Hearing Science and Hearing Disorders

M.E. Lutman

• Publisher: Academic Press
• Year: 2014
• Print ISBN: 9780124604407
• eBook ISBN: 9781483295169
• Topic: Medicine
• Subtopic: Audiology & Speech Pathology

Hearing Science and Hearing Disorders focuses on the nature of the processes in the inner ear and the nervous system that mediate hearing. Organized into eight chapters, this book first discusses the nature of speech communication, the extent of hearing problems, and the pathophysiology of hearing. Four core chapters follow, in which four areas of central importance to understanding hearing disorders and their effects are covered. These areas are assessment of auditory function, the scope for technological solutions, the nature of audio-visual speech perception, and the effects of deafness upon speech production. This book will be valuable to students; to academic and professional workers concerned with hearing, speech, and their disorders; and to scientifically or medically literate people in general.

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Hearing for Speech: the Information Transmitted in Normal and Impaired Hearing

Peter J. Bailey

Publisher Summary

This chapter discusses the information that is present in speech; it has been shown that such knowledge can be used to bring a degree of order and understanding into the difficulties that are faced by hearing-impaired listeners in understanding speech. Supra-segmental information can be distinguished from two types of segmental information—the voicing and place of articulation features. There are grounds for believing humans to be specialized for speech communication. Parallel specialization for speech perception would be plausible despite the absence of extensive experimental evidence for perceptual specialization. Several characteristics of speech render it an efficient code for the particular communicative role it that it serves. It can support high rates of information flow by contrast with writing and reading, but it is flexible to allow the other extreme in trading off speed against accuracy, as when people pronounce clearly and slowly a crucial, unpredictable, or rare word, or a proper name.

Contents

I INTRODUCTION

II WHAT FORMS CAN LINGUISTICALLY RELEVANT INFORMATION TAKE?

III ARTICULATORY AND ACOUSTIC BASES FOR PHONETIC CONTRASTS

A Acoustic theory of speech production

B Acoustic correlates of articulatory dimensions

IV REPRESENTATIONS OF ACOUSTIC AND AUDITORY SPEECH PATTERNS

V PROPERTIES OF HEARING IMPAIRMENT RELEVANT TO SPEECH PERCEPTION

VI DIVISION OF LABOUR BETWEEN PERCEPTION AND PRODUCTION FOR EFFICIENT SPEECH COMMUNICATION

VII SPEECH PERCEPTION IN HEARING IMPAIRED LISTENERS

VIII CONCLUDING SUMMARY

I INTRODUCTION

There are grounds for believing humans to be specialised for speech communication - most evidently for speech production by virtue of the shape and flexibility of movement of the vocal tract and its articulators. Parallel specialisation for speech perception would be plausible despite the absence of extensive experimental evidence for perceptual specialisation (Repp, 1982). More generally, there is no compelling evidence against the formal uniqueness of human linguistic behaviour (MacPhail, 1982). The implications of these points are: first, evolutionary pressures have rendered speech an efficient vehicle for linguistic communication (Lieberman, 1973), and secondly, speaker/hearers appear to be specialised to produce and perceive linguistic information in this form. Second, evolution of human culture has since made speech the main reason we have for hearing. To the extent that these observations are true it would seem perverse not to attempt to maximise access to speech for hearing-impaired people before trying other rehabilitative measures.

Several characteristics of speech render it an efficient code for the particular communicative role it serves. It can support high rates of information flow by contrast with writing and reading (intelligibility falls significantly only when speech rates exceed about 160 words per minute), but it is flexible enough also to allow the other extreme in trading off speed against accuracy, as when people pronounce clearly and slowly a crucial, unpredictable, or rare word, or a proper name. The redundancy which permits these high rates also ensures substantial resistance to the masking effects of extraneous noise, so that speech remains intelligible despite considerable distortion or signal-to-noise ratios worse than 0 dB. Speech is well adapted to carry extra-linguistic information such as emphasis and emotional content by means of variations in pitch, loudness and rhythm. Speech also provides meta-communicative cues such as those which cede or deny the floor to another interlocutor; these facilitate fluent conversation. At a more practical level, there is clear virtue in a mode of communication which leaves hands, feet and limbs free for other skilled behaviour. For all these reasons there is good justification for expressing our understanding of hearing impairment in specific relation to the requirements of understanding speech. Traditionally in audiology this has only been done in a rather notional way, on which recent developments in the acoustical control of test materials have begun to improve (e.g. Fourcin, 1980).

Much of the handicap of hearing impairment derives from a single intervening disability - the failure to understand speech easily, particularly when listening in a noisy environment. A common report from hearing-impaired people is that they can hear speech but cannot understand what is being said. My two main aims in this chapter are to characterise the main types of linguistically significant information present in speech, and to establish a basis for discussion of some factors which contribute to poor speech perception in hearing-impaired people. A complementary chapter (Summerfield, 1982; this volume) considers the limitations on, and implications of, speech perception using additional information derived from senses other than hearing primarily vision.

Ideally we might begin with a description of the process of speech understanding in normal hearing listeners. In that way, drawing on data on hearing impairment we might be able to deduce the effects of hearing impairment on speech processing and discuss the results of experiments which test such predictions. In a statistical fashion this has been done (see Haggard, 1982; this volume). But these purely quantitative formulations make no reference to the specific classes of information within speech or to the processes by which the information is analysed and interpreted and so, despite their usefulness, offer no element of explanation. Only with such understanding can hope be entertained of an effective approach to disability assessment, hearing aid design, and training in hearing tactics. Unfortunately, in spite of a considerable body of data on the relationships between speech production, speech acoustics and speech perception, there exists no adequate account of normal speech understanding that embraces the complex relationships reported and qualifies as a genuine “theory”. I shall concentrate, therefore, on a more descriptive level, clarifying the principles governing the form of acoustical patterns resulting from activities of the vocal apparatus, and examining how these patterns carry linguistically significant information for the listener. Many of the experiments I shall refer to have used listeners with normal hearing; this is in part because such experiments are the majority, but also because there are reasons to believe that improvements in auditory prostheses are now likely to follow from a better overall understanding of normal speech processes (Haggard, 1982; this volume). My general strategy will be to attempt to rationalise the patterns of speech perception deficits that are characteristic of certain kinds of hearing impairment, particularly hearing loss of sensorineural origin acquired post-lingually. Specifically, I shall comment on the relative efficacy with which various significant acoustical properties of speech are preserved in the listener’s impaired auditory system.

II WHAT FORMS CAN LINGUISTICALLY RELEVANT INFORMATION TAKE?

Most normal language users believe that understanding is an immediate and effortless consequence of listening to speech. Against this background, describing formally what is involved in successful speech understanding is surprisingly difficult. To describe spoken language demands a complex representation which can take many forms. A useful view of the speech communication process is as a set of sub-processes inside the brains of talkers and listeners. The first set is in the talker starting with the intention to communicate, and involves a series of normally hierarchical stages where implicit knowledge about word meanings, syntax, word-sound correspondence etc. is used to encode a message into an acoustic signal. The listener is supposed to decode the signal using an approximately matched set of hierarchical but inverse perceptual processing stages, beginning with an auditory representation and terminating in recovery of the talker’s message and hence “understanding”. Each processing stage is assumed to transform the message from one internal representation to another, preserving linguistically relevant information. A full account of linguistic communication would thus require a specification of each representation and a detailed description of the mechanism of each processing stage. This view is not an explanatory model of the process but a starting framework within which detailed models could be proposed. The psychological reality of a particular model has then to be established by experimental investigation.

Although normal and abnormal linguistic and phonetic structures can be described in a fashion that is logically rigorous (see Cowie and Douglas-Cowie, 1982; this volume) the only readily accessible data which can be measured in a physical sense are the optical correlates of speech and the acoustic speech signal; if one is interested in production, various physiological measures of articulatory behaviour may be added. However, if used in isolation, conventional techniques for acoustical analysis of speech do not illuminate directly its linguistically significant properties. This issue - the nature of acoustic correlates of linguistic units - is a central one for this chapter and will be considered in detail. We must begin, however, with a brief discussion of some ways of conceptualising the elements of a linguistic message.

I shall refer to the structures that generate speech - vocal cords, pharynx, soft and hard palate, tongue, teeth, jaw, lips, nasal passages etc. - as forming the vocal tract, and to the larger moving parts - lips, tongue and jaw - as the major articulators in the vocal tract. Measurements of articulator movement reveal intricate motor patterns; the simple demonstration of attending to all the detailed antics in your own vocal tract, while speaking this sentence aloud in slow motion will confirm that speaking is a complex act which demands precise control and coordination of a large number of muscles. Despite this complexity when expressed in terms of spatio-temporal coordinates of major articulators over time, a number of general principles of vocal tract action can be described which form the basis for a more manageable taxonomy of speech involving a set of intersecting articulatory classes. Articulatory classifications of speech elements are economical, and have historical respectability - they were employed by Sanskrit grammarians roughly 2600 years ago.

A relatively small number of articulatory dimensions is sufficient to carry linguistically significant contrasts. Vowels (for example, /i/ and /a/ in “deep, dark“), semi-vowels /w/ as in “wailed”), continuant consonants (/s/ as in “monster”) and interrupted consonants (/d/ as in “dark”, /g/ in “grotto”) form a natural ranking of articulations with increasingly narrow constriction of the vocal tract. Another important dimension is the position in the vocal tract where the maximum constriction occurs; the initial consonants in “gay“, “day” and “bay” involve constriction at increasingly more forward vocal tract locations, towards the front of the mouth. These two dimensions correspond roughly to those known to phoneticians as manner and place of articulation. The voicing contrast, referring to the initial presence or absence of vibration of the vocal cords, as between the initial consonants /b/ and /p/ in “bay” and “pay”, allows further subdivisions of some of the above categories. This taxonomy allows the phonemes of a language to be represented as an intersecting set of features and hence allows utterances to be represented as articulatorily-defined segments arrayed serially in time. Thus the initial segment in “bay” is an interrupted, voiced consonant with bilabial place of articulation, that is with vocal tract constriction at the lips. The adequacy of such a description of the content of an utterance in terms of a series of phonetic segments or phonemes (consonant, vowel, consonant etc.) having in turn distinctive features (interrupted, voiced etc.) depends on the purpose for which the description is used. It shares much with schemes one might use to classify the orthography of written language; segments correspond roughly to alphabetical characters and features to properties like presence or absence of a vertical stroke in a character. For speech, descriptions at this level are natural candidates for expressing economically some of the knowledge that language users have which makes them creative. For example, we can state simple prescriptive rules for the formation of the plural of English nouns never previously encountered. Although generally written with an “s”, the plural is realised phonetically in different ways, chiefly as /Iz/, /z/ or /s/ depending on the preceding segment. The ease with which this and similar rules can be stated in segmental terms contrasts sharply with their difficul...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Editorial Note
• List of Contributors
• Chapter 1: Hearing for Speech: the Information Transmitted in Normal and Impaired Hearing
• Chapter 2: Hearing Disorders in the Population: First Phase Findings of the MRC National Study of Hearing
• Chapter 3: Pathophysiology of the Peripheral Hearing Mechanism
• Chapter 4: The Scientific Basis for the Assessment of Hearing
• Chapter 5: Audio-visual Speech Perception, Lipreading and Artificial Stimulation
• Chapter 6: Speech Production in Profound Postlingual Deafness
• Chapter 7: New and Old Conceptions of Hearing Aids
• Chapter 8: Rehabilitation and Service Needs
• Glossary of Audiological, Acoustical and Phonetic Terms
• Subject Index