Factors affecting speech recognition in noise and hearing loss in adults with a wide variety of auditory capabilities

Studies concerning speech recognition in noise constitute a very broad spectrum of work<br/>including aspects like the cocktail party effect or observing performance of individuals in<br/>different types of speech-signal or noise as well as benefit and improvement with hearing aids.<br/>Another important area that has received much attention is investigating the inter-relations<br/>among various auditory and non-auditory capabilities affecting speech intelligibility. Those<br/>studies have focussed on the relationship between auditory threshold (hearing sensitivity) and a<br/>number of suprathreshold abilities like speech recognition in quiet and noise, frequency<br/>resolution, temporal resolution and the non-auditory ability of cognition.<br/>There is considerable discrepancy regarding the relationship between speech recognition in<br/>noise and hearing threshold level. Some studies conclude that speech recognition performance<br/>in noise can be predicted solely from an individual’s hearing threshold level while others<br/>conclude that other supra-threshold factors such as frequency and/or temporal resolution must<br/>also play a role. Hearing loss involves more than deficits in recognising speech in noise, raising<br/>the question whether hearing impairment is a uni- or multi-dimensional construct. Moreover,<br/>different extents of hearing loss may display different relationships among measures of hearing<br/>ability, or different dimensionality.<br/>The present thesis attempts to address these three issues, by examining a wide range of hearing<br/>abilities in large samples of participants having a range of hearing ability from normal to<br/>moderate-severe impairment. The research extends previous work by including larger samples<br/>of participants, a wider range of measures of hearing ability and by differentiating among levels<br/>of hearing impairment.<br/>Method: Two large multi-centre studies were conducted, involving 103 and 128 participants<br/>respectively. A large battery of tests was devised and refined prior to the main studies and<br/>implemented on a common PC-based platform. The test domains included measurement of<br/>hearing sensitivity, speech recognition in quiet and noise, loudness perception, frequency<br/>resolution, temporal resolution, binaural hearing and localization, cognition and subjective<br/>measures like listening effort and self-report of hearing disability. Performance tests involved<br/>presentation of sounds via circum-aural earphones to one or both ears, as required, at intensities<br/>matched to individual hearing impairments to ensure audibility. Most tests involved<br/>measurements centred on a low frequency (500 Hz), high frequency (3000 Hz) and broadband.<br/>The second study included some refinements based on analysis of the first study. Analyses<br/>included multiple regression for prediction of speech recognition in stationary or fluctuating<br/>noise and factor analysis to explore the dimensionality of the data. Speech recognition<br/>performance was also compared with that predicted using the Speech Intelligibility Index (SII).<br/>iii<br/>Findings: Findings from regression analysis pooled across the two studies showed that speech<br/>recognition in noise can be predicted from a combination of hearing threshold at higher<br/>frequencies (3000/4000 Hz) and frequency resolution at low frequency (500 Hz). This supports<br/>previous studies that conclude that resolution is important in addition to hearing sensitivity. This<br/>was also confirmed by the fact that SII (representing sensitivity rather than resolution) underpredicted<br/>difficulties observed in hearing-impaired ears for speech recognition in noise. Speech<br/>recognition in stationary noise was predicted mainly by auditory threshold while speech<br/>recognition in fluctuating noise was predicted by a combination having a larger contribution<br/>from frequency resolution. In mild hearing losses (below 40 dB), speech recognition in noise<br/>was predicted mainly by hearing threshold, in moderate hearing losses (above 40 dB) it was<br/>predicted mainly by frequency resolution when combined for two studies. Thus it can be<br/>observed that the importance of auditory resolution (in this case frequency resolution) increases<br/>and the importance of the audiogram decreases as the degree of hearing loss increases, provided<br/>speech is presented at audible levels. However, for all degrees of hearing impairment included<br/>in the study, prediction based solely on hearing thresholds was not much worse than prediction<br/>based on a combination of thresholds and frequency resolution. Lastly, hearing impairment was<br/>shown to be multi-dimensional; main factors included hearing threshold, speech recognition in<br/>stationary and fluctuating noise, frequency and temporal resolution, binaural processing,<br/>loudness perception, cognition and self-reported hearing difficulties. A clinical test protocol for<br/>defining an individual auditory profile is suggested based on these findings.<br/>Conclusions: Speech recognition in noise depends on a combination of audibility of the speech<br/>components (hearing threshold) and frequency resolution. Models such as SII that do not<br/>include resolution tend to over-predict somewhat speech recognition performance in noise,<br/>especially for more severe hearing impairments. However, the over-prediction is not great. It<br/>follows that for clinical purposes there is not much to be gained from more complex<br/>psychoacoustic characterisation of sensorineural hearing impairment, when the purpose is to<br/>predict or explain difficulty understanding speech in noise. A conventional audiogram and<br/>possibly measurement of frequency resolution at 500 Hz is sufficient. However, if the purpose<br/>is to acquire a detailed individual auditory profile, the multidimensional nature of hearing loss<br/>should not be ignored. Findings from the present study show that, along with loss of sensitivity<br/>and reduced frequency resolution ability, binaural processing, loudness perception, cognition<br/>and self-report measures help to characterize this multi-dimensionality. Detailed studies should<br/>hence focus on these multiple dimensions of hearing loss and incorporate measuring a wide<br/>variety of different auditory capabilities, rather than inclusion of just a few, in order gain a<br/>complete picture of auditory functioning.<br/>Frequency resolution at low frequency (500 Hz) as a predictive factor for speech recognition in<br/>noise is a new finding. Few previous studies have included low-frequency measures of hearing,<br/>which may explain why it has not emerged previously. Yet this finding appears to be robust, as<br/>it was consistent across both of the present studies. It may relate to differentiation of vowel<br/>components of speech. The present work was unable to confirm the suggestion from previous<br/>studies that measures of temporal resolution help to predict speech recognition in fluctuating<br/>noise, possibly because few participants had extremely poor temporal resolution ability.

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2010

Author dc:creator

• Athalye, S.A.

Advisor dc:contributor.advisor

• Lutman, M.E.