Energy-efficient waveform for electrical stimulation of the cochlear nerve

Abstract

The cochlear implant (CI) is the most successful neural prosthesis, restoring the sensation of sound in people with severe-to-profound hearing loss by electrically stimulating the cochlear nerve. Existing CIs have an external, visible unit, and an internal, surgically-placed unit. There are significant challenges associated with the external unit, as it has limited utility and CI users often report a social stigma associated with prosthesis visibility. A fully-implantable CI (FICI) would address these issues. However, the volume constraint imposed on the FICI requires less power consumption compared to today’s CI. Because neural stimulation by CI electrodes accounts for up to 90% of power consumption, reduction in stimulation power will result directly in CI power savings. To determine an energy-efficient waveform for cochlear nerve stimulation, we used a genetic algorithm approach, incorporating a computational model of a single mammalian myelinated cochlear nerve fiber coupled to a stimulator-electrode-tissue interface. The algorithm’s prediction was tested in vivo in human CI subjects. We find that implementation of a non-rectangular biphasic neural stimulation waveform may result in up to 25% charge savings and energy savings within the comfortable range of hearing for CI users. The alternative waveform may enable future development of a FICI.

Ramped pulse shapes are more efficient for cochlear implant stimulation in an animal model

“…Ramped shapes require less charge than rectangular shapes to evoke an eABR. Earlier studies have argued that ramped shapes require less charge to evoke responses similar in amplitude to those from rectangular pulses 4,5,17 . To test this hypothesis in vivo, we first plotted the amplitude of wave II as a function of charge injected per phase for all mice (Fig.…”

“…However, it is not the most efficient way to stimulate neurons because the charge injected is immediately counteracted by the following phase 2,3 . In fact, recent evidence gathered by using patch-clamp recordings in vitro 4 and supported by modeling work 5,6 suggests that the use of an alternative, non-rectangular pulse shape with a ramped slope (Fig. 1a) might be more efficient.…”

“…This would result in a more gradual recruitment of auditory nerve fibers with increasing current level, thereby mimicking acoustic hearing, and increasing the dynamic range. Finally, with regard to battery consumption, two studies have reported that ramped pulses are more power-efficient than rectangular pulses in models of the auditory nerve 5,17 . Importantly, Yip et al recently demonstrated the efficiency of novel non-rectangular pulse shapes in human CI experiments 5 .…”

“…Finally, with regard to battery consumption, two studies have reported that ramped pulses are more power-efficient than rectangular pulses in models of the auditory nerve 5,17 . Importantly, Yip et al recently demonstrated the efficiency of novel non-rectangular pulse shapes in human CI experiments 5 . In a loudness perception task, they found that an exponential waveform (not linear as used in Ballestro et al 4 ) with five discrete current steps produced up to 26% energy savings within a comfortable range of hearing compared to a rectangular waveform 5 .…”

“…Importantly, Yip et al recently demonstrated the efficiency of novel non-rectangular pulse shapes in human CI experiments 5 . In a loudness perception task, they found that an exponential waveform (not linear as used in Ballestro et al 4 ) with five discrete current steps produced up to 26% energy savings within a comfortable range of hearing compared to a rectangular waveform 5 . Lower power consumption reduces battery usage, which means that batteries in the sound processor behind the ear can be smaller and less visible, or that the CI users may need to change them less frequently.…”