signal from the measured signal, is operationally coupled to an analogue input of the AD-DA-interface of the processing unit, and

the AD-DA interface comprises a variable amplifier for amplifying an input to the AD-DA interface by a configurable amplification factor.

2. A hearing assistance device according to claim 1 wherein the processing unit is configured to compute at least one estimator of at least one statistical variable obtained from said measured signal.

3. A hearing assistance device according to claim 2 configured to use said at least one statistical estimator to identify said response from the auditory nerve.

4. A hearing assistance device according to claim 2 configured to generate signals or commands based on a criterion applied to said at least one estimator computed in the processing unit.

5. A hearing assistance device according to claim 1 comprising a switch unit comprising a number of switching elements allowing each of said multitude of electrodes to be selected as a stimulation electrode and/or as a recording electrode at a given point in time.

6. A hearing assistance device according to claim 5 wherein the control unit is configured to use said signals or commands to control the stimulation unit and the switch unit.

7. A hearing assistance device according to claim 1 wherein the control unit is configured to identify said response from the auditory nerve for a given stimulation electrode for a number of different levels of the stimulation signal, and to determine a threshold level of stimulation from said responses from the auditory nerve.

8. A hearing assistance device according to claim 1 wherein the control unit is configured to change a sampling rate and/or an amplification factor of the AD-DA interface, whereby signals from other parts of the nerve system between cochlea and the hearing centre of the brain can be extracted.