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.