Sunday, January 3, 2010

Do CIs Provide Normal Hearing?

Warning: another technical explanation ahead.

Many parents of deaf children--especially those who choose to have the cochlear implant (CI)--are under the impression that CIs will provide normal hearing. Although the CI gives profoundly deaf children much more access to sound than hearing aids ever could, they simply can not replace normal hearing.

Here is a (very basic) science lesson of how normal hearing works (photos and descriptions provided by Cochlear Corporation):

1. Sounds enter the ear canal and travel to the eardrum.

2. These sound waves cause the eardrum to vibrate, setting the bones in the middle ear into motion.

3. This motion is converted into electric impulses by tiny hair cells inside the inner ear (cochlea).

4. These impulses are sent to the brain, where they are perceived by the listener as sound.


And here is how the cochlear implant creates sound:


1. The external sound processor captures sounds, then filters and processes the sounds.

2. The sound processor translates the filtered sounds into digital information, which is then transmitted to the internal implant.

3. The internal implant converts the digital information into electrical signals, and sends them to a tiny, delicate curl of electrodes that sits gently inside the cochlea.

4. The electrical signals from the electrodes stimulate the hearing nerve, bypassing the damaged cells that cause hearing loss, allowing the brain to perceive sound.


This curl of electrodes has a maximum of 22 channels to replace the millions of channels for a normal, functioning cochlea.

Here's a link that I thought was helpful in demonstrating the difference:
http://www.hei.org/research/aip/audiodemos.htm
However, you should listen with skepticism because even though this is an accurate representation of what kinds of sounds the implant is capable of creating, we can never know what happens between the implant and the brain. The brain--especially that of a child as young as Brynn--can do remarkable things with the limited information a CI can give. And most deaf adults who lost their hearing late in life report hearing speech mostly the same as before the implant.

Music, however, has mixed reviews. The last audio demonstration on this site is one of Matt's and my absolute favorite songs--but after listening to it in CI-mode, we can fully understand why many CI recipients report not liking music as well as before they were deafened. It seemed down right scary to us even at the highest channel available!

It's still amazing, however, that many children, when implanted early, report really enjoying music. I know of a CI recipient, for example, that plays the violin, another one that plays the trumpet, another one that can sing on-key (most of the time), and several more that can play the piano! Apparently they can learn to sing along with lyrics to popular songs just as well as their hearing peers.

So how is the implant inferior, if it's possible that the brain really can gather and interpret more information than the CI is actually giving?

Well, there are a few limiting factors:

First, the range of sound that a normal person can hear is between 0-120 dB. So, in layman's terms, this is between a leaf falling and a jet engine. The CI, however, can only deliver between a 70-80 dB range of sound. This means, that in order to focus on speech sounds, some of the quieter sounds must be sacrificed and the louder sounds are processed as if they were the same loudness as other things much quieter. So, again in layman's terms, this means a CI recipient is not going to hear faint whispers; and that a jet engine will sound only as loud as a diesel truck.

Because the louder sounds are thus "squished" into the same dB (loudness) range, it becomes very difficult for CI recipients to distinguish between these sounds. A characteristic environment that is very difficult for CI recipients is a noisy restaurant. So many sounds get squished together that deciphering the voice of the person who is speaking becomes very difficult.

Second, the human cochlea has millions of tiny hairs that move in accordance with the waves of sound coming in the ear (see above diagram). These hairs not only help us hear everything from whispers to jet engines, but they also help us distinguish between an oboe playing a middle C at 30 dB and a piano playing a middle C at 30 dB. Same loudness, same frequency, different tone quality. As far as I can tell, the CI doesn't really have any way of providing different tone qualities. So everything should just sound like middle C at 30 dB. Even still, many CI recipients can still decipher the difference between an oboe and a piano, so go figure. Although music is obviously still experienced differently for CI users than for individuals with normal hearing, this still demonstrates the remarkable fact that the brain is one extraordinary instrument--

And some people don't believe in an Intelligent Designer.

I digress.

One interesting story--we met a guy who became deaf in his 20s. Now in his grey-haired years, he was telling us about the first time they turned the implant on. He said the first hour was pure torture. He couldn't decipher anything that was said and everything sounded like static. He worried that he had made the biggest mistake of his life. But a few hours after that, he had a business phone call and he said he didn't miss a thing.

Other late-deafened adults report that everyone sounds either like Alvin and the Chipmunks or a scary horror film for the first few weeks. But then, things get more normal. The CI is the same as it was the day it was turned on. But the brain interprets the signals differently.

Anyway, I hope you can tell by now that the brain has more to do with the success of a CI than the CI itself. The CI is imperfect at best, completely unusable at worst--and it all depends on the brain.

The brain of Brynn, for example, is so malleable due to her age that the CI has remarkable odds of giving her a near-normal hearing life. The brain of a late-deafened adult has a history of hearing naturally and can quickly adjust to the CI's inferior signals.

The brain of a deaf adult, however, with no prior history of hearing, can not make any sense at all of the signals. Some even report dizzy spells, head pain, vertigo, or even the taste of iron in their mouths. Although the deaf adults who wear their implants for a long time say that these strange side-effects eventually disappear, their ability to process speech sounds never develops like it would for Brynn.

So to anyone considering a cochlear implant for their child: do it early or don't do it at all.

4 comments:

Alyson said...

Other late-deafened adults report that everyone sounds either like Alvin and the Chipmunks or a scary horror film for the first few weeks.

The latter is definitely how it sounded for me!

Matt + Natalie said...

Alyson??? I didn't know you had a CI! I know a little of Ben's story--and that he doesn't wear his hearing aids anymore--but I had no idea you had a CI! When were you deafened?

anna w. said...

we will be praying for you all! especially little Brynn. Do you mind me asking, how long until you can start knowing the results of the implant?

Matt + Natalie said...

Anna, your questions has many answers. But the most straightforward would be likely when we turn the implant on, which is scheduled for Feb 11th. If she can hear sound at that time--and lets us know by giving us some definitive reaction--then we can be confident that she is at least hearing with her CI. Now, how she will grow in her abilities to process that sound, and subsequently learn to speak, is a whole separate ball of wax and really only years upon years will tell us that. Hope that answers your question.