Searching for the Sound of a Cochlear Implant: Evaluation of Different Vocoder Parameters by Cochlear Implant Users With Single-Sided Deafness

Abstract

Cochlear implantation in subjects with single-sided deafness (SSD) offers a unique opportunity to directly compare the percepts evoked by a cochlear implant (CI) with those evoked acoustically. Here, nine SSD-CI users performed a forced-choice task evaluating the similarity of speech processed by their CI with speech processed by several vocoders presented to their healthy ear. In each trial, subjects heard two intervals: their CI followed by a certain vocoder in Interval 1 and their CI followed by a different vocoder in Interval 2. The vocoders differed either (i) in carrier type—(sinusoidal [SINE], bandfiltered noise [NOISE], and pulse-spreading harmonic complex) or (ii) in frequency mismatch between the analysis and synthesis frequency ranges—(no mismatch, and two frequency-mismatched conditions of 2 and 4 equivalent rectangular bandwidths [ERBs]). Subjects had to state in which of the two intervals the CI and vocoder sounds were more similar. Despite a large intersubject variability, the PSHC vocoder was judged significantly more similar to the CI than SINE or NOISE vocoders. Furthermore, the No-mismatch and 2-ERB mismatch vocoders were judged significantly more similar to the CI than the 4-ERB mismatch vocoder. The mismatch data were also interpreted by comparing spiral ganglion characteristic frequencies with electrode contact positions determined from postoperative computed tomography scans. Only one subject demonstrated a pattern of preference consistent with adaptation to the CI sound processor frequency-to-electrode allocation table and two subjects showed possible partial adaptation. Those subjects with adaptation patterns presented overall small and consistent frequency mismatches across their electrode arrays.

Pitch Matching in Cochlear Implant Users With Single-Sided Deafness: Effects of Electrode Position and Acoustic Stimulus Type

“…In the present study, we have investigated two types of broadband stimuli (NBN and PSHC), which were 1/3-octave wide and had 36-dB per octave spectral slopes. These slopes are broadly consistent with previous vocoder studies that aimed to simulate the sound of CI (reviewed in Mesnildrey and Macherey, 2015; Karoui et al, 2019). However, it is likely that the spectral slope corresponding to that of the excitation spread of a CI electrode will vary across electrodes and across subjects.…”

“…PSHCs are pulsatile broadband stimuli that can simulate the broad spread of excitation and their pulse rate can be adjusted to minimize intrinsic modulations after auditory filtering (Mesnildrey et al, 2016). Furthermore, a recent evaluation in SSD subjects showed that speech processed by a vocoder using PSHC carriers was judged more similar to speech processed by the clinical CI processor than sine- or noise-vocoded speech (Karoui et al, 2019).…”

Approximations to the Voice of a Cochlear Implant: Explorations With Single-Sided Deaf Listeners

“…Most recently, Karoui et al (2019) assessed the similarity of three vocoders (sine, noise, and pulse-spreading harmonic complex) and three output maps (no upshift in frequency, two equivalent rectangular bandwidth [ERB] upshift, and four ERB upshift) to the sound of Cochlear Corporation CIs. The pulse-spreading harmonic complex was judged more similar to a CI than either sine or noise and either no shift or a two ERB shift were judged more similar than the four ERB shift.…”

Brain plasticity and hearing disorders

“…A cochlear implant (CI) is a surgically implanted electronic device that provides a sense of sound for profound to total hearing loss patients. This neuroprosthesis bypasses the defunct cochlea by transferring acoustic vibrations of a sound into electric pulses of the electrodes to stimulate the auditory nerve, and subsequently the auditory cortex creating a somewhat synthetic sound perception of the environment [2].…”