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.

FIG. 1.

Channel deactivation methods and associated frequency allocations for all conditions. In cases for which the participant had electrode(s) deactivated in their clinical map, we chose to activate the closest available electrode to maintain the greatest spatial separation between activated electrodes. For example, if E1 elicited a non-auditory percept, E2 would be activated instead for the 8, 16, and all on conditions.

FIG. 2.

Mean outcomes for 11 participants with scala tympani straight electrode arrays and 7 participants with translocated precurved electrode arrays across all tested channel conditions for CNC words (A), AzBio sentences in quiet (B), and at +5 dB SNR (C), as well as sound quality ratings for CNC words (D), AzBio sentences in quiet (E), and noise (F). Error bars are +1 SEM. Horizontal dashed lines represent mean scores for 11 participants with precurved electrode array (Berg et al., 2019).

FIG. 3.

Mean scores for 11 participants with scala tympani straight electrode arrays and 7 participants with translocated precurved electrode arrays across all tested channel conditions for vowels (A), consonants (B), place of consonants (C), manner of consonants (D), voicing of consonants (E), and QSMD (F). Error bars are +1 SEM. Horizontal dashed lines represent mean scores for 11 participants with precurved electrode array (Berg et al., 2019).