On balancing neural precision, loudness, and potential effects on speech perception with Cochlear-Implant current-focussing strategies

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 is also sensitive to stimulation from electrodes located further away. Shaping the stimulation voltage into a very sharp profile (e.g. using tripolar as opposed to the more-common monopolar stimulation) has been proposed as a solution, but with only limited success. We argue that this is due to a combination of factors, including the fact that envelopes of adjacent CI channels are highly correlated in response to many sounds including speech, that some neural spread of excitation is necessary for sufficient loudness, and that, at comfortably loud levels, monopolar and sharp stimulation modes can yield very similar output voltage at cochlear 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. Based on computational modelling and on psychophysical measures of charge summation, we show that the difference between monopolar and Hilltop stimulation persists at the level of the spiral ganglion neurons, and therefore warrants investigations with more complex, multi-channel 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.