Two Ears Are Not Always Better than One: Mandatory Vowel Fusion Across Spectrally Mismatched Ears in Hearing-Impaired Listeners

Abstract

Hearing loss and auditory prostheses can alter auditory processing by inducing large pitch mismatches and broad pitch fusion between the two ears. Similar to integration of incongruent inputs in other sensory modalities, the mismatched, fused pitches are often averaged across ears for simple stimuli. Here, we measured parallel effects on complex stimulus integration using a new technique based on vowel classification in five bilateral hearing aid users and eight bimodal cochlear implant users. Continua between five pairs of synthetic vowels were created by varying the first formant spectral peak while keeping the second formant constant. Comparison of binaural and monaural vowel classification functions for each vowel pair continuum enabled visualization of the following frequency-dependent integration trends: (1) similar monaural and binaural functions, (2) ear dominance, (3) binaural averaging, and (4) binaural interference. Hearing aid users showed all trends, while bimodal cochlear implant users showed mostly ear dominance or interference. Interaural pitch mismatches, frequency ranges of binaural pitch fusion, and the relative weightings of pitch averaging across ears were also measured using tone and/or electrode stimulation. The presence of both large interaural pitch mismatches and broad pitch fusion was not sufficient to predict vowel integration trends such as binaural averaging or interference. The way that pitch averaging was weighted between ears also appears to be important for determining binaural vowel integration trends. Abnormally broad spectral fusion and the associated phoneme fusion across mismatched ears may underlie binaural speech perception interference observed in hearing aid and cochlear implant users.

Comparison of Place-versus-Pitch Mismatch between a Perimodiolar and Lateral Wall Cochlear Implant Electrode Array in Patients with Single-Sided Deafness and a Cochlear Implant

“…This was in contrast to evidence pointing in the direction that it is important to deliver auditory information at the correct cochlear place [Oxenham et al, 2004;Zhou et al, 2016;Reiss et al, 2016]. Possibly, reorganization of the neural system to the new frequency-place situation [Eggermont, 2017] may result in little impact of mismatch on performance.…”

Binaural pitch fusion: Comparison of normal-hearing and hearing-impaired listeners

“…While slightly broadened fusion may be a beneficial adaptation in minimizing the perception of diplacusis due to slight differences in pitch perception between the ears, as well as for sound localization in the presence of small placemismatches (Blanks et al, 2008), extremely broad fusion on the order of octaves may be detrimental for speech perception and considered maladaptive. As demonstrated previously, broad fusion across mismatched frequencies often leads to averaging of spectral information between ears (Reiss et al, 2014b;Reiss et al, 2016) and can worsen vowel discrimination . Broad fusion may also lead to an inability to separate voices in a room full of talkers, i.e., the "cocktail party effect," if voices of different pitches and thus their speech are fused together, leading to various forms of speech fusion (Cutting, 1976).…”

Low-Frequency Pitch Perception in Cochlear Implant Recipients With Normal Hearing in the Contralateral Ear

“…In binaural pitch fusion, stimuli from the two sides are integrated, and subjective differences across ears recede in perceptual saliency. Fusion is thought to be a prerequisite to the binaural benefits associated with spatial hearing Kan et al, 2013), but binaural fusion has also been argued to hurt performance when disparate spectral cues are integrated across ears (Reiss, Eggleston, Walker, & Oh, 2016;Reiss, Ito, Eggleston, & Wozny, 2014). Participants in the present cohort described the sound quality of speech with their CI as "normal" when listening with the CI plus the NH ear (CI+NH) yet "robotic" when listening with the CI alone.…”