In-vitro characterization of a cochlear implant system for recording of evoked compound action potentials.

BACKGROUND: Modern cochlear implants have integrated recording systems for measuring electrically evoked compound action potentials of the auditory nerve. The characterization of such recording systems is important for establishing a reliable basis for the interpretation of signals acquired in vivo. In this study we investigated the characteristics of the recording system integrated into the MED-EL PULSARCI100 cochlear implant, especially its linearity and resolution, in order to develop a mathematical model describing the recording system. METHODS: In-vitro setup: The cochlear implant, including all attached electrodes, was fixed in a tank of physiologic saline solution. Sinusoidal signals of the same frequency but with different amplitudes were delivered via a signal generator for measuring and recording on a single electrode.Computer simulations: A basic mathematical model including the main elements of the recording system, i.e. amplification and digitalization stage, was developed. For this, digital output for sinusoidal input signals of different amplitudes were calculated using in-vitro recordings as reference. RESULTS: Using an averaging of 100 measurements the recording system behaved linearly down to approximately -60 dB of the input signal range. Using the same method, a system resolution of 10 µV was determined for sinusoidal signals. The simulation results were in very good agreement with the results obtained from in-vitro experiments. CONCLUSIONS: The recording system implemented in the MED-EL PULSARCI100 cochlear implant for measuring the evoked compound action potential of the auditory nerve operates reliably. The developed mathematical model provides a good approximation of the recording system.

Results with a cochlear implant channel-picking strategy based on "Selected Groups".

A novel channel-picking strategy for cochlear implants (CIs) which considers the spatial distribution and the spectral relevance of the channels selected for stimulation is described. In the proposed strategy, the available channels are subdivided into groups, designated as "Selected Groups" (SG), and within each group, a specified number of active channels with the largest amplitudes are selected for stimulation. The hypothesis is that most of the spectral information that can be perceived by CI listeners is conveyed by taking the highest filter band outputs within a stimulation area represented by a group of neighboring channels. Two experiments were conducted in subjects with MED-EL implant systems, measuring recognition of sentences in speech-shaped noise. In experiment 1, the SG group size was varied from two to four while selecting one active channel per group and keeping the pulse phase durations constant. Results showed no significant difference in sentence recognition between continuous interleaved sampling and SG configurations up to a group size of three. In experiment 2, phase durations were doubled, using groups of two channels with one active channel each. This resulted in a reduction of pulse amplitudes by about 40%. Intelligibility of sentences in noise was unaffected, making a substantial reduction of implant supply voltages feasible. In all experiments, the stimulation frame rate was kept constant in order to avoid rate-change effects.