Hearing without your ears ---- how it works

From: North County Times - Escondido,CA,USA - Apr 24, 2004

By: BRADLEY J. FIKES - Staff Writer

Into the water you go. The gurgling fills your ears. Then, with a flick of a finger, classical music to suit your underwater ballet. Or, hear the voice of your swimming coach, as clear as if on land.

It's the Aqua Sphere FM Radio Snorkel, from Vista-based Aqua Sphere. Just as joggers can carry an iPod to hear their favorite tunes, the Radio Snorkel is aimed at water-lovers who now practice their sport without auditory enhancement.

The device works in a way familiar to anyone who has ever put a tuning fork to their head: Sound vibrations conduct to the inner ear through the bone, bypassing the ear, eardrum and the inner ear bones. The Radio Snorkel transmits sound through the mouthpiece. Bite on it, and seemingly out of nowhere, you hear music ---- the vibrations that conduct the sound are imperceptible.

The principle of bone conduction has been known for a long time, but is now being increasingly applied in devices such as the Radio Snorkel.

Toward the end of last year, Sanyo announced that it would start selling in Japan a cell phone that works through bone conduction, the TS41, cheekily called the "Bone Phone." It's supposed to be most useful in areas with loud noise, where air conduction of sound is impractical.

Bone conduction is also a reminder that hearing, which seems a straightforward process, is actually extremely complicated, a seemingly random jumble of components that (in most cases) work so well that we rarely think about it.

Tracing it back

Hearing, as with all other sensations, is perceived in the brain, carried by nerves that transmit sensory signals from elsewhere in the body. Specific regions of the brain in both hemispheres perform the job of somehow placing these signals into our consciousness. At any step along the way, a malfunction can leave a person partially or wholly deaf.

Here is the process of hearing in brief, as detailed by researchers in the field:

The nerve that carries the electrical signals created by sound to the brain is logically enough called the auditory nerve. If the nerve is severed, people lose their hearing.

The auditory nerve runs from the brain to various kinds of "hair cells," which connect at their root to nerve fibers that lead to the auditory nerve. The hair cells originate the signals that the auditory nerve carries to the brain. At the top, these hair cells have minute hairs called sterocilia. When these sterocilia are compressed by vibrations, they trigger the cells to produce electrical signals. If the hair cells are damaged, such as by loud noise, hearing loss results.

The hair cells are embedded in the cochlea, a fluid-filled organ with spirals resembling that of a snail shell. This marks the beginning of what is called the inner ear.

The cochlea is constructed in an extremely complex manner, with inner chambers, canals and partition that direct pressure vibrations to the appropriate hair cells, which are sensitive to vibrations of differing pitch and amplitude. Damage to the cochlea impairs hearing.

Juncture point

The cochlea is the connecting point for sound transmitted through the air and sound transmitted through bone, said Ed Givelberg, an associate scientist in the department of electrical engineering and computer science at UC Berkeley.

"The cochlea is situated in a cavity in the temporal bone, which is the hardest bone in your body," Givelberg said. "The temporal bone conducts vibrations very well."

You don't even have to try the tuning fork on the skull trick to experience bone conduction, Givelberg said. It happens every time you talk, because you hear the sound of your voice largely through bone conduction.

Givelberg has studied the cochlea's structure closely. He has created a mathematical model of its geometry. However, when asked for specifics, Givelberg figuratively threw his hands up in the air, saying the mathematical details are too complicated for an easy summary.

In a general way, Givelberg said, the cochlea functions through interactions of its fluid-filled interior, elastic tissues inside the fluid and the hair cells.

Completing the journey, the cochlea hooks up with a tiny bone called the stapes, which links with another tiny bone called the incus, which connects with another called the malleus. The malleus contacts the eardrum, which vibrates in sympathy with the air pressure of sound.

These three bones are part of the middle ear, and that outside the eardrum is considered the outer ear.

Evolutionary perspective

Another way of looking at hearing is to examine how it arose in our vertebrate ancestors. Research indicates that the ability to hear sound through the air was added on to an existing vibration-detection system that first appeared in fish.

Modern-day fish actually have two ways of detecting vibration. One is a sensory organ called the lateral line that runs along the length of the fish's body. The second is a variation of the inner ear, less complex than the cochlea, but with a similar function. This function has become progressively more complex and sensitive in reptiles and mammals.

In fish, small bones called otholiths are moved by water vibrations, in turn moving hair cells that emit electrical signals picked up by the fish brain. The vibrations are originally picked up by bones in the fish's skull, which is more dense than the surrounding water.

Certain fish refine this system by connecting their inner ear with an air-filled swim bladder, which gives them greater sensitivity to vibrations. That is because the difference in density is greater between water and air than between water and bone.

Moving to land, some reptiles have added an outer ear and eardrum to the inner ear, along with a recognizable cochlea. Others, primarily burrowing reptiles and snakes, pick up vibrations through their body. Snakes are particularly good at this, because they have vibration receptors in their skin.

Boning up

In mammals, the inner ear structure changes in a most unusual way, as documented in fossil records of the transition from reptiles to mammals. Reptiles use just one bone, the stapes, to transmit sound from the eardrum to the inner ear. Mammals use three, which are linked together to magnify the vibrations, thus increasing sensitivity to sound.

Where did the two extra middle ear bones found in mammals come from? The jaws of their reptilian ancestors, according to researchers who have found a series of forms with intermediate shaped jaws.

The late scientist Stephen Jay Gould described the change this way in his book "Dinosaur in a Haystack":

"Only one bone, called the dentary, builds the mammalian jaw, while reptiles retain several small bones in the rear portion of the jaw. We can trace, through a lovely sequence of intermediates, the reduction of these small reptilian bones, and their eventual disappearance or exclusion from the jaw, including the remarkable passage of the reptilian articulation bones into the mammalian inner ear (where they became our malleus and incus, or hammer and anvil)."

Most astonishingly, Gould wrote, this transitional series includes a species with two jaw joints, corresponding to both those found in reptiles and mammals. Since only one point of articulation is needed, there is no mechanical obstacle to allowing the old reptilian joint to become unhinged and the superfluous bones moved up the jaw into the inner ear.

When it goes wrong

For us modern-day mammals, hearing through the air has become our primary method of detecting sound, because it is so sensitive. For example, our two ears allow us to determine the direction of a sound. That is because sound waves usually don't hit both eardrums at the same time; the minute lag allows the brain to calculate the direction.

Just as humans are the masters of thinking, bats are the most skilled at using sound. Bats can fly in total darkness by echolocation, "illuminating" their path by detecting the echoes of their ultrasonic squeals. This acuity is so great that many species of bats catch insects through echolocation.

But complexity has its disadvantages, namely that there are more things that can go wrong along the circuitous path from a sound's origin to its perception. Damage to the auditory nerve, the hair cells, the cochlea, the middle ear bones or other subsidiary structures can cause differing degrees of deafness.

For humans, some forms of partial deafness can be alleviated with hearing aids, which not only amplify sound, but electronically process it so speech can be more clearly perceived. But if the hair cells are destroyed, hearing aids are of no use.

Cochlear implants, called by some a "bionic ear" bypass, convert sounds into electrical impulses that are delivered to nerve endings in the cochlea. Hearing is not as good as natural hearing, because the implant only uses a few electrodes, and there are thousands of specialized hair cells.

More information is available at the Bionic Ear Institute (www.bionicear.org).

But when the auditory nerve connections or the nerve itself is destroyed, no amount of cochlear bionics will help. The only possible treatment is to somehow turn sound into electrical impulses that the brain can interpret, and deliver them straight to the brain. That is what the Los Angeles-based House Ear Clinic in Los Angeles announced in January.

Two patients were given an implant of what the House Ear Clinic calls a Penetrating Electrode Auditory Brainstem Implant, or PABI. This device uses cochlear implant technology, modified to deliver its electrical signals to a part of the brainstem called the cochlear nucleus. It has restored limited hearing in the patients.

And cycling back to the origins of hearing, the Scripps Research Institute in La Jolla recently announced that it had discovered the function of a gene implicated in deafness. The gene, named cadherin 23, makes "tip link" proteins found on the hair cells, said Ulrich Mueller, an associate professor in the Department of Cell Biology at the institute.

A parallel study by Teresa Nicholson and colleagues at the Oregon Hearing Research Canter and Vollum Institute validated the findings of Mueller's team. The study found that when the cadherin 23 gene was deleted, tip links never form. The study was performed on zebra fish.

The research teams' results were published this month in the scientific journal Nature.

Contact staff writer Bradley J. Fikes at (760) 739-6641 or bfikes@nctimes.com.

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