Cochlea-Implant with Neural Response Telemetry

Cochlear implants are used for the rehabilitation of deaf or severely hearing impaired persons. In cochlear implants, acoustic sensations are caused by direct electrical stimulation of the Acoustic Nerve. Among a variety of stimulation strategies, in the last few years strategies based on high rate pulsatile stimulation have proven to be the most successful approaches. However, due to problems caused by channel interaction, these strategies use non- simultaneous stimuli. New progress in the design of stimulation electrodes seems to allow for a new class of stimulation strategies, which apply high rate pulsatile stimuli simultaneously. One goal of this project is to explore this new class of high rate simultaneous pulsatile stimulation strategies and at the same time establish the fundamentals for a cochlear implant which is capable for implementing those strategies. It is hoped that these new strategies provide more natural firing patterns of the acoustic nerve. A second goal of the present project is the incorporation of a neural response measurement tool in a cochlear implant. Successful stimulation requires the knowlegde of basic patient-specific stimulation parameters, such as stimulation thresholds (THL) or stimulation levels for most comfortable loudness (MCL) for each of the stimulation channel. These parameters are currently assessed in fitting sessions, where the patient is asked about his/her subjective hearing impressions. This method is not useful for all patients, e.g., in small children or congenitally deaf persons. To allow for an objective measurement of stimulation parameters, the so called "electrically evoked action potentials" (EAP) shall be measured in the implant. EAPs are the responses of the nerve to a stimulation signal, where the amplitudes of EAPs are in the range between 10m V - 1000m V. Information about THLs and MCLs, as well as information about the state of the nervous tissue within the cochlea are expected by the so called "amplitude growth function" and the "recovery function", two functions derived from EAP measurements.

Neural Response Telemetry (NRT) integrated into a Cochlear Implant (CI) provides an approach for objective measurement of patient specific stimulation parameters. The neural responses of inner ear structures to electrical stimulation pulses, the so called "Electrically Evoked Action Potentials" (EAPs) can be measured, quantized, and transmitted to external devices. The direct measurement of EAPs seems to be one step towards an automatic fitting procedure. It is assumed that the higher the EAP amplitude is, the more single neurons are firing, and the louder the hearing impression is. However, it is quite difficult to measure EAPs in practical applications, since the EAP amplitudes are extremely small (within the range of 10V to 2000V). In clinical applications, NRT might be a useful tool for patient fitting especially in cases, where direct communication with the patient is not possible, e.g., in very young children. Cochlear Implants are a means to help profoundly deaf or severely hearing impaired persons. With a CI, hearing sensations are elicited by means of direct electrical stimulation of particular regions of the cochlea. Considerable progress has been achieved in the last decade. Today, most patients implanted with a modern CI can expect a high level of speech understanding, and many CI-patients are able to use the telephone again. In the scope of the project a concept for a cochlear implant incorporating an NRT system has been developed. The CI can be used to implement ultra high-rate pulsatile stimulation strategies. Stimulation pulses in arbitrary electrodes can be applied strictly non-overlapping in time, or, as a novel possibility, fully overlapping in time (parallel stimulation). The essential technical specifications are summarized below: RF-link: Channel coding: Amplitude Shift Keying (ASK) RF-carrier: about 11MHz (dependent on coupling) Bit rate: 600kbit/s, self-clocking bit format Stimulation parameters: Max. # of channels: 16 Basic pulsatile waveforms: biphasic or triphasic Max. stimulation rate for standard CIS: 31kpulses/sec Max. # of channels for simultaneous stimulation: 16 Stimulation mode: monopolar against remote ground electrode NRT-system: Duration of measurement window: 1.7ms A/D conversion: 1.2 MHz 1. order S-modulator (adaptive) Max. input signal amplitude: 2200V Duration of read-back: 6.9ms The CI-concept described has been developed in close cooperation with MED-EL corporation located in Innsbruck. Thus, the main results of the current project could be directly used by MED-EL to manufacture a next generation Cochlear Implant. MED-EL meanwhile is one of the three world-wide leading manufacturers of Cochlear Implants, with 14 subsidiaries and about 350 employees world-wide.