Cochlear Implants

The cochlear implant is the most significant technical advance in the treatment of hearing impairment since the development of the hearing aid around the turn of the century.

Designed to restore some sense of hearing for:

• Children or adults who receive little or no benefit from hearing aids.

• Loss must be: (a) profound, (b) bilateral, and (c) sensorineural (sensory but not neural is more accurate).

• Problem: Auditory nerve is intact, but hair cell transducers are not functioning.

Purpose of implant: Generate electrical signals that do the job that is normally carried out by the damaged hair cells.

Normal Ear

Deafness

Normally functioning hair cells damaged or not present; there is some atrophy of 8th N dendrites in most cases.Dorman (2004)

Transmitting Coil

Electrode Array

Receiving Coil

Note: There is also a speech processor, which is not shown in this picture.

The speech processor uses a bank of bandpass filters (or a Fourier analyzer) to separate the signal into some number of frequency bands (e.g., 22) before passing it along to the array of electrodes.

What part of the auditory system is the speech processor replacing?

Speech Processor

Speech Processor

(Note: CI users may actually have two eyes.)

Dorman (2004)

Several speech processor designs are in use. This figure shows a simplified version of a common processor design.

https://www.boystownhospital.org/patientServices/Pages/CochlearImplantCenter.aspx

http://medimoon.com/2014/05/children-with-cochlear-implants-at-risk-for-deficits-in-executive-function/

http://infanthearing.vihsp.org.au/following-diagnosis/cochlear-implants

http://advancingyourhealth.org/highlights/2014/08/27/cochlear-implants-hearing-loss/

CI user with a diagram showing (roughly) what things look like on the other side of the skull.

http://www.pddcs.co.uk/2012/07/what-does-cochlear-implant-look-like-in.html

 NIH Publication No. 11-4798 (2013-11-01). "Cochlear Implants". National Institute on Deafness and Other Communication Disorders. "According to the Food and Drug Administration (FDA), as of December 2012, approximately 324,200 people worldwide have received implants. In the United States, roughly 58,000 adults and 38,000 children have received them."

Brief Digression

1. In the U.S. the largest population of CI users consist of:

a. children

b. Adults

2. Most deaf adults are:

a. prelingually deaf

b. postlingually deaf

This figure shows which electrodes are receiving the most electrical current for some sample speech sounds for a 22-electrode implant.

(Note that electrode 1 stimulates the basal end and electrode 22 stimulates the apical end.)

Fre

qu

ency

→

apex →

base →

voicing →

F1&F2 →

F2&F3

↓

F1

↓

Notice that these electrode stimulation patterns resemble spectrograms, with two exceptions: (1) only a single time slice for each sound is shown (these patterns will extend for 100 ms

or more in connected speech) , and (2) with only 22 channels, the frequency resolution is not nearly as good as you would see in a spectrogram.

Fre

qu

ency

→

apex →

base →

voicing →

F1&F2 →

F2&F3

↓

F1

↓

1 ch

2 ch

3 ch

4 ch

6 ch

8 ch

music

(orig)

• Implant based primarily on place theory. If we make the most generous assumption that place theory is correct (it is probably incomplete), how many stimulating electrodes should there be?

• The CI restores some sense of hearing, but it is nowhere near the hearing sensation that is produced in a normal ear. (There are no “bionic parts” that work as well as the original.)

• Subjects vary wildly in the amount of benefit they derive from a CI -- mostly for unknown reasons.

GOOD NEWS, BAD NEWS

First the Bad News

Despite these drawbacks, CIs work. Research shows:

• Ability to understand speech improves.

• Children acquire speech and language skills more quickly.

• Post-lingually deafened adults maintain their speech production skills better – especially control of pitch and loudness, which improves immediately most of the time.

Now the Good News

Addendum: How the Implant Works: The Standard Story

This explanation obviously requires that 8th N dendrites be present. Are they?

A postmortem study * of 16 temporal bones from CI patients showed that some had very few dendrites and most had none. None.

*Linthicum FH, Fayad J, Otto SR, Galey FR, House WF (1991). "Cochlear Implant Histopathology," Am J Otol 12 No 4 1991 245-311.

How do you get tonotopic organization out of a system like this in which the full bundle of 8th N cell bodies is stimulated all at once rather than in a location-specific way?