Speech recognition with cochlear implants as a function of the number of channels: Effects of electrode placement

Abstract

This study investigated the effects of cochlear implant (CI) electrode array type and scalar location on the number of channels available to CI recipients for maximum speech understanding and sound quality. Eighteen post-lingually deafened adult CI recipients participated, including 11 recipients with straight electrode arrays entirely in scala tympani and 7 recipients with translocated precurved electrode arrays. Computerized tomography was used to determine electrode placement and scalar location. In each condition, the number of channels varied from 4 to 22 with equal spatial distribution across the array. Speech recognition (monosyllables, sentences in quiet and in noise), subjective speech sound quality, and closed-set auditory tasks (vowels, consonants, and spectral modulation detection) were measured acutely. Recipients with well-placed straight electrode arrays and translocated precurved electrode arrays performed similarly, demonstrating asymptotic speech recognition scores with 8–10 channels, consistent with the classic literature. This finding contrasts with recent work [Berg, Noble, Dawant, Dwyer, Labadie, and Gifford. (2019). J. Acoust. Soc. Am. 145, 1556–1564] that found precurved electrode arrays well-placed in scala tympani demonstrate continuous performance gains beyond 8–10 channels. Given these results, straight and translocated precurved electrode arrays are theorized to have less channel independence secondary to their placement farther away from neural targets.

The Acoustic Change Complex in Response to Frequency Changes and Its Correlation to Cochlear Implant Speech Outcomes

“…One of the major issues about CI user’s hearing ability is that their spectral resolution is poor and they cannot perform well in tasks that heavily depend on pitch cues such as speech perception in noisy backgrounds, music melody recognition, voice pitch differentiation, and talker identification ( Galvin et al, 2007 ; Sagi and Svirsky, 2017 ; Gaudrain and Başkent, 2018 ; Fowler et al, 2021 ). Unlike normal acoustic hearing with a healthy cochlea that transmits the temporal and spectral information of sounds through approximately 3,000 inner hair cells, CI users’ hearing is constrained at the peripheral stage not only by the limitation of CI signal processing algorithms that discard temporal fine structures but also by the use of only up to 22 electrodes for sound delivery ( Gaudrain and Başkent, 2018 ; Berg et al, 2020 ). CI users’ capability to detect frequency changes is further exasperated by deafness ( Moore, 1985 ).…”

“…Understanding the source of this variability is critical for customized rehabilitation ( Fu and Galvin, 2008 ). The variability appears to be related to a variety of potential factors including patient demographics (e.g., patient’s age, age of implantation, duration of deafness, duration of CI use, and etiology of hearing loss), cochlear abnormalities, surgical issues, electrode insertion (e.g., insertion depth and location), clinical mapping (e.g., frequency-place mismatch), device maintenance, neural status (e.g., survival of spiral ganglion neurons, and cortical neural plasticity), and higher-level cognitive functions (e.g., verbal working memory, attention, executive function, and learning processes, Blamey et al, 1992 ; Alexiades et al, 2001 ; Doucet et al, 2006 ; Finley et al, 2008 ; Reiss et al, 2008 ; Grasmeder et al, 2014 ; Jeong and Kim, 2015 ; Moberly et al, 2018 ; Berg et al, 2020 ; Heutink et al, 2020 ; Kim et al, 2021 ). With so many influencing factors, it is difficult to predict the likelihood of CI success using only demographic data ( Lachowska et al, 2014 ).…”

The recognition of time-compressed speech as a function of age in listeners with cochlear implants or normal hearing

“…These measures can provide objective evidence toward the strength or weakness of the electrode-to-neuron interface, which affects the integrity of the signal received by the brain but cannot measure any potential central or cognitive changes. Another factor that affects the electrode-to-neuron interface and speech recognition in listeners with CIs is the placement of the electrode arrays as determined by CT scans (e.g., Berg et al, 2020 , 2021 ). Better simulations that could help match performance between the two listener groups are likely dependent on the stimuli or other individual factors such as array type, insertion depth, and array placement (e.g., Croghan et al, 2017 ; Berg et al, 2019 , 2020 ).…”

Incidence of Complete Insertion in Cochlear Implant Recipients of Long Lateral Wall Arrays

“…Previous investigations have reported variable incidence of partial insertion with long arrays, which may negatively influence speech recognition outcomes due to a reduction in the number of effective intracochlear channels. 37-40 Understanding morphological features associated with partial insertions may help to identify CI candidates who would benefit from complete insertion of a shorter array. The aims of the present study were to describe the rate of complete insertion among patients implanted with a 31.5-mm flexible lateral wall array and to identify morphological features associated with cases of partial insertion.…”