Excess Vowel Fusion in Hearing-Impaired Listeners Across Fundamental Frequency and Spatial Separation in the Free Field: An Alternative Mechanism for Difficulties With Speech in Noise.
Michelle R Molis, Briana N Martinez, Nicole Dean and 1 others
Hearing-impaired listeners still fuse two simultaneous vowels into a single blended sound even when the speakers are on either side of the listener, and they benefit less from voice pitch differences than normal-hearing listeners.
The effect is smaller than under headphones, and individual tone-fusion range did not predict it.
Key findings
1Both normal-hearing and hearing-impaired listeners fused different vowels into a single-vowel response when the two speakers had the same voice pitch, and fusion decreased as the pitch difference grew. Hearing-impaired listeners showed more fusion and less improvement in identifying both vowels as the pitch difference increased, with a significant group-by-pitch-difference interaction on identification accuracy.
2Vowel fusion was less frequent under loudspeaker (free-field) presentation than under the headphone conditions of the earlier study, consistent with the idea that the small interaural timing differences present in natural listening partially reduce the blending.
3No significant correlation was found between each listener's individual binaural tone-fusion range and their rate of vowel fusion in the free field, in contrast to the significant correlations reported in the earlier headphone studies.
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What it does not show
Only ten listeners per group, so individual variation is large and the null correlation with tone fusion may reflect a narrow spread of fusion ranges rather than a true absence of relationship. Stimuli were synthetic vowels, not natural connected speech, so the degree to which the fusion pattern generalises to real conversational speech is uncertain. The hearing-impaired group was heterogeneous in degree of loss, duration of loss, and hearing-aid use, making it hard to attribute the group difference to a single factor. The /æ/→/ɛ/ confusion pattern seen in many normal-hearing listeners may reflect regional dialect differences (most participants were from the Pacific Northwest) rather than a fusion effect, and the authors note this cannot be ruled out. No direct within-subject comparison with the headphone condition was made in this experiment, so the claim that fusion is 'less frequent' than under headphones rests on between-study comparison rather than a controlled contrast.
Declared interests
Funded by the National Institutes of Health (R01 DC013307) and the VA Rehabilitation Research and Development Service (NCRAR C2361-C). The authors declared no conflicts of interest.
The easy way to misread this
Do not read the null correlation between tone-fusion range and vowel fusion as evidence that binaural fusion is irrelevant to speech-in-noise difficulty. The authors themselves note the narrow spread of fusion ranges in this small sample and the different tone frequencies used (2000 Hz pure tones here versus 100–200 Hz complex tones in the study that did find a strong correlation), so the absence of a correlation here does not overturn the earlier finding.
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