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

Field: Health Professions (Health Sciences)

Studies concerning speech recognition in noise constitute a very broad spectrum of work \nincluding aspects like the cocktail party effect or observing performance of individuals in \ndifferent types of speech-signal or noise as well as benefit and improvement with hearing aids. \nAnother important area that has received much attention is investigating the inter-relations \namong various auditory and non-auditory capabilities affecting speech intelligibility. Those \nstudies have focussed on the relationship between auditory threshold (hearing sensitivity) and a \nnumber of suprathreshold abilities like speech recognition in quiet and noise, frequency \nresolution, temporal resolution and the non-auditory ability of cognition. \nThere is considerable discrepancy regarding the relationship between speech recognition in \nnoise and hearing threshold level. Some studies conclude that speech recognition performance \nin noise can be predicted solely from an individual’s hearing threshold level while others \nconclude that other supra-threshold factors such as frequency and/or temporal resolution must \nalso play a role. Hearing loss involves more than deficits in recognising speech in noise, raising \nthe question whether hearing impairment is a uni- or multi-dimensional construct. Moreover, \ndifferent extents of hearing loss may display different relationships among measures of hearing \nability, or different dimensionality. \nThe present thesis attempts to address these three issues, by examining a wide range of hearing \nabilities in large samples of participants having a range of hearing ability from normal to \nmoderate-severe impairment. The research extends previous work by including larger samples \nof participants, a wider range of measures of hearing ability and by differentiating among levels \nof hearing impairment. \nMethod: Two large multi-centre studies were conducted, involving 103 and 128 participants \nrespectively. A large battery of tests was devised and refined prior to the main studies and \nimplemented on a common PC-based platform. The test domains included measurement of \nhearing sensitivity, speech recognition in quiet and noise, loudness perception, frequency \nresolution, temporal resolution, binaural hearing and localization, cognition and subjective \nmeasures like listening effort and self-report of hearing disability. Performance tests involved \npresentation of sounds via circum-aural earphones to one or both ears, as required, at intensities \nmatched to individual hearing impairments to ensure audibility. Most tests involved \nmeasurements centred on a low frequency (500 Hz), high frequency (3000 Hz) and broadband. \nThe second study included some refinements based on analysis of the first study. Analyses \nincluded multiple regression for prediction of speech recognition in stationary or fluctuating \nnoise and factor analysis to explore the dimensionality of the data. Speech recognition \nperformance was also compared with that predicted using the Speech Intelligibility Index (SII). \niii \nFindings: Findings from regression analysis pooled across the two studies showed that speech \nrecognition in noise can be predicted from a combination of hearing threshold at higher \nfrequencies (3000/4000 Hz) and frequency resolution at low frequency (500 Hz). This supports \nprevious studies that conclude that resolution is important in addition to hearing sensitivity. This \nwas also confirmed by the fact that SII (representing sensitivity rather than resolution) underpredicted \ndifficulties observed in hearing-impaired ears for speech recognition in noise. Speech \nrecognition in stationary noise was predicted mainly by auditory threshold while speech \nrecognition in fluctuating noise was predicted by a combination having a larger contribution \nfrom frequency resolution. In mild hearing losses (below 40 dB), speech recognition in noise \nwas predicted mainly by hearing threshold, in moderate hearing losses (above 40 dB) it was \npredicted mainly by frequency resolution when combined for two studies. Thus it can be \nobserved that the importance of auditory resolution (in this case frequency resolution) increases \nand the importance of the audiogram decreases as the degree of hearing loss increases, provided \nspeech is presented at audible levels. However, for all degrees of hearing impairment included \nin the study, prediction based solely on hearing thresholds was not much worse than prediction \nbased on a combination of thresholds and frequency resolution. Lastly, hearing impairment was \nshown to be multi-dimensional; main factors included hearing threshold, speech recognition in \nstationary and fluctuating noise, frequency and temporal resolution, binaural processing, \nloudness perception, cognition and self-reported hearing difficulties. A clinical test protocol for \ndefining an individual auditory profile is suggested based on these findings. \nConclusions: Speech recognition in noise depends on a combination of audibility of the speech \ncomponents (hearing threshold) and frequency resolution. Models such as SII that do not \ninclude resolution tend to over-predict somewhat speech recognition performance in noise, \nespecially for more severe hearing impairments. However, the over-prediction is not great. It \nfollows that for clinical purposes there is not much to be gained from more complex \npsychoacoustic characterisation of sensorineural hearing impairment, when the purpose is to \npredict or explain difficulty understanding speech in noise. A conventional audiogram and \npossibly measurement of frequency resolution at 500 Hz is sufficient. However, if the purpose \nis to acquire a detailed individual auditory profile, the multidimensional nature of hearing loss \nshould not be ignored. Findings from the present study show that, along with loss of sensitivity \nand reduced frequency resolution ability, binaural processing, loudness perception, cognition \nand self-report measures help to characterize this multi-dimensionality. Detailed studies should \nhence focus on these multiple dimensions of hearing loss and incorporate measuring a wide \nvariety of different auditory capabilities, rather than inclusion of just a few, in order gain a \ncomplete picture of auditory functioning. \nFrequency resolution at low frequency (500 Hz) as a predictive factor for speech recognition in \nnoise is a new finding. Few previous studies have included low-frequency measures of hearing, \nwhich may explain why it has not emerged previously. Yet this finding appears to be robust, as \nit was consistent across both of the present studies. It may relate to differentiation of vowel \ncomponents of speech. The present work was unable to confirm the suggestion from previous \nstudies that measures of temporal resolution help to predict speech recognition in fluctuating \nnoise, possibly because few participants had extremely poor temporal resolution ability.