On Balancing Sharpness of Excitation Patterns, Loudness, and Potential Effects on Speech Perception with...

Cochlear implant (CI) users struggle to understand speech under noisy conditions. This has long been attributed to channel interactions: a given neuron not only responds to electrical stimulation from the closest electrode but also is sensitive to stimulation from electrodes located further away. Shaping the stimulation voltage into a very sharp profile (e.g. using tripolar instead of the more common monopolar stimulation) has been proposed as a solution, only with limited success. We argue this is due to several factors: the strong correlation between envelopes of adjacent CI channels with speech sounds, the necessity to recruit enough neurons for sufficient loudness, and, at loud levels, the similarity between monopolar and sharp stimulation output voltages at locations that are at least 3-4 electrodes away from a given stimulation electrode.

We describe a "Hilltop" method to effectively reduce the voltage more than 3-4 electrodes away from a given electrode, while keeping the same percept of place pitch as with monopolar stimulation. We assess this method psychophysically in human CI users (N = 7 males, 9 females) and with computational modelling.

Based on computational modelling and on psychophysical measures of charge summation, we show that the voltage difference between monopolar and Hilltop stimulation persists at the level of the spiral ganglion neurons.

The results warrant investigations with more complex, multichannel stimuli. We also describe possible limitations of this strategy, including the presence of sidelobes of opposite effective polarity in some participants, and suggest ways to mitigate their effects.