EAP recordings in Cochlear Implant patients

A Cochlear implant (Cl) prosthesis can help people, who are deaf due to a nonfunctioning inner ear, to restore auditory sensations. Each CI system must be fitted to the characteristics of the patient, e.g. by adjusting the stimulation current range and the stimulation pulse width. This procedure can be done easily for postlingual deafened patients, who can describe their subjective hearing impressions sufficiently, but it is much more difficult for infants and prelingually deafened patients. Due to the miniaturization of Cls it is now possible to implant Cls in children even under the age of two, giving these children the opportunity to learn talking at the same age normal hearing infants do. The number of patients who can`t describe their hearing impressions sufficiently is therefore increasing, thus objective adjustment methods become more and more important. The recording of the Electrically evoked Compound Action Potential (EAP) is currently one of the most promising methods. Building on the results of a previous FWF project (P11088-ÖTE) it is planned to continue research in the field of EAP measurements, using and optimizing the equipment available from of the previous project. The goal of this project is to specifically determine the parameters necessary for EAP measurements at high stimulation rates, and for EAP recordings for simultaneous stimulation, to estimate the peripheral influence of channel interaction. Finally the EAP recording technique must be refined and modified so that the hardware can be miniaturized and integrated into an application specific integrated circuit being developed to include all the electronic circuitry of a new cochlear implant.

Cases of profound hearing loss and deafness can often be treated successfully with an inner ear auditory prostheses, the so called cochlear implant (CI). Contrary to conventional hearing aids cochlear implants are based on the principle of direct electrical stimulation of the auditory nerve. The stimulation patterns are derived from the speech signal using various signal processing schemes. The level of speech intelligibility that can be reached depends, among other parameters, on patient specific speech processor settings. Thresholds and maximum stimulation levels have to be found. The adjustment of these values can be very difficult especially with children, and the use of objective measures might turn out to be helpful. Other uses of such measurements like technical and medical diagnoses and for research purposes are advantageous. The measurement of the electrically evoked compound action potential (CAP) as a response of the auditory nerve on the electrical stimulation turns out to be suitable, because the potentials of the fibers of the auditory nerve are measured directly. The neural response is independent of how awake the patient is and is not distorted by muscle artifacts. Also, at the location of the CAP measurement, signal processing by the brain has not yet happened. The small amplitude of the neural response in combination with the existence of unavoidable artifacts requires designing special measurement schemes. The main problem is the stimulation artifact, driving the amplifier into saturation, and leaving a residual charge on the electrodes which is superimposed on the nerve response (residual artifact). Contrary to usual measurement systems, which use "fast overload recovery" amplifiers to overcome amplifier saturation the method presented here is to disconnect the amplifier from the electrodes during stimulation (blanking amplifier). Evaluation of this concept is done based on patient measurements using a test system with a newly developed application specific integrated circuit (ASIC) containing a blanking amplifier and a first order S- modulator. For example, a dependence of the refractory behavior of the auditory nerve of the polarity of biphasic stimulation pulses could be found if stimulating with double pulse sequences. This behavior can also be seen when simulating a single nerve fiber. An essential part of the new chip is a system for measuring CAPs, implementing the measurement concept with a blanking amplifier and an analog to digital converter. Beyond that the chip permits the application of biphasic and triphasic stimulation pulses. Because of stimulus artifacts it was not possible to record EAPs at the same intracochlear contact which was used for stimulation. Consequently, stimulating and recording electrodes had to be different, and this raised the question about the relevance of the stimulating and recording electrode positions: How does the electrode configuration used influence the EAP threshold? Since the EAP threshold was derived from the EAP amplitude growth function (AGF), AGFs obtained for different electrode configurations were compared. EAP recordings obtained for a fixed position of the stimulating electrode but with various recording electrode positions lead to amplitude growth functions (AGFs) with similar EAP threshold and morphology, for which only the EAP magnitude (and hence the slope of the AGF) varied with the recording contact. In contrast, for a certain recording electrode position the onset of the EAP growth function often varied significantly between different stimulating electrodes, resulting in different EAP thresholds. This indicates that the characteristics of the evoked intracochlear potential mainly represent the neural excitation originating from the stimulating electrode, and are therefore assumed to be sensitive to the characteristics of the local neural population. The position of recording contact is of minor importance and influences only the magnitude of the recorded EAPs: With increasing distance between stimulating and recording contacts the response becomes smaller.