Recent research published in the Journal of the Association for Research in Otolaryngology states that binaural tone fusion range was significantly correlated with vowel fusion rates in both Normal-hearing (NH) listeners and hearing-impaired (HI) individuals.
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 and may similarly lead to fusion and averaging of speech streams across ears.
Hence, Lina A.J Reiss and colleagues from the Department of Otolaryngology, Oregon Health and Science University, Portland, USA carried out the present study to examine the relationship between speech fusion and vowel identification using dichotic speech stimuli.
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.
The results showed that-
a. 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.
b. 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.
c. Binaural tone fusion range was significantly correlated with vowel fusion rates in both NH and HI listeners.
d. Confusion patterns with dichotic vowels differed from those seen with concurrent monaural vowels, suggesting different mechanisms behind the errors.
Therefore, the authors concluded 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."
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