Identification and multiplicity of double vowels in cochlear implant users

Fusion of dichotic consonants in normal-hearing and hearing-impaired listenersa)

Hearing-impaired (HI) listeners have been shown to exhibit increased fusion of dichotic vowels, even with different fundamental frequency (F0), leading to binaural spectral averaging and interference. To determine if similar fusion and averaging occurs for consonants, four natural and synthesized stop consonants (/pa/, /ba/, /ka/, /ga/) at three F0s of 74, 106, and 185 Hz were presented dichotically-with ΔF0 varied-to normal-hearing (NH) and HI listeners. Listeners identified the one or two consonants perceived, and response options included /ta/ and /da/ as fused percepts. As ΔF0 increased, both groups showed decreases in fusion and increases in percent correct identification of both consonants, with HI listeners displaying similar fusion but poorer identification. Both groups exhibited spectral averaging (psychoacoustic fusion) of place of articulation but phonetic feature fusion for differences in voicing. With synthetic consonants, NH subjects showed increased fusion and decreased identification. Most HI listeners were unable to discriminate the synthetic consonants. The findings suggest smaller differences between groups in consonant fusion than vowel fusion, possibly due to the presence of more cues for segregation in natural speech or reduced reliance on spectral cues for consonant perception. The inability of HI listeners to discriminate synthetic consonants suggests a reliance on cues other than formant transitions for consonant discrimination.

Level differences impact the fusion of concurrent vowels dissimilarly within versus across ears

This study investigated how level differences affect the fusion and identification of dichotically and monaurally presented concurrent vowel pairs where the vowels differed in level by 0, 4, 8, or 12 dB. With dichotic presentation, there was minimal variation in fusion and identification-vowels were nearly always fused and were identified consistently across level differences. Conversely, with monaural presentation, fusion and identification varied systematically across level differences-with the more intense vowel dominating fused percepts. The dissimilar effect of level difference for dichotic versus monaural presentation may arise from differences in energetic masking and/or divergent mechanisms underlying sound segregation and integration.

Onset Asynchrony: Cue to Aid Dichotic Vowel Segregation in Listeners With Normal Hearing and Hearing Loss

PURPOSE

The effect of onset asynchrony on dichotic vowel segregation and identification in normal-hearing (NH) and hearing-impaired (HI) listeners was examined. We hypothesized that fusion would decrease and identification performance would improve with increasing onset asynchrony. Additionally, we hypothesized that HI listeners would gain more benefit from onset asynchrony.

METHOD

A total of 18 adult subjects (nine NH, nine HI) participated. Testing included dichotic presentation of synthetic vowels, /i/, /u/, /a/, and /ae/. Vowel pairs were presented with the same or different fundamental frequency (f _o; f _o = 106.9, 151.2, or 201.8 Hz) across the two ears and one onset asynchrony of 0, 1, 2, 4, 10, or 20 ms throughout a block (one block = 80 runs). Subjects identified the one or two vowels that they perceived on a touchscreen. Subjects were not informed that two vowels were always presented or that there was onset asynchrony.

RESULTS

The effect of onset asynchrony on fusion and vowel identification was greatest in both groups when Δf _o = 0 Hz. Mean fusion scores across increasing onset asynchronies differed significantly between the two groups with HI listeners exhibiting less fusion across pooled Δf _o. There was no significant difference with identification performance.

CONCLUSIONS

As onset asynchrony increased, dichotic vowel fusion decreased and identification performance improved. Onset asynchrony exerted a greater effect on fusion and identification of vowels when Δf _o = 0, especially in HI listeners. Therefore, the temporal cue promotes segregation in both groups of listeners, especially in HI listeners when the f _o cue was unavailable.

An Alternative Explanation for Difficulties with Speech in Background Talkers: Abnormal Fusion of Vowels Across Fundamental Frequency and Ears

Normal-hearing (NH) listeners use frequency cues, such as fundamental frequency (voice pitch), to segregate sounds into discrete auditory streams. However, many hearing-impaired (HI) individuals have abnormally broad binaural pitch fusion which leads to fusion and averaging of the original monaural pitches into the same stream instead of segregating the two streams (Oh and Reiss, 2017) and may similarly lead to fusion and averaging of speech streams across ears. In this study, using dichotic speech stimuli, we examined the relationship between speech fusion and vowel identification. Dichotic vowel perception was measured in NH and HI listeners, with across-ear fundamental frequency differences varied. Synthetic vowels /i/, /u/, /a/, and /ae/ were generated with three fundamental frequencies (F₀) of 106.9, 151.2, and 201.8 Hz and presented dichotically through headphones. For HI listeners, stimuli were shaped according to NAL-NL2 prescriptive targets. Although the dichotic vowels presented were always different across ears, listeners were not informed that there were no single vowel trials and could identify one vowel or two different vowels on each trial. When there was no F₀ difference between the ears, both NH and HI listeners were more likely to fuse the vowels and identify only one vowel. As ΔF₀ increased, NH listeners increased the percentage of two-vowel responses, but HI listeners were more likely to continue to fuse vowels even with large ΔF₀. Binaural tone fusion range was significantly correlated with vowel fusion rates in both NH and HI listeners. Confusion patterns with dichotic vowels differed from those seen with concurrent monaural vowels, suggesting different mechanisms behind the errors. Together, the findings suggest that broad fusion leads to spectral blending across ears, even for different ΔF₀, and may hinder the stream segregation and understanding of speech in the presence of competing talkers.