Interpreting the Effect of Stimulus Parameters on the Electrically Evoked Compound Action Potential and on Neural Health
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JARO (2020) DOI: 10.1007/s10162-020-00774-z D 2020 The Author(s)
Research Article
Journal of the Association for Research in Otolaryngology
Interpreting the Effect of Stimulus Parameters on the Electrically Evoked Compound Action Potential and on Neural Health Estimates TIM BROCHIER,1
COLETTE M. MCKAY,2
AND
ROBERT P. CARLYON1
1
Cambridge Hearing Group, MRC Cognition and Brain Sciences Unit, University of Cambridge, 15 Chaucer Road, Cambridge, CB2 7EF, UK 2 Bionics Institute, 384-388 Albert Street, East Melbourne, VIC 3002, Australia Received: 27 March 2020; Accepted: 14 October 2020
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 inputoutput 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 Correspondence to: Tim Brochier & Cambridge Hearing Group, MRC Cognition and Brain Sciences Unit & University of Cambridge & 15 Chaucer Road, Cambridge, CB2 7EF, UK. email: [email protected]
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. Keywords: cochlear implants, neural health, neural survival, ECAP, electrophysiology, inter-phase gap
INTRODUCTION 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
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