Cochlear Implants: Fundamentals and Application - part 1 potx

The cochlear implant is a device that bypasses a nonfunctional inner ear andstimulates the hearing nerves with patterns of electrical currents so that speechand other sounds can be exper

Cochlear Implants: Fundamentals and

Application

Graeme Clark

Springer

COCHLEAR IMPLANTS

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Graeme Clark

Department of Otolaryngology and

The Bionic Ear Institute

The University of Melbourne

Cochlear implants : fundamentals and applications / Graeme Clark.

p cm — (Modern acoustics and signal processing)

Includes bibliographical references and index.

ISBN 0-387-95583-6 (alk paper)

1 Cochlear implants 2 Deaf—Rehabilitation I Title II AIP series in modern acoustics and signal processing.

RF305.C536 2003

617.8 ⬘9—dc21 2002030584 ISBN 0-387-95583-6 Printed on acid-free paper.

䉷 2003 Springer-Verlag New York, Inc.

AIP Press is an imprint of Springer-Verlag New York, Inc.

All rights reserved This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer-Verlag New York, Inc., 175 Fifth Avenue, New York, NY 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden.

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A member of BertelsmannSpringer Science ⳭBusiness Media GmbH

at the University of Sydney, the University of Melbourne, and The Bionic EarInstitute I would like also to express my appreciation to our children Sonya,Cecily, Roslyn, Merran, and Jonathan; their spouses Ian, Peter, and Marissa; andour grandchildren Elise, Monty, Daniel, Noah, and Rebekah for their encourage-ment and enriching our lives.

I have been very impressed by the emergence of the bionic ear as a practical proposition, but even more by the promise for the future that it seems to embody It makes use of the arrangement in the cochlea for pitch recognition to bring electronic technology into direct functional relationship with the nervous system and the hu- man consciousness Maybe that unique relationship has no other parallel in the nervous system, and thus that direct link between electronics and physiology will find no other application to medi- cine Nevertheless, I feel it may represent a new benchmark in the understanding of neural and mental function in terms of their physi- cal components.

—Professor Emeritus Sir Macfarlane Burnett, A.K., O.M., K.B.E., M.D., Ph.D., Lond., F.A.A., F.R.S., Nobel Laureate (Physiology or Medicine)—The First Patron of the Bionic Ear Institute, 1985

Soun is nought but air y-broke

—Geoffrey Chaucerend of the 14th centuryTraditionally, acoustics has formed one of the fundamental branches of physics

In the twentieth century, the field has broadened considerably and become creasingly interdisciplinary At the present time, specialists in modern acousticscan be encountered not only in physics departments, but also in electrical andmechanical engineering departments, as well as in mathematics, oceanography,and even psychology departments They work in areas spanning from musicalinstruments to architecture to problems related to speech perception Today, sixhundred years after Chaucer made his brilliant remark, we recognize that soundand acoustics is a discipline extremely broad in scope, literally covering wavesand vibrations in all media at all frequencies and at all intensities

in-This series of scientific literature, entitled Modern Acoustics and Signal cessing (MASP), covers all areas of today’s acoustics as an interdisciplinary field

Pro-It offers scientific monographs, graduate-level textbooks, and reference materials

in such areas as architectural acoustics, structural sound and vibration, musicalacoustics, noise, bioacoustics, physiological and psychological acoustics, speech,ocean acoustics, underwater sound, and acoustical signal processing

Acoustics is primarily a matter of communication Whether it be speech ormusic, listening spaces or hearing, signaling in sonar or in ultrasonography, weseek to maximize our ability to convey information and, at the same time, tominimize the effects of noise Signaling has itself given birth to the field of signalprocessing, the analysis of all received acoustic information or, indeed, all infor-mation in any electronic form With the extreme importance of acoustics for bothmodern science and industry in mind, AIP press, now an imprint of Springer-Verlag, initiated this series as a new and promising publishing venture We hopethat this venture will be beneficial to the entire international acoustical commu-nity, as represented by the Acoustical Society of America, a founding member of

vii

viii Series Preface

the American Institute of Physics, and other related societies and professionalinterest groups

It is our hope that scientists and graduate students will find the books in thisseries useful in their research, teaching, and studies As James Russell Lowellonce wrote, “In creating, the only hard thing’s to begin.” This is such a beginning

Robert T Beyer Series Editor-in-Chief

The cochlear implant is a device that bypasses a nonfunctional inner ear andstimulates the hearing nerves with patterns of electrical currents so that speechand other sounds can be experienced by profoundly deaf people It is the cul-mination of investigations that started in the 19th century, and as such it is thefirst major advance in helping profoundly deaf children to communicate since thesign language of the deaf was developed at the Paris Deaf School 200 years ago

It is also the first direct interface to the central nervous system to restore sensoryfunction for use on a regular clinical basis

I became interested in helping deaf people hear when I was 10 years old,because my father had a severe hearing loss and I knew how difficult it was forhim to cope as a pharmacist and as a family man In 1966 I left my practice as

an ear, nose, and throat surgeon in Melbourne to do research and to learn how itmight be possible to help people with a profound hearing loss These were thepatients I had to turn away from my clinic, saying that a hearing aid would be oflittle help but that one day medical research might provide an alternative For methis meant first undertaking basic studies to learn about the differences betweenacoustic and electrical stimulation of the auditory neural pathways

When it became clear from these and other basic studies that the best chance

of providing speech understanding was through multiple electrode stimulation,many scientific challenges were to lie ahead As previous attempts to producespeech understanding with electrical stimulation had been unsuccessful, and asreproducing the coding of sound was not seen as feasible, the research facedrigorous scientific criticism The first criticism came from auditory neuroscience,where research had shown the complexity of the inner ear and central brain path-ways Not surprisingly it was believed that inserting a relatively small number ofelectrodes into the inner ear to stimulate groups of nerve fibers would fail toproduce sufficient information for speech understanding The second criticismcame from the biological and clinical disciplines Here the concern was that im-plantation would damage the very nerves it was intended to stimulate In addition,

it was thought the electrode could be a pathway for middle ear infection to induce

x Preface

dangerous infection in the inner ear These biological and clinical criticisms werealso well founded The delicacy of the inner ear had been appreciated in earsurgery, and the risk of infection was ever present in young children The abovetwo major criticisms required answers before clinical studies could be done onpatients

It was also essential to determine from a small group of volunteers how thecomplex signals of speech could be presented as patterns of electrical stimulationthat could be understood This seemed at the time an almost insurmountablechallenge Research that followed established that speech processing could in fact

be achieved safely for profoundly deaf adults, who had hearing before going deaf.After the benefits were shown for adults, it was appropriate to initiate research tosee if children born deaf or deafened early in life could obtain sufficient speechunderstanding to enable them to manage successfully in a hearing world Woulddeaf children be able to develop the right central neural connections, as they hadreceived no auditory stimulation during the plastic phase of brain development?Indeed children who were born deaf were shown to develop speech at a levelcomparable to that in adults who had prior exposure to sound Furthermore, itwas discovered that if they were operated on at a young age, they could developgood speech sounds as well as language

Providing hearing and speech understanding for children born deaf then led to

an intense ethical debate The signing deaf community had developed an effectivecommunication system and support network to help one another Communitymembers were upheld by a strong belief in their self-worth, which is so necessary

to manage in a world of sound where people with good hearing did not fullyappreciate the great difficulty they had For a time the implant was seen as anideological threat to their beliefs and as undermining this well-knit group, andfor a number of years the efficacy of the procedure was questioned It requiredmany controlled studies and the opinion of educators who had experience withthe achievements of children with hearing aids before the benefits of the implantfor children were fully appreciated

The cochlear implant has been the result of research in many disciplines, cluding surgical anatomy, surgical pathology, biology, biophysics, neurophysi-ology, psychophysics, speech science, engineering, surgery, audiology, rehabili-tation, and education Few medical advances have required the integration of somany disciplines The scientific questions in these disciplines had to be addressed

in-in a logical, systematic, and sequential manner, and are discussed in-in this book

As a result of this research, cochlear implantation has grown from a smallnumber of isolated experimental studies done by a few, to a diverse disciplineinvestigated by many Its scientific credibility has been recognized through itsinclusion in international physiological, acoustical, surgical, otolaryngological,audiological, education, speech science and technology, and engineering societymeetings In addition, there are many international meetings devoted solely to thetopic of cochlear implants The growth in knowledge in the last 30 years has beenrapid This can be seen in the number of papers that include cochlear implants inthe title, abstract, or subject heading: in the 1960s, one; in the 1970s, 72; in the

1980s, 679; in the 1990s, 1,935 There have been many other relevant tions Not only have there been a very large number of scientific papers, but therealso have been monographs and book chapters.

publica-Initially the field drew on basic sciences for its development, and then graduallyestablished its own body of scientific and clinical knowledge This has continued

to the point that now electrical simulation of the auditory system can justly claim

to be making scientific contributions to the disciplines that helped establish it, inparticular neurophysiology, biology, psychophysics, speech science, and the clini-cal disciplines of surgery, audiology, and rehabilitation

One aim of this book is to show how the numerous disciplines have contributedand how they have interrelated This book presents the fundamentals of the re-search as well as the clinical outcomes so that the reader will have a more com-plete understanding of the discipline It is intended for a general reader, and thosewith a more specialized background can refer to the references In presenting thefundamentals, research at the University of Melbourne/Bionic Ear Institute andelsewhere is cited Clinical studies cannot be divorced from the basic research.The two must guide each other and the main aim should be to help people Thisrequires excellent basic research, but it should be focused and not an end in itself

In this book this interaction is presented at all opportunities

Finally, the basic and clinical research would not have reached the wider munity without the biomedical and engineering expertise of industry The workhas been much more demanding than developing a pacemaker, as more complexelectronics have had to be encapsulated in a smaller implanted package Further-more, the interface with the auditory nervous system is a very intricate bioengi-neering achievement For this reason this book not only presents the basic andclinical research, but also discusses how these have supported the industrialachievement

com-Graeme Clark

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I am greatly indebted to the tremendous support received from Sue Davine,David Lawrence, Helen Reid, and John Huigen in the preparation of this book.

It has been a major undertaking, and Sue has completed many hours of typingand compiling the text David has produced diagrams and figures of a very highstandard, researched topics, and provided invaluable help Helen has diligentlysearched for references and found them, and John has helped to coordinate thiscombined effort I would also like to thank in particular Andrew Vandali, AnthonyBurkitt, David Grayden, Ian Rutherfurd, Jim Patrick, Joanna Parker, Mark Har-rison, Peter Busby, Peter Seligman, Richard Dowell, Thomas Stainsby, and Chrisvan den Honert for kindly reading sections of the text and for their helpful com-ments The imperfections are mine alone Thanks also to Russell Brooks forcompiling the index

The cochlear implant research in Australia has been a team effort, and it hasbeen a privilege to have worked with young and talented research students and

to have seen them develop into mature scientists It has also been a valued perience to have been closely involved with the staff members of Cochlear Lim-ited, a number of whom were research colleagues Without the close relationshipbetween the basic and focused research in Melbourne and the industrial researchand development in Sydney, the Nucleus device would not have become available

ex-to tens of thousands of severely and profoundly deaf people in more than 120countries

I would also like to pay tribute to the many scientists and clinicians from othercenters who have also worked with dedication to achieve hearing and speech fordeaf people There has been great collegiality internationally in this relativelysmall field that has crossed political and other divides to make the world a betterplace for people with a hearing disability

Finally, the work would not have been possible without the belief and support

of many benefactors, governments, trusts, and foundations Sometimes simplyhaving their encouragement at difficult times was enough

Graeme Clark

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Definition xxxi

Normal Hearing xxxi

Deafness xxxi

Overall Concept of the Bionic Ear xxxii

Training in the Use of the Bionic Ear xxxiii

Fundamental Objections and Questions xxxiii

Answers to the Fundamental Objections xxxiv

1 A History 1 Pre-science 1

Eighteenth Century 1

Nineteenth Century 3

Twentieth Century 3

1900 to 1930s: Early Hearing Aids 3

1930s to 1940s: Initial Indirect Electrical Stimulation in the Human 4

1950s to 1960s: Initial Direct Electrical Stimulation in the Human 6

1960s: Fundamental Research in the Experimental Animal 9

1970s: Fundamental Research in the Experimental Animal and Human 12

1980s: Fundamental Research, Industrial Development, and Clinical Trials 23

xvi Contents

1990s: Continuing Fundamental Research and Industrial

Development 45

References 46

2 Surgical Anatomy 58 Overview 58

Temporal Bone 59

Components 59

Embryology 60

Mastoid Air Cell System and Variations 61

Blood Supply and Innervation 61

Infant and Young Child 62

External Ear 64

Pinna 64

External Auditory Meatus 65

Middle Ear 65

Ossicles 65

Muscles 66

Relationships 67

Posterior Tympanotomy 70

Round Window and Niche 71

Inner Ear 73

Osseous 73

Membranous 77

Histology of the Cochlea 79

Embryology 82

Central auditory system 83

Overview 83

Auditory Nerve 86

Cochlear Nucleus 87

Superior Olivary Complex 89

Lateral Lemniscus 91

Inferior Colliculus 91

Superior Colliculus 92

Medial Geniculate Body 92

Auditory Cortex 93

References 93

3 Surgical Pathology 100 Inflammation 100

Classification 101

Etiology 101

Pathophysiology 101

Insertion Trauma 104

Tissue Responses in the Cochlea of the Experimental Animal 104

Tissue Responses in the Human 109

Bio-compatibility of Materials 112

Methods of Investigation 112

Tissue Response 113

Infection 116

Otitis Media 116

Labyrinthitis and Meningitis 117

Experimental Animal Studies 122

Host Factors and Foreign Bodies 136

Clinical Protocol 139

Deafness and the Central Auditory Pathways 140

Spiral Ganglion 140

Cochlear Nucleus 141

Pons and Midbrain 142

Human Brainstem 143

Prenatal (Congenital) and Postnatal Hearing Loss 144

Genetic and Chromosomal 144

Acquired 148

References 151

4 Neurobiology 160 Overview 160

Definition of Terms 160

Current and Charge 160

Voltage 161

Resistance 161

Capacitance 161

Impedance 161

Electrode/Tissue Interface 162

Polarization 162

Charge Transfer 162

Charge Density 163

Equivalent Circuits 163

Impedance 165

Corrosion-Stimulus Parameters 168

Mechanisms 168

Stimulus Parameters 169

Scanning Electron Microscope Evaluation of Electrodes 171

Electrical Parameters and Neural Stimulation 172

Electrochemically Safe Stimulus Parameters 172

Charge Density and Charge per Phase 173

Biochemical Effects 173

Neural Preservation 174

Electrical Stimulation of the Cochlear Nerve 175

Acute studies on the Effects of Low Rates of Stimulation 175

xviii Contents

Chronic Studies on the Effects of Low Rates of Stimulation 176

Acute Studies on the Effects of High Rates of Stimulation 183

Chronic Studies on the Effects of High Rates of Stimulation 187

Electrical Stimulation of the Cochlear Nucleus 189

Acute Studies on the Effects of Low Rates of Stimulation 189

Chronic Studies on the Effects of Low Rates of Stimulation 189

References 190

5 Electrophysiology 199 General Neurophysiology 199

Action Potentials 199

Strength-Duration Curves 202

Electrical Models of the Nerve Membrane 203

Convergence and Divergence 204

Auditory Physiology 205

Physics of Sound 206

External and Middle Ear Function 207

Cochlea 208

Auditory Neurophysiology 211

Electrophonic Hearing (Electrical Stimulation of the Cochlea) 233

Mechanisms 233

Electrophonic Hearing and Cochlear Implantation 235

Electrical Stimulation of the Cochlear Nerve 236

Temporal Coding 237

Place Coding 262

Intensity Coding 272

References 274

6 Psychophysics 296 Acoustic Stimulation 296

Pitch and Timbre 297

Loudness 302

Critical Band and Ratio 305

Musical Acoustics 308

Bilateral Hearing 311

Electrical Stimulation 315

Temporal Information 315

Temporal Information: Prelinguistically Deaf 323

Place Information 326

Place Information: Prelinguistically Deaf 338

Loudness 341

Intensity Information: Prelinguistically Deaf 352

Musical Perception 353

Bimodal Stimulation 356

Bilateral Stimulation 358

References 365

7 Speech (Sound) Processing 381 Acoustic 381

Articulators and Vocal Tract Shape 382

Speech Analysis 382

Speech Perception and Production 385

Binaural Hearing 395

Acoustic Models of Cochlear Implant Speech-Processing Strategies 395

Channel Vocoders and Fixed Filters 395

Formant Vocoders 397

Acoustic Representation of Electrical Stimulation 398

Speech Cues 401

Channel Numbers 402

Speech in Noise 404

Channel Selection 404

Electrical Stimulation: Principles 405

Channel Numbers 406

Channel Selection 407

Speech in Noise 408

Speech Processing Strategies 409

Multiple-Channel Strategies: Fixed Filter Schemes 411

Multiple-Electrode Strategies: Formant and Spectral Cue Extraction 415

Adaptive Dynamic Range Optimization (ADRO) 432

Dual Microphones 433

Bimodal Speech Processing 435

Bilateral Speech Processing 438

References 442

8 Engineering 454 Electronic and Communications Engineering 456

Principles 456

Speech Processors 465

Receiver-Stimulators 484

Bioengineering 502

Design Principles 502

Design Realization 518

Conclusion 536

References 537

9 Preoperative Selection 550 Aims 550

Adults 550

Children 551

xx Contents

Clinical Protocol 552

Medical History and Examination 553

Aims 553

History 553

Physical Examination 554

Diagnosis-Etiology 554

Adults 554

Children 555

Audiology 559

Pure Tone Thresholds 559

Impedance Audiometry 562

Hearing Aid Evaluation 563

Cochlear Microphonics and ABR Tests for Neuropathy 564

Communication 566

Speech Perception 566

Speech Production 568

Language 568

Special Investigations 569

Radiology 569

Electrical Stimulation of the Promontory 572

Vestibular Assessment 574

Management 575

Hearing and Speech Perception 575

Predictive Factors 576

Preoperative Counseling 586

References 586

10 Surgery 595 Overview 595

Brief History 596

Aims 596

Position Multiple Electrodes Close to the Auditory Nerves 596

Implant Electrode with Minimal Trauma to the Inner Ear 597

Locate the Receiver-Stimulator to Allow Optimal Use of a Microphone, Speech Processor and Transmitting Coil 597

Implant Receiver-Stimulator to be Unaffected by Growth Changes 597

Implant Operation Performed Safely 597

Fundamentals and Clinical Practice 597

Preoperative Measures 598

Incision 599

First Stage Mastoid Cell Removal 603

Creation of a Bed for the Receiver-Stimulator 603

Creation of Gutter for the Lead Wire Assembly 606

Exposure of the Round Window via a Posterior Tympanotomy 607

Cochleostomy (Opening into the Inner Ear) 608Insertion of Arrays 612Sealing the Opening 617Perilymph “Gusher” 618Fixing the Electrode Array and Receiver-Stimulator 619Flap and Wound Closure 620Radiology 620Postoperative Care 621Complications and Management 621Intraoperative Complications 621Postoperative Complications 623Special Cases 639Ossified Cochlea 639Secretory (Serous) Otitis Media 642Tympanic Membrane Perforation and Chronic Suppurative OtitisMedia 642Open Mastoid 642Congenital or Genetic Malformation of the Cochlea 643Transmastoid Labyrinthectomy and Acoustic Neuroma 643Insertion and Reinsertion 644Pedestal (Plug and socket) 644Magnetic Resonance Imaging (MRI) 645References 645

Aims 654Principles 655Plasticity in the Experimental Animal 655Plasticity—Psychophysics 656Plasticity—Cross-Modality in Humans 661Analytic Versus Synthetic Training 661Mapping and Fitting Procedures in Adults and Children 663Physiological and Psychophysical Principles 663Producing a MAP 665Signal Gain 668Loudness Summation 670Patient Preference 670Training in Adults and Children 670General 671Predictive Factors 672Strategy and Time Course for Learning 674Analytic 676Synthetic 676Environmental Sounds 677Background Noise 677

xxii Contents

Music 678Telephone 678Television 679Mapping and Fitting Children 679Preprogramming Training 680Conditioning 680Initial Setting 681Follow-up Device Settings .682Neural Response Telemetry 683Training in Children 684General 684Personnel 684Pragmatics 686Speech Perception 686Perception of Environmental Sounds 688Speech Production 689Language 690Education of Children 693Acoustic Environment 693Strategies 694Program for Implanted Children 695Counseling of Adults and Children 696References 697

Aims 707Development of Tests 707Speech and Sound Perception: Test Principles 708Variability of Materials and Responses 708Prerecorded Versus Live Voice 709Training Effects and Experience 710Closed-Set Tests 710Speech Features (Consonants and Vowels) 712Open-Set Tests 713Speech Reading 714Speech Tracking 715Speech in Noise 716Environmental Sounds 717Test Batteries 717Questionnaires 718Bimodal and Bilateral Speech Processing 718Speech Production: Test Principles 720Imitative and Spontaneous Speech 720Computer Aided Speech and Language Assessment procedure

(CASALA) 721

Language: Test Principles 721Receptive Language 722Expressive Language 722Pragmatics 724Speech perception with Cochlear Implants 724Predictive Factors 724Speech-Processing Strategies for Postlinguistically Deaf Adults 726Speech-Processing Strategies for Pre- and Postlinguistically DeafChildren 738Speech production with Cochlear Implants 744Single-Channel System (3M/House) 744Nucleus Multiple-Channel (F0/F1/F2) and Multipeak Strategies 744Language Development for Pre- and Postlinguistically Deaf

Children 747Receptive Language 747Expressive Language 750Cognition 751References 752

Speech and Language Benefits 767Biological Safety 767Social Benefits 768Personal 768Family 768School 769Economic Benefits 769Economic Measures 769Cost-Effectiveness 770Cost-Benefit Analysis 770Quality of Life 771Ethics 773Human Experimentation 773Rights of Children 779Attitudes of Hearing-Impaired People 782References 783

Improved Sound Fidelity and Speech Processing 787Selection of Information 788Optimal Rate Stimulation 788Improved Coding 789Improved Speech Perception in Noise 795Bimodal Speech Processing 795Bilateral Speech Processing 797Dual Microphones 797Improved Speech and Language in Children 797Totally Implantable Cochlear Prosthesis 801Auditory Nerve Preservation and Regeneration 802References 809

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A apical

AAA auditory association area

ABF adaptive beam forming

ABR auditory brainstem

ADC analog-to-digital converter

ADRO adaptive dynamic range

optimization

AGC automatic gain control

AM amplitude modulation

AN auditory nerve

ANF auditory nerve fiber

ANSI American National

and Language Assessment

CC common cavity

CD characteristic delayCELF clinical evaluation of

language fundamentals

CF crista fenestra

CG common ground

CI cochlear implantsCID Central Institute for the

DeafCIS continuous interleaved

sampler

CM cochlear microphonicsCMOS Complementary metal

oxide semiconductorCMV cytomegalovirus

CN cochlear nucleusCNC consonant-nucleus-

consonantCNS central nervous systemCRC Cooperative Research

CenterCSF cerebrospinal fluidCSIRO Commonwealth Scientific

Industrial ResearchOrganization

CT chorda tympani

xxvi Abbreviations

CT (scan) computed tomography

CUNY City University of New

DNA deoxyribonucleic acid

DNLL dorsal nuclei lateral

lemniscus

DRSP differential rate speech

processing strategy

DSP digital signal processor

EABR evoked auditory brainstem

response

ECAP electrically evoked

compound action potential

EcoG electrocochleography

ED electrode decoder

EE excitatory contralateral

and excitatory ipsilateral

EEPROM electrically erasable

programmable read-only

memory

EI excitatory contralateral

and inhibitory ipsilateral

ENT ear, nose, and throat

EOWPVT Expressive One-Word

Picture Vocabulary Test

EPROM erasable programmable

F1 first formant frequency

F2 second formant frequency

FDA Food and Drug

AdministrationFEP fluoroethylene propyleneFET field effect transistorFFT fast Fourier transformFGF fibroblast growth factor

FM frequency modulation

FN facial nerveGASP Glendonald Auditory

Screening Procedure

H helicotrema

HA hearing aidHCRC Human Communication

Research CenterHINT Hearing in Noise Test

Hk hook region

HL hearing lossHMM hidden Markov modelHRP horseradish peroxidase

IC inferior colliculusICC central nucleus of the

inferior colliculusIDE Investigational Device

differenceIMPEBAP implant evoked brainstem

auditory potentialIMSPACP Imitated Speech Pattern

TestINLL intermediate nuclei lateral

lemniscusINSERM Institut National de la

Sante´ et de la RechercheMe´dicale

IP interleaved pulseIPSP inhibitory postsynaptic

potential

IPSyn index of productive

syntax

ITD interaural time difference

JFET junction field effect

transistor

JLD just discriminable level

difference

JND just noticeable difference

KEMAR Knowles Electronic

Manikin for Acoustic

LDL loudness discomfort level

LIF leukemia inhibitory factor

LiP listening progress profile

LL lateral lemniscus

LQ language quotient

LSO lateral superior olive

LVAS large vestibular aqueduct

syndrome

M middle turn

MAA minimum audible angle

MAC Minimal Auditory

MLU mean length of utterance

MNTB medial nucleus of the

trapezoid body

MOS metal oxide

semiconductorMOSFET metal oxide

semiconductor field effecttransistor

MPP multiple pulse per periodMRI magnetic resonance

imagingmRNA messanger ribonucleic

acidMSO medial superior oliveMSP miniature speech

processorMSTP Monosyllables, Spondees,

Trochees, andPolysyllables TestMTP monosyllable, trochee,

polysyllableMUSL Melbourne University

Sentence ListsNH&MRC National Health and

Medical Research Council

of AustralianHL normal hearing levelNID National Institute of

DeafnessNIH National Institutes of

HealthNINCDS National Institute of

Neurological andCommunicative Disordersand Stroke

NINDB National Institute of

Neurological Diseases andBlindness

NINDS National Institute of

Neurological Diseases andStroke

NRT neural response telemetryNST Nonsense Syllable Test

NU Northwestern University

OC organ of CortiOCG output current generatorOHC outer hair cell

PAA polyacrylic acid

PAT Parametric Artificial

PLE phonetic level evaluation

PLS Preschool Language Scale

PMA premarket approval

PMMA Primary Measures of

the DeafROC receiver-operating curveROM read-only memoryRTI Research Triangle

Institute

RW round window

SA stimulus artifactSAS simultaneous analog

system

SC superior colliculi

SC supporting cellSEM scanning electron

microscopeSERT Sound Effects

Recognition Test

SG spiral ganglion cells

SI Synchronization IndexSII Speech Intelligibility

IndexSIT Speech Intelligibility Test

SL spiral ligament

SM scala mediaSMSP Spectral Maxima Sound

ProcessorSNR signal-to-noise ratioSOC superior olivary complex

SP summating potentialSPEAK speech processing

strategy: SMSP with 20filters

SPL sound pressure levelSpL spiral laminaSPP single pulse per periodSPS simultaneous pulsatile

stimulationSQUID superconducting quantum

interference deviceSRT speech reception thresholdSSEP steady state evoked

potential

ST scala tympani

StV stria vascularis

SUKL Sˇta´tny U´ stav pre Kontrolu

Liecˇiv (State Institute for

Drug Control, Slovakia)

lemniscusVNTB ventral nucleus of the

trapezoid bodyVOT voice onset timeVRA visual reinforcement

audiometryXIC commissure of the inferior

colliculus

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Normal Hearing

Hearing occurs when sound is transmitted down the ear canal, through the middleear, to the inner ear The inner ear is a very small, coiled, snail-like structureembedded in bone that houses the sense organ of hearing (organ of Corti) Theorgan of hearing rests on a membrane (basilar membrane) lying across the coil.This membrane vibrates selectively to different sound frequencies, so that it acts

as a sound filter High frequencies produce maximal vibrations at the beginning

of the coil near an opening from the middle ear called the round window Lowfrequencies produce maximal vibrations at the other end of the coil

The sense organ of hearing in the inner ear consists of cells with hairs thatprotrude into a gelatinous membrane When these hairs move back and forth inresponse to sound, their vibrations are converted into electrical currents Thisprocess results from chemical and physical changes in these hair cells Theseelectrical currents stimulate the hearing nerves and produce patterns of excitation.These patterns or stimulus codes are transmitted to the higher brain centers wherethey are interpreted as sound The patterns of electrical responses are processed

as pitch and loudness, as well as meaningful signals such as speech

Deafness

A person who has a progressive sensorineural deafness loses the hair cells in theinner ear As a result the hearing becomes faint and distorted and the sound has

xxxii Introduction

FIGURE1 The cochlear implant The components are as follows: a, microphone; b, the-ear speech processor; c, body-worn speech processor; d, transmitting aerial; e, receiver-stimulator; f, electrode bundle; g, inner ear (cochlea); h, auditory or cochlear nerve.(Reprinted with permission from Clark, G.M 2000b Sounds from silence St Leonards,NSW, Allen & Unwin.)

behind-to be amplified for enough cells behind-to respond When most of the hair cells are absent,

no amount of amplification with a hearing aid will help the person hear speech,

as there is no hearing organ to excite the remaining hearing nerves leading to thebrain centers At best the person will hear muffled sounds These people areprofoundly deaf and were the first who stood to benefit from the bionic ear

Overall Concept of the Bionic Ear

Research that commenced at the University of Sydney in 1967 and continued atthe University of Melbourne in 1970 led to a multiple-electrode cochlear implant,which was developed industrially by Cochlear Proprietary Limited in 1982 As

illustrated in Figure 1, it consists of a directional microphone (a) that converts

sound into electrical voltages that are sent to a small speech processor worn

behind the ear (b) or a larger, more versatile one attached to a belt (c) The speech

processor filters this waveform into frequency bands The outputs of the filtersare referred to a map of the patient’s electric current thresholds and comfortablelistening levels for the individual electrodes A code is produced for the stimulusparameters (electrode site and current level) to represent the speech signal at eachinstant in time This code, together with power, is transmitted by radio waves via

a circular aerial (d) through the intact skin to the receiver-stimulator (e) implanted

in the mastoid bone The receiver-stimulator decodes the signal and produces a

pattern of electrical stimulus currents in a bundle of electrodes (f) inserted around the first turn of the inner ear (g) to stimulate the auditory nerve fibers (h) A

pattern of hearing nerve activity in response to sound is produced, and provides

a meaningful representation of speech and environmental sounds The electrodebundle, lies close to, but not attached to, the spiral ganglion cells in the inner earand their peripheral hearing nerve fibers

Training in the Use of the Bionic Ear

After recovery from the cochlear implant operation, the patient attends trainingsessions in how to understand the sensations created by electrical simulation Thefirst task is to establish thresholds and maximum comfortable levels for electricalstimulation on each electrode pair The thresholds and maximum comfortablelevels are programmed into the map of the patient’s speech processor Auditorytraining exercises involve listening to speech and repeating what is heard Thespeech material may be sentences, words, or vowels and consonants The exer-cises allow the audiologist to assess the performance of the patient and at thesame time provide training The task must not be too difficult or the patient may

be discouraged The patient is also counseled on how to use the device, for ample what to expect if the batteries become flat Later, training is given in theuse of the telephone Auditory training for children concentrates on improvingnot only their ability to perceive and understand speech and environmentalsounds, but also their speech production, receptive and expressive language, andcommunication The speech material used for the training is age appropriate Thetraining is integrated into the child’s educational program at either a preschool orschool level The children need to be taught by auditory-oral or auditory-verbalmethods to take advantage of the new auditory information they are receiving Incertain situations the use of total communication where signed English is com-bined with an auditory stimulus will be required Sign language for the deaf mayalso be used in certain children after individual assessment regarding their com-munication needs

ex-Fundamental Objections and Questions

In the 1960s and 1970s many believed that successful electrical stimulation ofthe hearing nerve to help people understand speech was not possible in the fore-seeable future A fundamental objection, which was reasonable, was that the innerear hair cells and their nerve connections were too complex and numerous toreproduce the temporal and spatial pattern of responses in the hearing nerve byelectrical stimulation with just a small number of electrodes There are some20,000 inner and outer hair cells required for normal hearing

A second objection was that a bionic ear would destroy the very hearing nerves

xxxiv Introduction

in the inner ear it was intended to stimulate For example, a Teflon strip withsharp edges can cut through the inner ear basilar membrane and lead to near-totalloss of the inner ear nerve cells in the vicinity of the injury It was also believedthat the electrode could be a pathway for middle ear infection to initiate infection

of the inner ear, which could in turn spread to the meningeal lining of the brain

A third objection was that speech was too complex to be presented to thenervous system by electrical stimulation for speech understanding

A fourth objection was that there would not be enough residual hearing nerves

in the inner ear after they died back due to deafness to transmit essential speechinformation There can be an 80% loss of the hearing nerve ganglion cells andtheir fibers after the destruction of inner ear hair cells in deafness

A fifth objection was that children born deaf would not develop appropriatenerve-to-brain cell connections, through lack of exposure to sound during theearly critical phase of development, for electrical stimulation to give adequatehearing The number of nerve connections on brain cells can be significantlyreduced when compared to that in people with normal hearing

There were other important questions: (1) Would the electrical stimulus rents damage the hearing nerves? (2) Were the candidate materials for the im-plantable electrodes and receiver/stimulator toxic to tissue? (3) Would middle earinfection spread along the electrode bundle to produce infection in the inner earwith possible life-threatening infection around the linings of the brain (menin-gitis)? (4) Could electrodes be inserted into the inner ear far enough so that thehearing nerves responsible for the place coding of speech frequencies would bestimulated? (5) What type of patients should be selected? (6) How should theoperation be performed? (7) Would the perception of pitch on a multiple-electrode

cur-or place-coding basis be possible? (8) Would the perception of pitch on a coding basis be possible? (9) What electrical currents would produce loudness?(10) Would patients have memory for sounds and speech after prolonged deaf-ness? (11) Could speech be processed so that patients could understand conver-sations? (12) Would speech and music sound natural? (13) If a speech-processingscheme was achieved for English, would it be effective in other languages?(14) How important a factor was the child’s age at implantation with regard tolearning to understand speech?

time-Answers to the Fundamental Objections

The first fundamental objection, that the inner ear hair cells and their nerve nections were too complex and numerous to reproduce the temporal and spatialpattern of responses in the hearing nerve by electrical stimulation with just a smallnumber of electrodes, was studied by determining how well electrical stimulationcould reproduce the coding of sound The temporal coding of frequency wasexamined in the experimental animal by determining how well groups of braincells could respond at increasing rates of stimulation The voltages from braincells and brainstem field potentials at increasing rates of stimulation showed the

con-electrical activity in the auditory brainstem was markedly suppressed by stimulusrates at 100 pulses/second Behavioral studies in the experimental animal showedthat rates of stimulation in excess of 200 to 600 pulses/second could not bediscriminated.

The experimental animal findings thus indicated that the reproduction of thetemporal coding of frequency by electrical stimulation with a single-electrodecochlear implant could reproduce speech frequencies only from 200 to 600 cycles/second, which is much less than the 4000 cycles/second needed for speech intel-ligibility Therefore, the best chance of helping deaf people understand speechwas to use multiple-electrode stimulation to provided more information for speechunderstanding

To achieve the place coding of frequency through multiple-electrode tion required determining where to place the electrodes in the inner ear so thatthe current would most easily pass through separate groups of hearing nerve fibersconnected to the different frequency regions of the brain Research showed thatthe compartment below the sense organ of hearing (scala tympani) and close tothe ganglion cells at the center of the inner ear spiral was the correct location.Research also demonstrated that electrical currents could be partly localized togroups of nerve fibers within the inner ear without it short-circuiting away throughfluid by pushing electrical current out one electrode and pulling it back fromanother (bipolar stimulation)

stimula-The animal experiments referred to above demonstrated that both temporal andplace frequency coding or pitch perception could be only partially reproduced byelectrical stimulation In other words, a cochlear implant is like a bottleneckbetween the world of sound and the central hearing pathways of the brain.The second fundamental objection was that if an electrode was implanted inthe inner ear, which was particularly important for multiple-electrode stimulation,

it would damage the very nerves it was intended to stimulate It was found,however, in the experimental animal, that if no excessive force was used with itsinsertion, the hearing nerves were preserved The risk of injury was reduced to aminimum if the electrode bundle had the right mechanical properties It needed

to be smooth, tapered, flexible at the tip, and stiffer toward the proximal end.Infection could be restricted from entering the inner ear if the electrode entrypoint was sealed with a graft of fascia, and care was taken to prevent infection

of the middle ear during the healing phase of the tissue over the first few weekspostoperatively

The density of the electrical charge passing through electrodes with electricalstimulation was also known to damage nerve fibers The safe limits for use with

a cochlear implant had to be tested, too It was found to be safe if the current had

a positive and negative phase to reduce the buildup of direct current (DC), andthe charge density was below approximately 32 microcoulombs per square cen-timeter per phase

The third objection, that speech was too complex to be presented to the nervoussystem by electrical stimulation for speech understanding, would have to be ad-dressed by multiple-electrode stimulation to transmit as much information as pos-

xxxvi Introduction

sible through the bottleneck This required studies on patients to determine howeffective multiple-electrode stimulation would be, as speech perception is an es-pecially human skill and could not be evaluated on the experimental animal.Studies on patients required developing a fully implantable receiver-stimulator toreceive information transmitted through the intact skin, rather than a plug andsocket, which was more likely to break and become infected

A prototype receiver-stimulator to use on patients was produced by the versity of Melbourne from 1974 to 1978 using hybrid technology that connected

Uni-a number of silicon chips together on Uni-a silicon substrUni-ate or wUni-afer The wUni-aferswere placed in a watertight or hermetically sealed container The prototypereceiver-stimulator was implanted in the first profoundly deaf adult patient onAugust 1, 1978, with the banded electrode array passing around the inner ear tolie near, but not in direct contact with, the nerves relaying speech frequency tothe brain

Perceptual studies were then undertaken on the first and subsequent patients todetermine if the findings on the temporal and place coding of frequency in theexperimental animal were applicable to humans The patient studies confirmedthat rate of stimulation was not effective in transmitting frequency or pitch in-formation over the range required for speech understanding Pitch ratios wereplotted against repetition rate, and it was shown that when the pitch of a stimuluswas compared with a reference rate of 100 pulses/s, the pitch ratios increaselinearly up to 300 pulses/s, and then reached a plateau; 300 pulses/s is much lessthan the 4000 pulses/s needed for speech understanding

The studies on the place coding of frequency showed that with localized trical stimulation the patients could perceive only timbre, not true pitch In thehigh-frequency areas of the inner ear the sensation was sharp, and on the lowerfrequency side it was dull However, the patients could rank the timbre according

elec-to the site of stimulation

The perceptual studies on the patients confirmed the findings on experimentalanimals that electrical stimulation with the cochlear implant was a bottleneck forinformation from the outside world to the central auditory pathways The firstresearch to transmit information through the bottleneck selected speech frequen-cies using fixed filters with similar properties to the tuning of the inner ear Whenthe outputs were used to stimulate the hearing nerves simultaneously, the resultwas poor Simultaneous stimulation produced overlap of currents resulting inunpredictable variations in loudness However, a speech-processing strategy wasdiscovered that gave the patients the ability to understand connected or runningspeech when presented with speech reading or even using electrical stimulationalone The clue to this speech-processing strategy came when the first patientreported vowel sounds when each electrode was stimulated on a place-codingbasis The vowels corresponded to those perceived by normal-hearing peoplewhen similar areas of the inner ear were excited by single-formant frequencies.Formants are concentrations of energy at particular frequencies or vocal tractresonances They are important for intelligibility, especially the second formant.This research led to the University of Melbourne’s inaugural speech-processing

strategy, which extracted the second formant frequency using a bank of filters.The voltages from the filters stimulated electrodes at appropriate frequency re-gions around the inner ear The stimuli were perceived as timbre The soundpressure for the formant frequency was coded as current level and perceived asloudness The fundamental or voicing was perceived as pitch This speech-processing strategy was tested at a number of centers in the United States andEurope, and in 1985 was the first multiple-electrode cochlear implant to be ap-proved by the U.S Food and Drug Administration (FDA) This inaugural speechprocessing strategy enabled patients who had hearing before going deaf to un-derstand running speech when combined with lipreading and some speech usingelectrical stimulation alone.

The research at the University of Melbourne then focused on which furtherspeech elements to extract and present on a place-coding basis It was found thatpicking the energy in the first as well as the second formant peak, and presentingthis nonsimultaneously on a place-coding basis, gave improved results Then itwas discovered that selecting energy in the high-frequency bands in the thirdformant region as well as the first and second formants (Multipeak) gave furtherimprovement The most recent strategies [spectral maxima speech processor(SPEAK) and advanced combination encoder (ACE)], implemented in the Nu-cleus 24 system, select the six to eight frequency bands with the greatest energyfrom a 16 to 20 band pass filter bank and present the information as a place code

As the strategy selects the six to eight maximal outputs from the band pass filters,the sites of stimulation within the cochlea may lie close together, leading to anoverlap in the electrical current with unpredictable variations in loudness Thishas been minimized by using a constant rate of stimulation on all electrodes Inthis case, the rate of stimulation is not used to convey voicing, but voicing isconveyed through the amplitude variations in the signal The present scores in-dicate that the average person can now communicate effectively over the tele-phone Furthermore, the scores are now better than the average scores obtained

by severely to profoundly deaf persons with some residual hearing using a hearingaid Another strategy, the continuous interleaved sampler (CIS) developed at Re-search Triangle, North Carolina, produces similarly good results

The fourth objection, that there would not be enough residual hearing nerves

in the inner ear after they died back due to deafness for understanding speech,was partly resolved by finding the good results referred to above However, theresidual hearing nerve population could have been responsible for the significantvariability in the scores The relationship between the hearing nerve populationand speech perception was studied by ranking speech perception scores versusthe cause of deafness, and the hearing nerve population versus cause of deafness.The rankings for both speech perception and hearing nerve population versuscause of deafness were different This suggested that speech perception is notstrongly related to the population of nerves or ganglion cells So die back afterdeafness is not a significant factor in performance with the present cochlear im-plant systems

The final major objection was that children might not be able to develop the

xxxviii Introduction

right nerve-to-brain cell connections for speech understanding through electricalstimulation when their brains are at a malleable stage It was therefore of criticalimportance to learn whether speech perception performance in children who hadhearing before going deaf was comparable to that for children who were borndeaf and thus had no prior exposure to sound It was important to determine inparticular whether exposure to sound during a critical period when the brainconnections are plastic would be a necessity for adequate perception or whetherappropriate connections could develop in the absence of exposure to sound Thespeech perception abilities of two groups of children (those born without exposure

to sound and those becoming deaf after exposure to sound) were compared Theirbest perception skills ranged from mere detection of sound to recognition of words

in sentences from an open set The recognition of closed and open sets of words

by children born without hearing (prelinguistically deaf) and deaf after hearing(postlinguistically deaf) improved dramatically after operation Although open-set recognition was better for the postlinguistic subjects, the performance of thechildren born deaf was sufficiently good to feel confident that prior exposure tosound was not necessary for good speech perception

Although the results showed children born deaf could develop the right to–brain cell connections for speech understanding with a cochlear implant, itwas still not clear how young they should be to get the best results For this reasonthe sentence scores for a group of children at the clinic of the Royal VictorianEye and Ear Hospital were plotted versus age at implantation There was consid-erable variability in responses, but a curve fitting showed that performance im-proved the younger the age at operation It also indicated that the scores might

nerve-be nerve-better if the operation was performed when the child was under the age of 2years

There are special safety issues to be considered when implanting children underthe age of 2: the effects of head growth; middle ear infection, which is especiallycommon at this age; and electrical stimulation on a maturing nervous system.This research was undertaken through a 5-year contract to the U.S NationalInstitutes of Health and showed no cause for concern in operating on this group

of children, provided that care was taken to seal the electrode entry point with afascial graft, and that the operation did not take place in the presence of anincipient middle ear infection

The research at the University of Melbourne and the Bionic Ear Institute hasbeen germinal to the development of the multiple-electrode bionic ear manufac-tured by Cochlear Limited The research was also fundamental in helping Co-chlear Limited to achieve the largest share of the world market in the 1980s andthrough the 1990s

Isra-Eighteenth Century

The 18th century saw considerable social change as well as the flowering ofscience This was the climate in which the first efforts to help deaf children weremade by l’Abbe´ de l’Epe´e at the Paris Deaf School (Fig 1.1) in approximately

1794, as well as Heineke in Germany L’Abbe´ de l’Epe´e developed a languagebased on a system of signs, while Heineke saw learning to speech read as thebetter way to help The need to help deaf people communicate could have beenthe reason count Alessandro Volta, an Italian physics professor, soon after de-veloping the battery, carried out on himself in the late 1790s the first experiment

on electrical stimulation of the auditory nerve His results were read on June 26,

1800, before the Royal Society meeting in London presided over by the Rt Hon.Sir Joseph Banks, who was the botanist on Captain Cook’s voyage of discovery

to Australia in 1770 The report is recorded in the Philosophical Transactions of

the Royal Society of London for the year 1800, part I, p 427: