Available online 16 May 2024, 109026

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Results show a strong linear relationship between modulation sensitivity and pitch resolution for cochlear implant users and peers with no known hearing loss.

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Cochlear implant users have poor modulation sensitivity, compared to people with no known hearing loss, when listening to sounds processed by their clinical sound processors. However, many cochlear implant users can detect the smallest controllable modulation depth provided by their implants when clinical sound processing is bypassed. This suggests clinical sound processing could be improved to provide better modulation sensitivity.

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Computational models of neural synchrony in the auditory nerve predict pitch resolution of cochlear implant users and peers with no known hearing loss.

ABSTRACT

Cochlear implants are medical devices that have restored hearing to approximately one million people around the world. Outcomes are impressive and most recipients attain excellent speech comprehension in quiet without relying on lip-reading cues, but pitch resolution is poor compared to normal hearing. Amplitude modulation of electrical stimulation is a primary cue for pitch perception in cochlear implant users. The experiments described in this article focus on the relationship between sensitivity to amplitude modulations and pitch resolution based on changes in the frequency of amplitude modulations. In the first experiment, modulation sensitivity and pitch resolution were measured in adults with no known hearing loss and in cochlear implant users with sounds presented to and processed by their clinical devices. Stimuli were amplitude-modulated sinusoids and amplitude-modulated narrow-band noises. Modulation detection and modulation frequency discrimination were measured for modulation frequencies centered on 110, 220, and 440 Hz. Pitch resolution based on changes in modulation frequency was measured for modulation depths of 25%, 50%, 100%, and for a half-waved rectified modulator. Results revealed a strong linear relationship between modulation sensitivity and pitch resolution for cochlear implant users and peers with no known hearing loss. In the second experiment, cochlear implant users took part in analogous procedures of modulation sensitivity and pitch resolution but bypassing clinical sound processing using single-electrode stimulation. Results indicated that modulation sensitivity and pitch resolution was better conveyed by single-electrode stimulation than by clinical processors. Results at 440 Hz were worse, but also not well conveyed by clinical sound processing, so it remains unclear whether the 300 Hz perceptual limit described in the literature is a technological or biological limitation. These results highlight modulation depth and sensitivity as critical factors for pitch resolution in cochlear implant users and characterize the relationship that should inform the design of modulation enhancement algorithms for cochlear implants.

Section snippetsINTRODUCTION

When sound is transferred to the cochlea, different frequencies are distributed along the length of the cochlea in accordance with the organ's tuning properties. This tonotopic representation of frequency is exploited in one of the most successful neural interfaces: the cochlear implant. Commercial multi-electrode implants directly stimulate different regions of the auditory nerve by spectrally separating incoming sound and allocating different frequencies to specific electrodes along the

Overview

Experiment 1 characterizes modulation sensitivity and pitch resolution in adults with no known hearing loss and in adult cochlear implant users as perceived with their clinical sound processors. Modulation detection was measured for amplitude-modulated sinusoids and narrow-band noise for modulation frequencies of 55, 110, 220, and 440 Hz. Modulation frequency discrimination was measured for amplitude-modulated sinusoids and narrow-band noise for modulation frequencies near 110, 220, and 440 Hz.

Overview

Experiment 2 was designed to compare modulation sensitivity and pitch resolution for single-electrode stimulation. The goal of Experiment 2 is to characterize loss of modulation encoding caused by sound processing for cochlear implants. To this end, analogous measures of modulation sensitivity and pitch resolution were conducted as in Experiment 1 but using amplitude-modulated pulse trains delivered to a single electrode. Results are analyzed to compare modulation sensitivity and pitch

DISCUSSION

The purpose of this study was to characterize modulation encoding, modulation sensitivity, and pitch resolution based on modulation frequency in cochlear implant users and participants with no known hearing loss. The interplay of neural encoding, modulation sensitivity, and purely temporal pitch perception has been deeply explored for more than a half a century (Miller & Taylor 1948; Viemeister & Burns 1976; Patterson et al. 1978; Burns & Viemeister 1981). During this time, cochlear implants

CRediT authorship contribution statement

Andres Camarena: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Raymond L. Goldsworthy: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

ACKNOWLEDGEMENTS

This work was supported by funding from the National Institutes of Health: NIH NIDCD 1 R01 DC018044 Encoding Temporal Fine Structure for Cochlear Implants. The authors are grateful for the tireless efforts of research participants and the supportive environment of the Bionic Ear Laboratory. The authors thank Julianne Papadopoulos for helpful comments on this manuscript.

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