Perceiving speech in single and multi-talker babble in normal and impaired hearing

Most speech is heard in the background of other sounds, particularly other people talking. Listeners with normal hearing have remarkable abilities to filter out extraneous sounds and listen only to the desired talker. This is known among researchers as the ?cocktail party effect?, because these abilities are so important at a noisy cocktail party. We are only beginning to understand the complex processes going on in the ears and brains of people as they perform this remarkable feat, far beyond anything current-day computers can manage. One fact is clear ? people with hearing impairment find this situation very challenging, even though they might function very well with their hearing aids in a quiet room. The situation is even worse for users of cochlear implants, devices which stimulate the hearing nerve directly with tiny electrical currents in cases where ordinary hearing aids are not adequate.
Any attempts to improve this situation through more sophisticated electronics in cochlear implants and hearing aids will depend on a thorough understanding of how the abilities of hearing-impaired listeners differ from those of normal listeners. In our laboratory model of a cocktail party (not including drinks!), both normal hearing and hearing-impaired participants will be tested on their perception of sentences in the presence of other background sounds. These other sounds will consist of a ?babble? of talkers in which the number of people talking simultaneously will vary from 1 to 16. We will test a number of ideas about why hearing-impaired people find this more difficult than normal listeners.
Our focus will be on one crucial factor ? ?babble? can fluctuate a lot in level, especially if there is only one other talker speaking. This creates ?dips? in the interfering sounds in which the target speech is more intense, thus easier to hear. Normal-hearing people can very effectively ?listen in the dips? of interfering sounds, but hearing-impaired listeners do this poorly. Many ideas have been advanced in explanation, including the way in which people with hearing impairment hear the pitch of voices, and their apparent inability to fuse together bits of sound information separated in time. What we hope to do is more fully explain how people with normal hearing manage this so well, whereas people with hearing impairment do not. We are hopeful that this deeper understanding will lead to new ideas for hearing aids that will enable hearing-impaired people to understand speech better in noisy environments.

Understanding speech in the presence of masking sounds, such as other voices, is particularly challenging for those with impaired auditory systems. A major source of difficulty is a reduction or absence of masking release: unlike normally-hearing (NH) listeners, hearing-impaired (HI) listeners and cochlear implant (CI) users do not show substantially better speech perception with interfering sounds that fluctuate in level, such as speech, than with steady-state maskers. Attempts to identify the source of these deficits have been hindered by the fact that the processes underlying masking release in normal hearing are not completely understood, particularly in the ecologically valid situation where target speech is presented against a background of varying numbers of competing voices (n-talker babble). In NH listeners speech perception varies non-monotonically as the number of competing talkers increases from one to many, reflecting the interaction of different factors that have not been isolated in previous research, including: opportunities to glimpse the target afforded by fluctuations in masker level; difficulties in attending to the target rather than the masking speech; and interference in auditory object formation arising from the presence of multiple fundamental frequency (F0) contours. A major element of the proposed work is the development of novel stimulus configurations to allow independent assessment of the role of such factors in changes in performance with increasing numbers of interfering talkers. It is anticipated that the contribution of different factors will vary across different listener groups. Possible limitations on CI and HI listeners? ability to benefit from glimpsing will be examined with specific investigations of 1) the role of periodicity, including consideration of whether it is cued by temporal fine structure or envelope cues; 2) deficits in the ability to fuse segments of speech information over brief temporal gaps; and 3) effects of overall signal-to-noise ratio on the utility of glimpsing. Other issues to be addressed include effects of the presence of multiple F0 contours in the masker, and the role of masker intelligibility. The latter will be investigated using novel stimuli that allow comparison between intelligible and unintelligible maskers that both incorporate the spectro-temporal complexity of speech. The proposed work will both extend our basic scientific knowledge of speech perception in noise and also have important practical applications in the development of technology to improve outcomes for CI and hearing-aid users, and the enhancement of diagnostic tests probing disabilities such as auditory processing disorder.