bionicdeepa


Cochlear Implant - What's that then?

It is an electrical device that stimulates the auditory nerve directly inside the ear.

This nerve carries the impulse to the brain for processing. In a normal ear, sound waves vibrate the eardrum that in turn vibrates the cochlear via those tiny bones that you can't remember the name of, except maybe the anvil. The cochlear is the thing that looks a bit like a snail shell but it is made of cartilage. When it vibrates, it sends a ripple around the spiral of the cochlear. The lower the frequency of sound the further around the spiral the wave travels. Along the way, tiny hairs are shaken by the wave at different intervals around the spiral and it is these hairs that are attached to the nerve. As they shake, the nerve is stimulated and our clever brain can understand what it all means.

The cochlear implant system is really several different components. The actual implanted bit is a receiver and wire with 16 electrodes that curl inside the cochlear itself. The receiver is recessed into the skull just behind the ear during surgery and then the skin is stitched closed again over the top. It is only fitted to one ear due to cost ($70,000) but there have been some bi-lateral trials.

 

 

On the outside, Deepa will wear a processor and microphone that looks like a hearing aid. The mic picks up sound and the processor filters it into 16 frequency channels before passing it to the internal electrode via an external transmitter that magnetically locates over the internal receiver. It transmits through the skin - no plugs. The external parts are easily removable returning Deepa to peace and quiet land whenever she feels like a break from the hearing world. Like every night about 11:00pm. This gives time for the batteries to recharge ready for the morning.

 

So how much better will it be compared to the hearing aids?

Most deafness is caused by the tiny hairs in the cochlear being either deformed or missing. Although 99% of the ear is working fine, the stumpy hairs of a deaf person just don't have the sensitivity to pass on the fine detail of information to the nerve. They are especially useless with higher frequencies. But it does not mean that Deepa hears nothing now. If I shout into her ear, she does hear a faint sound but according to her tests, she does not hear anything quieter than 90db. That is like one of those jack hammers at close range.

Hearing aids help Deepa a lot by amplifying all sounds by 90db. So in a quiet room, she would hear me typing on this keyboard or clicking my fingers, etc. This surprises most hearing people and is quite confusing at first.

If she can hear quiet sounds like that, then what is the problem?

The thing that a hearing aid can not do is make the hairs respond to high frequencies. They literally do not exist for Deepa. If you were lying in a bath with your head underwater and someone was shouting at you from downstairs, then this is probably close to what Deepa hears with her hearing aid. It is just good enough to tell the difference between a motorbike passing outside and a dog barking etc. but you will never understand speech.

Last night, I did an audio test with Deepa. I progressively removed the high frequencies of an audio track until we found the point where she noticed a difference, then I put just enough back so that they both sounded the same to her. Listen to the bad one first several times and see if you can understand the words.

Bad = Deepa with Hearing Aid

Good = Normal hearing

To Deepa, there is no difference between these two tracks. Anyone who has spoken to Deepa can probably understand her after a little practice but it is still obvious that she has a deaf voice. I thought it was interesting that in the bad recording you probably did not recognize Deepa's voice as being different from the others. Also, when you listen to the bad sound track, did you recognize that it was music at the end? Maybe you got it by the rhythm but notice the complete absence of the high frequency guitar and symbols. All that is left is a bass player with his Marshall amp turned up to eleven.

What about music? Will the implant open up an amazing new musical world? Well maybe, maybe not. Deepa currently enjoys music a lot which again is a surprise to many. Although she can't hear the high frequencies, she still gets the emotional content of music and can easily describe the mood of what is playing. Although one time when I was playing Charlie Parker in the car, she did ask to put music on instead of talk radio. Her favorite type of music is something with strong vocals like Frank Sinatra. Here is a simulation of what Deepa might be hearing with her hearing aids but I know this is not very accurate as she definitely hears more of the vocal than this.

Deepa's hearing aid - quality

normal hearing - quality

And here are some sound samples from a Cochlear Implant program aimed at speech. Listen to the 16 channel first to see if you can recognize them. Which would you prefer - Hearing aid or Implant?

16 channel
16 channel
Normal
Normal

And just for fun what if Deepa wore her hearing aid in the left ear and the implant in the right? I don't know if the pitch would match from implant to hearing aid but if it did, it might sound like this. Wear headphones for the proper effect.

Implant + Hearing Aid Mix
Implant + Hearing Aid Mix

You might also think about it like this picture below. The 'normal' disk represents a full range of sound from low to high frequencies at the same volume. If we imagine that blue is low frequency and yellow/orange contains the most useful information for understanding speech, then Deepa with her hearing aid can only get sounds in the blue range and cannot understand the words. If the implant's 16 channels were programmed to pick up sound across the widest range, it might look like the third disk which would be better, although still a bit steppy, for music with its use of all frequencies but the processors are normally programmed for the speech range so you get the most useful detail to understand words but sacrifice the other sounds. But what if Deepa combines both hearing aid and CI? If the hearing aid is doing well with the low frequencies that would allow the CI to concentrate on the higher frequecies. And more channels in less range would mean more detail.

 

That symbol sound in the first example is very similar to the high frequency sounds we use in speech. Unfortunately for Deepa, it is this range that really gives the understanding to spoken words. When we whisper, we are only using the high frequency components of speech but there is almost no loss of comprehension. Switch it the other way around and you will start loving foreign films for their subtitles.

If any of you were smart enough to understand the words, then remember you have the advantage of a very high resolution database of speech already in your head from years of perfect hearing. It is much easier to recognize a bad quality copy of something you already know. Recognize these?

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You can roll over the image with the mouse to see if you were right. You probably got the first one but blew it on the second even though both pictures are equally degraded and each only use 16 colors. This is all because of familiarity and previous exposure to the high quality original.

So what about the cochlear implant then? In a way, the current technology is very similar to the degraded pictures above. Using the image analogy again, I think Deepa's current hearing with hearing aids might be like the image on the left below, so the implant will be a big improvement but obviously a long way from natural hearing. The main difference is that for the first time Deepa will hear high frequencies and if her brain can adapt after 34 years, then she has a chance to understand speech.

...

The fact that the implant only has 16 electrodes stimulating the nerve compared to about 30,000 fine hairs in a normal working cochlear explains why Deepa is not going to suddenly understand speech the moment it is switched on (which will be January 9th, 2003). She does not have the previous experience of hearing perfect quality sound and so she has no reference for the low quality sound coming from the implant. When a person goes deaf later in life and then has the implant, they are much better at understanding the new sound just like you probably recognized the Mona Lisa. Also, if children have the implant very young (1 - 2 years old) they adapt exceptionally well as the brain is still forming and understanding of the world. This amazing technology really does allow some of them to live as a hearing people.

How would you get on? Here are some simulations of cochlear implant sound. Remember Deepa's implant should be better with 16 channels...

4 channel
4 channel
8 channel
8 channel
8 channel
12 Channel
   

 

I wish they had recorded "I am a Dalek - exterminate." But that would be childish.

For Deepa it will be a long slow process dealing with the new stimulation before it all starts to make sense. I imagine and hope that in 5 years time Deepa will be able to have simple conversations on the phone with people she knows very well. Follow the progress on her diary page.