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The performance of cochlear implant (CI) listeners is limited by several factors among which the lack of spatial selectivity of the electrical stimulation. Recently, many studies have explored the use of multipolar strategies where several electrodes are stimulated simultaneously to focus the electrical field in a restricted region of the cochlea. These strategies are based on several assumptions concerning the electrical properties of the inner ear that need validation. The first, often implicit, assumption is that the medium is purely resistive and that the current waveforms produced by several electrodes sum linearly. In experiment 1, several impedance measurements were carried out in vitro and in eight CI users using sinusoidal and pulsatile waveforms to test this hypothesis. High-resolution voltage recordings (1.1 MHz sampling) were obtained and showed the resistivity assumption to be valid at 46.4 kHz, the highest frequency tested. However, these measures also revealed the presence of parasitic capacitive effects arising from the device at high frequency that could be deleterious to these strategies.Multipolar strategies also require an estimation of the contribution of each electrode to the overall electrical field. This can be partly obtained by measuring the impedance matrix. However, measuring the voltage on active electrodes (i.e. the diagonal of the matrix) is not straightforward because of the polarization of the electrode-fluid interface. Existing multipolar strategies use linear extrapolation from measurements made at neighboring electrodes to infer this value. In experiment 2, we use a simple model including a constant phase element in order to isolate the polarization component of the contact impedance. We show that this model can fit the high-resolution impedance measurements better than previous approaches in the CI field that used resistor-capacitance circuit models despite using the same number of variables. Implications for the design of multipolar strategies are discussed.

Noise- and sine-carrier vocoders are often used to acoustically simulate the information transmitted by a cochlear implant (CI). However, sine-waves fail to mimic the broad spread of excitation produced by a CI and noise-bands contain intrinsic modulations that are absent in CIs. The present study proposes pulse-spreading harmonic complexes (PSHCs) as an alternative acoustic carrier in vocoders. Sentence-in-noise recognition was measured in 12 normal-hearing subjects for noise-, sine-, and PSHC-vocoders. Consistent with the amount of intrinsic modulations present in each vocoder condition, the average speech reception threshold obtained with the PSHC-vocoder was higher than with sine-vocoding but lower than with noise-vocoding.

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