Abstract
Variations in the condition of the neural population along the length of the cochlea can degrade the spectral and temporal representation of sounds conveyed by CIs, thereby limiting speech perception. One measurement that has been proposed as an estimate of neural survival (the number of remaining functional neurons) or neural health (the health of those remaining neurons) is the effect of stimulation parameters, such as the interphase gap (IPG), on the amplitude growth function (AGF) of the electrically evoked compound action potential (ECAP). The extent to which such measures reflect neural factors, rather than non-neural factors (e.g. electrode orientation, electrode-modiolus distance, and impedance), depends crucially upon how the AGF data are analysed. However, there is currently no consensus in the literature for the correct method to interpret changes in the ECAP AGF due to changes in stimulation parameters. We present a simple theoretical model for the effect of IPG on ECAP AGFs, along with a re-analysis of both animal and human data that measured the IPG effect. Both the theoretical model and the re-analysis of the animal data suggest that the IPG effect on ECAP AGF slope (IPG slope effect), measured using either a linear or logarithmic input-output scale, does not successfully control for the effects of non-neural factors. Both the model and the data suggest that the appropriate method to estimate neural health is by measuring the IPG offset effect, defined as the dB offset between the linear portions of ECAP AGFs for two stimuli differing only in IPG.
Highlights
Cochlear implants (CI) provide a sense of hearing to people with sensorineural hearing loss by directly stimulating auditory nerve fibres via an implanted array of electrodes
Similar conclusions apply to other stimulus manipulations, such as phase duration or stimulus polarity, but we focus on the effect of interphase gap (IPG) to illustrate the general principle because animal physiological data are available to help elucidate the relation to neural health and because of the emerging interest in the effect of IPG as a clinical tool
We argue that a better method, which has been used by Kim et al (2010) and by McKay and Smale (2017), is to measure the average dB offset between the linear portions of the evoked compound action potential (ECAP) amplitude growth function (AGF) for the different IPGs (Fig. 9)
Summary
Cochlear implants (CI) provide a sense of hearing to people with sensorineural hearing loss by directly stimulating auditory nerve fibres via an implanted array of electrodes. Variations in neural health along the length of the cochlea can degrade the spectral and temporal representation of sounds conveyed by CIs, thereby limiting speech perception. In order to explain and potentially remedy cases of poor speech perception by CI users, several methods to estimate neural health have been proposed. Measurements derived from the electrically evoked compound action potential (ECAP) have been correlated with SGN density in animals (Shepherd and Javel 1997; Prado-Guitierrez et al 2006; Ramekers et al 2014) and with speech perception in humans (Kim et al 2010). ECAP measures are objective and time-efficient and can be measured with the device itself, without the need for additional hardware. These factors make ECAPs a viable clinical method for estimating neural health. The aim of this paper is to review ECAP-based methods of estimating neural survival and neural health, to identify the most
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