Re-Engineering Hearing:Introduction to Cochlear Implant Design

Dr. Ryan Clement

04/30/08

GE 106: Freshman Engineering 3

Class Overview

• Introduction to Cochlear Implants – Anatomy and Physiology of Hearing– Speech and Sound (Frequency Spectrum)– Cochlear Implant Design Principles

• Brainstorming design session– New design concept for a cochlear electrode array

• Ethics discussion exercise– The unexpected response from the deaf community

Key Concepts

• Sound waves to neural impulses

• Sounds and speech basics– Frequency Spectrum– Spectrograms

• Electrical Stimulation of Cochlea

Normal Hearing- Anatomy

External ear

Middle ear

Inner ear(cochlea)

How Do We Normally Hear?

Sound enters ear…Ear drum

the eardrum and middle ear bones vibrate…

which creates a fluid wave in the cochlea…

Transduced to neural impulses for brain to interpret

The Cochlea: Sound to Neural Impulses

http://www.iurc.montp.inserm.fr/cric/audition/

Cross-section of Cochlea

http://www.bcm.edu/oto/research/cochlea/Hearing/

Transverse Sound Wave

Hair Cells

Auditory Nerves

To B

rain

Sound and FrequencyP

ress

ure

TimeHigh Frequency

Low Frequency

Tones and the Cochlea

High frequency

Low frequency

Frequency Spectrum of Tones

Pow

er/M

agni

tude

Frequency

Power SpectrumGives an indication of what frequencies are present in complex signals

Time Domain

Frequency Domain

Summed signal

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-2

-1

0

1

2

+

Complex Sound SpectrumComplex sounds can be decomposed into individual frequency components…

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-2

-1

0

1

2

0 500 1000 1500 2000 2500 3000 3500 40000

100

200

300

400

500

Signal vs. Time

Frequency Spectrum

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

200Hz

400Hz

800Hz

1600Hz

3200Hz

200Hz400Hz800Hz1600Hz 3200Hz

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

0 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.01-1

-0.5

0

0.5

1

Speech- Frequency Spectrum of Vowels

a e i

o u

6 kHz0 kHzFrequency

Pow

er

Spectrograms“Discover your true north”

Power spectrum plotted vs. time

1.5 2.0 2.5

Time (seconds)

10k

1k

100Fre

quen

cy (

Hz)0 0.5 1 1.5 2 2.5 3 3.5

x 104

-0.2

-0.1

0

0.1

0.2

Speech signal vs. time

Di- -s- -c o v- er y- our t- r- ue N- o r th

The Cochlea is a Frequency Analyzer!

Un-coiled Cochlea

LowFreq

HiFreq

Sensorineural Hearing Loss

• The loss of inner hair cells = deafness

• Causes– Heredity– Genetic– Aging process– Ototoxic drugs– Excessive exposure

to loud sounds

Microscopic view of hair cells on basilar membrane.

Outer Hair Cells

Inner HairCells

Electric Activation with Cochlear Implant

Hair Cells

Auditory Nerves

(The University of Melbourne)

Cochlear Implant FactsCochlear Implant FactsQuick Facts:

• candidacy: severe-to-profound sensorineural deafness

• mechanism: electrical stimulation bypasses impaired cochlea

• ~100,000 recipients worldwide (~21,000 in the U.S.) *

• 50% children (12 mo-17 years); 50% adults *

• Manufacturers:Cochlear Corporation: NucleusTM

Advanced Bionics: ClarionTM

Med-EL: Combi-40+TM

AllHear: AllHearTM single channelAntwerp Bionic Systems: LauraTM

(now owned by Cochlear)MXM Laboratories: DigisonicTM

* FDA survey of venders 11/2001 http://www.nidcd.nih.gov/health/pubs_hb/coch.htm#c

Cochlear Corporation: NucleusTM

FDAapproved

Cochlear Electrode Array

CochlearElectrode

Cochlea

AuditoryNerve

Cochlear Corporation’s Nucleus Electrode

What Do Cochlear Implants Sound Like?

1 Channel

4 Channel

8 Channel

2 Channel

base ap

ex

Auditory neuron cell bodies

Electrode 1 Electrode 2

Activated CellsActivated Cells

Independent Activation!

Electrode 1 Electrode 2

Channel interaction

Incr

ease

d D

ista

nce

Design Problem

10 Minute Design Session!

New Electrode Concept

• Break into groups of 4-5• Create a new electrode

design that places the electrodes closer to the neuron cell bodies

• Try to think of 2 strategies• Sketch the basic ideas• Discuss the pros/cons of

each

electrodeCell body

Actual Solutions- Advanced Bionics

Electrode Positioner*

*been attributed to 9 menengitis-related deaths and is no longer used

Actual Solutions- Cochlear Corporation

The Nucleus Contour features a self-curling electrode array

A stylet keeps the array straight during insertion, and then is removed by the surgeon

The array then hugs the center of the cochlea (where the auditory neurons are)

COCHLEAR IMPLANT ETHICS

Deaf Community

Two communities of the deaf:

- signers (Gallaudet, American Sign Language)

- vocalists (Central Inst for the Deaf, Alex. G. Bell)

History of device controversy

• if children are taught “signing” from early on, they develop “easy, facile” communication with other signers

•vocalists are taught to use whatever residual communication they have, augmented by hearing aids – large, bulky oral speech never “perfect”, communication-limited

Cochlear Implant (Prosthesis)Pre- and Post-lingual deafness

•adults who have developed language skills prior to CI use learn to use the prosthesis more easily

•some infants, even with extensive training never learn to use the implant to communicate

•adult-onset deafness, difficult to learn sign-language, not controversial to employ CI

Anti-cochlear implant arguments – for children

•dangerous surgery, possible infections, weakens skull

•just another hearing aid providing limited communication

•vocalist children “mainstreamed” often frustrated & angry

•superior social development of “signers” w/ signing family

Ethical Dilemmas- Cochlear Implants

Imagine that you and your spouse are deaf. Your child is born without hearing. Would you go forward with the implant? What are all the issues you would consider?

Would your reasoning change if you both could hear?

Speech Processing

Performance Results

0

10

20

30

40

50

60

70

80

90

100

Med El CIS-Link Nucleus 22 Clarion v1.2

Scores on NU6/CNC words

Nucleus 24 ClarionHi-Focus

C II Hi Res.

Per

cen

t C

orr

ect

Dorman 2002

Factors Effecting Cochlear Implant Performance

• Duration of deafness• Age of onset of deafness• Age at implantation• Duration of cochlear implant use• Emersion in hearing world• Other:

– Number of remaining auditory nerve fibers– Electrode placement and insertion depth– Dynamic range

• Commitment!! (family and individual)