Thursday, October 29, 2009

E, N to the T

Today, I had my Clinical Skills session on Otolaryngology, popularly known as ENT (ear, nose, throat) or Head & Neck Surgery. All I have to say about this is... ENT is freaking awesome. If I were keen to become a surgeon (and I am not), I would definitely apply for this specialty. The awesomeness of my ENT experience did not include having a scope passed up my nose and down into my mouth while my throat muscles were frozen, although this did happen as well.

The thing about medicine is that a lot of the time, it feels like we're poking around in the dark. We understand some of the mechanisms of how things happen, but we don't understand why they happen. We know that blocking receptor X with illicit result Y or that region A of the brain subserves function B, but the specific details are a little foggy.

For instance, you can align the structures of a severed hand and restore some function, but you can't guarantee that even by lining up the two sides of an interrupted nerve, the individual neurons will find their original sheaths or even end up going to the right place. We can only attack things at a relatively gross level.

In general, we satisfy ourselves with our cause-and-effect knowhow because, well, we can help people with it. Still, sometimes it feels a bit crude. That is, unless you're an ENT.

Back in first-year anatomy, an ENT resident came to help us with our dissections for a few sessions. ENT, she boasted to us, requires finesse. Neurosurgery involves lopping off whole sections of the brain plus or minus. In ENT, she explained, individual nerves and highly populated regions of the body needed to be dissected with the utmost microscopic care. True? Maybe so.

Basically, what has me gushing about ENT today are two little words: cochlear implant. What is a cochlear implant? It is an ear for the deaf. So sophisticated and yet so logical is this device that if medicine boiled down just to cochlear implants, one would think that we have the human body licked - none of the fog that surrounds much of our other medical knowhow.

Here's how it works. In the normal ear, sound enters through the external ear canal and is transmitted to a spiral sea-shell shaped organ called the cochlea. On the cochlea are thousands of tiny hairs called stereocilia, and these hairs do a little dance to transmit sound to your neurons which in turn pass the sound along to your brain. The cochlea is arranged "tonotopically," which is to say it is organized in a way that the stereocilia on one end code for low pitch and the stereocilia on the other end code for high pitch. Aging, loud sounds, and a variety of diseases cause these stereocilia to die. They do not grow back, and the result is hearing loss.

In a person with moderate hearing loss, a hearing aid, which amplifies the sound may be sufficient to restore normal function. But what if all of those stereocilia are gone? What if you are cut and dry deaf? This is where the cochlear implant comes in.

The cochlear implant begins with a microphone placed near the external ear canal. The microphone collects sounds from the outside and transmits it to a miniature processor. The processor does a little math and then transmits commands to a tiny filament covered with electrodes which has been carefully installed surgically around the cochlea. The electrodes fire at different places along the length of the cochlea, essentially taking over the function of the stereocilia. Voila, an artificial ear. What was once deaf, now can hear.

I can't even begin to express to you just how absolutely amazing this device is. I mean, we are essentially hijacking the brain's neurons, bypassing the ear, and restoring a sensory modality that was previously completely lost! Nowhere else in the body can I think of an instance where our understanding of function is so sophisticated that neurons can be externally stimulated to elicit a complex function. Sure, we install pacemakers to hearts, but these do little more than beat to a rhythm. Nowhere else in the body can we restore a sensation that has been completely eradicated - not sight, not taste, not touch, not smell. Freaking awesome. I think I'm in love.

Now, the cochlear implant does have it's drawbacks - it's not quite as sophisticated as what you were born with. While your native ear had thousands of stereocilia, the cochlear implant maxes out at somewhere in the range of 32 channels. Amazingly enough, this is sufficient for the brain to interpret everyday speech at more or less the same level as normal. Where the implant falters is in the appreciation of music (which tends to sound like noise, a significant quality of life issue), localization (figuring out where a sounds is coming from), and discriminating sounds amongst a high level of background noise.

But hearing is believing. Below, I've included a demo of what it sounds like to hear through a cochlear implant. A particular phrase is repeated but heard as though you were listening through 1, 2, 4, 8, 16, and 32 channels in sequence. The final iteration presents the original sound.


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Once you've listened to the file once, go back and listen to it again. The brain is a highly adaptable organ, and you'll likely find that you can understand the phrase at an even lower number of channels than you could originally.

Below I've included another sound file which demonstrates what it sounds like to hear music through a cochlear implant. Again, the musical excerpt plays as through 4, 8, 16 and 32 channels before the original sound is revealed. You'll see that at 4 and 8 channels, the music sounds something like an airplane taking off. Even at 32, it doesn't come close to approximating the actual melody.


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Oh dear, what a science rush.

3 comments:

Michael said...

Rhapsody in Blue!! <3

Also, while the work is still very preliminary, similar things can be done by projecting things onto the retinal fields or the primary visual cortex... nearly all of our senses (smell and procioception being two particular distinctions) are mapped topographically, after all.
This is an example of such a 'bionic eye'

While both those are cool, I think the future is in creating new senses for us, rather than being limited by what we're born with. For example, we know that many other animals have an innate sense of magnetic north. Projects like Northpaw try to duplicate this sense... from what I've read on it, you do eventually develop a new 'awareness' of direction.

Michael said...

Err I meant 'exceptions', not 'distinctions'.

English fail FTL.

Michael said...

Also... holy freaking shiat this is scary awesome:
finger magnet implants