Variations in neural health along the cochlea can degrade the spectral and temporal representation of sounds conveyed by cochlear implants (CIs). We evaluated and compared one electrophysiological measure and two behavioural measures that have been proposed as estimates of neural health patterns, in order to explore the extent to which the different measures provide converging and consistent neural health estimates. All measures were obtained from the same 11 users of the Cochlear Corporation CI. The two behavioural measures were multipulse integration (MPI) and the polarity effect (PE), both measured on each of seven electrodes per subject. MPI was measured as the difference between thresholds at 80 pps and 1000 pps, and PE as the difference in thresholds between cathodic- and anodic-centred quadraphasic (QP) 80-pps pulse trains. It has been proposed that good neural health corresponds to a large MPI and to a large negative PE (lower thresholds for cathodic than anodic pulses). The electrophysiological measure was the effect of interphase gap (IPG) on the offset of the ECAP amplitude growth function (AGF), which has been correlated with spiral ganglion neuron density in guinea pigs. This ‘IPG offset’ was obtained on the same subset of electrodes used for the behavioural measures. Despite high test–retest reliability, there were no significant correlations between the neural health estimates for either within-subject comparisons across the electrode array, or between-subject comparisons of the means. A phenomenological model of a population of spiral ganglion neurons was then used to investigate physiological mechanisms that might underlie the different neural health estimates. The combined experimental and modelling results provide evidence that PE, MPI and IPG offset may reflect different characteristics of the electrode-neural interface.
We measured the sustained neural response to electrical stimulation by a cochlear implant (CI). To do so, we interleaved two stimuli with frequencies F1 and F2 Hz and recorded a neural distortion response (NDR) at F2-F1 Hz. We show that, because any one time point contains only the F1 or F2 stimulus, the instantaneous nonlinearities typical of electrical artefact should not produce distortion at this frequency. However, if the stimulus is smoothed, such as by charge integration at the nerve membrane, subsequent (neural) nonlinearities can produce a component at F2-F1 Hz. We stimulated a single CI electrode with interleaved sinusoids or interleaved amplitude-modulated pulse trains such that F2 = 1.5F1, and found no evidence for an NDR when F2-F1 was between 90 and 120 Hz. However, interleaved amplitude-modulated pulse trains with F2-F1~40 Hz revealed a substantial NDR with a group delay of about 45 ms, consistent with a thalamic and/or cortical response. The NDR could be measured even from recording electrodes adjacent to the implant and at the highest pulse rates (> 4000 pps) used clinically. We then measured the selectivity of this sustained response by presenting F1 and F2 to different electrodes and at different between-electrode distances. This revealed a broad tuning that, we argue, reflects the overlap between the excitation elicited by the two electrodes. Our results also provide a glimpse of the neural nonlinearity in the auditory system, unaffected by the biomechanical cochlear nonlinearities that accompany acoustic stimulation. Several potential clinical applications of our findings are discussed.
1Variations in neural health along the cochlea can degrade the spectral and temporal 2 representation of sounds conveyed by cochlear implants (CIs) . We evaluated and compared 3 several methods that have been proposed as estimates of neural health patterns, in order to 4 explore the extent to which the different measures provide converging and consistent neural 5 health estimates. All measures were obtained from the same 11 users of the Cochlear 6 Corporation CI. The two behavioural measures were multipulse integration (MPI) and the 7 polarity effect (PE), both measured on each of seven electrodes per subject. MPI was 8 measured as the difference between thresholds at 80-pps and 1000-pps, and PE as the 9 difference in thresholds between cathodic-and anodic-centred quadraphasic (QP) 80-pps 10 pulse trains. It has been proposed that good neural health corresponds to a large MPI and to a 11 large negative PE (lower thresholds for cathodic than anodic pulses). The 12 electrophysiological measure was the effect of interphase gap (IPG) on the offset of the 13 ECAP amplitude growth function (AGF), which has been correlated with spiral ganglion 14 nerve density in guinea pigs. This "IPG offset" was obtained on the same subset of electrodes 15 as for the behavioural measures. Despite high test-retest reliability, there were no significant 16 correlations between the neural health estimates for either within-subject comparisons across 17 the electrode array, or between-subjects comparisons of the means. A phenomenological 18 model of a population of spiral ganglion neurons was then used to investigate physiological 19 mechanisms that might underlie the different neural health estimates. The combined 20 experimental and modelling results provide evidence that PE, MPI, and IPG offset reflect 21 different characteristics of the electrode-neural interface.22
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.