Polarity Sensitivity as a Potential Correlate of Neural Degeneration in Cochlear Implant Users.

Cochlear implant (CI) performance varies dramatically between subjects. Although the causes of this variability remain unclear, the electrode-neuron interface is thought to play an important role. Here we evaluate the contribution of two parameters of this interface on the perception of CI listeners: the electrode-to-modiolar wall distance (EMD), estimated from cone-beam computed tomography (CT) scans, and a measure of neural health. Since there is no objective way to quantify neural health in CI users, we measure stimulus polarity sensitivity, which is assumed to be related to neural degeneration, and investigate whether it also correlates with subjects' performance in speech recognition and spectro-temporal modulation detection tasks. Detection thresholds were measured in fifteen CI users (sixteen ears) for partial-tripolar triphasic pulses having an anodic or a cathodic central phase. The polarity effect was defined as the difference in threshold between cathodic and anodic stimuli. Our results show that both the EMD and the polarity effect correlate with detection thresholds, both across and within subjects, although the within-subject correlations were weak. Furthermore, the mean polarity effect, averaged across all electrodes for each subject, was negatively correlated with performance on a spectro-temporal modulation detection task. In other words, lower cathodic thresholds were associated with better spectro-temporal modulation detection performance, which is also consistent with polarity sensitivity being a marker of neural degeneration. Implications for the design of future subject-specific fitting strategies are discussed.

Impedance measures for a better understanding of the electrical stimulation of the inner ear.

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. OBJECTIVE: 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. The second assumption relates to the estimation of the contribution of each electrode to the overall electrical field. These individual contributions are usually obtained by stimulating each electrode and measuring the resulting voltage with the other inactive electrodes (i.e. 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. In existing multipolar strategies, the diagonal terms of the matrix are therefore inferred using linear extrapolation from measurements made at neighboring electrodes. APPROACH: In experiment 1, several impedance measurements were carried out in vitro and in eight CI users using sinusoidal and pulsatile waveforms to test the resistivity and linearity hypotheses. In experiment 2, we used an equivalent electrical model including a constant phase element in order to isolate the polarization component of the contact impedance. MAIN RESULTS: In experiment 1, high-resolution voltage recordings (1.1 MHz sampling) 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 at high frequency that could be deleterious to multipolar strategies. Experiment 2 showed that the electrical model provides a better account of the high-resolution impedance measurements than previous approaches in the CI field that used resistor-capacitance circuit models. SIGNIFICANCE: These results validate the main hypotheses underlying the use of multipolar stimulation but also suggest possible modifications to their implementation, including the use of an impedance model and the modification of the electrical pulse waveform.

Pulse-spreading harmonic complex as an alternative carrier for vocoder simulations of cochlear implants.

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.

Simulating the dual-peak excitation pattern produced by bipolar stimulation of a cochlear implant: effects on speech intelligibility.

Several electrophysiological and psychophysical studies have shown that the spatial excitation pattern produced by bipolar stimulation of a cochlear implant (CI) can have a dual-peak shape. The perceptual effects of this dual-peak shape were investigated using noise-vocoded CI simulations in which synthesis filters were designed to simulate the spread of neural activity produced by various electrode configurations, as predicted by a simple cochlear model. Experiments 1 and 2 tested speech recognition in the presence of a concurrent speech masker for various sets of single-peak and dual-peak synthesis filters and different numbers of channels. Similarly as results obtained in real CIs, both monopolar (MP, single-peak) and bipolar (BP + 1, dual-peak) simulations showed a plateau of performance above 8 channels. The benefit of increasing the number of channels was also lower for BP + 1 than for MP. This shows that channel interactions in BP + 1 become especially deleterious for speech intelligibility when a simulated electrode acts both as an active and as a return electrode for different channels because envelope information from two different analysis bands are being conveyed to the same spectral location. Experiment 3 shows that these channel interactions are even stronger in wide BP configuration (BP + 5), likely because the interfering speech envelopes are less correlated than in narrow BP + 1. Although the exact effects of dual- or multi-peak excitation in real CIs remain to be determined, this series of experiments suggest that multipolar stimulation strategies, such as bipolar or tripolar, should be controlled to avoid neural excitation in the vicinity of the return electrodes.