Engineers investigate why the cochlea is coiled

Jun 14, 2012 by Lisa Zyga feature
Simulated beam patterns of straight and spiral cochleas: While the straight cochlea generates beam patterns without any vertical variation, the spiral cochlea generates beam patterns that vertically localize the sound source. Image credit: Xun Huang, et al. ©2012 EPLA

(Phys.org) -- The word “cochlea” comes from the Latin for “snail shell.” While this inner ear component has a clear spiral shape, it’s currently unclear why that is. In the 1980s, scientists supposed that the shape evolved solely for space conservation, but in the past few years researchers have questioned this view and proposed that the spiral curvature could have perceptual benefits as well. In a recent study, a team of engineers has added evidence to this view by performing simulations showing that a coiled cochlea can detect the source of a sound in the vertical direction significantly better than a straight cochlea.

The researchers, Xun Huang, Chi Xu, and Long Bai, at Peking University in Beijing, China, have published their study on the coiled ’s ability to provide vertical sound localization in a recent issue of EPL.

The idea that the cochlea’s spiral shape has perceptual benefits is somewhat surprising, since currently most cochlear models used in research simplify the cochlea as a straight system. This simplification seems justified because numerical simulations have shown that a straight cochlea and coiled cochlea have a similar mechanical oscillation amplitude, which describe the vibrations that get sent to the brain. However, recent research has suggested that there may be significant differences in the way that straight and coiled cochlea respond to sound.

“One previous paper stated that the spiral’s graded curvature enhances the cochlea’s mechanical response to low frequencies,” Huang told Phys.org. “Our work is the first one in sound localization. Basically, most previous works focus on ‘low level’ functions, such as mechanical response. We show that sound localization may be one of the potential functions of the cochlea’s coiled geometry, which was assumed solely developed for saving space.”

As all individuals with good hearing can attest, the ears can detect the source of a sound in both the horizontal and vertical planes. Horizontally, a sound source produces a time and volume difference between the left and right ears because the ears have different horizontal positions. But since the left and right ears are positioned at the same height on the head, a sound source in the sagittal plane (the vertical plane that divides the body into left and right halves) produces no difference between the two ears based on the source’s vertical position. Instead, two parts of the ear called the pinna and tragus help mammals achieve vertical sound localization.

However, recent experiments with the brown bat (Eptesicus fuscus) have suggested the existence of an alternative mechanism for vertical sound localization. After researchers disabled the function of the bat’s tragus, its vertical sound localization capability was weakened, but only temporarily. After a period of adaptation, the capability was partially restored due to an alternative mechanism.

As the researchers show here, one potential alternative way of achieving vertical sound localization could be the coiled shape of the cochlea. To begin, they proposed that the human hearing system might adopt a method similar to adaptive beamforming, a technique used for directional signal processing, to process sound signals for the brain. In their simulations, the researchers modeled the thousands of hair cells in the 3.5-cm-long cochlea as a sensor array in the context of beamforming. Then they compared how this sensor array perceived sound pressure when arranged in a straight vs. spiral geometry.

Their results revealed that straight and spiral cochleas generate different beam patterns, i.e., acoustic images of the sound sources. For all sound sources, the straight cochlea generates a beam pattern that spans the entire vertical range, providing no vertical sound localization. In contrast, the coiled cochlea generates different and much narrower beam patterns for different sound sources, providing significantly improved vertical sound localization. For horizontal sound localization, the straight and coiled cochleas show comparable performance.

The finding that vertical sound localization can be improved purely by geometric changes supports the argument that the cochlea’s coiled shape is useful not just for conserving space. The results could be helpful for designing cochlear implants and echolocation systems, in which sound waves are used to detect objects.

“From the very beginning, we were actually studying bat flight control based on sound echolocation,” Huang said. “As you can see, vertical sound localization is very important for bat flight. However, the biological information of a bat’s cochlea is not so accessible. Hence, we used the human cochlea as a test case.”

Huang added that they are currently designing experiments in an anechoic chamber to test the results of these simulations.

Explore further: Serial time-encoded amplified microscopy for ultrafast imaging based on multi-wavelength laser

More information: Xun Huang, et al. “Is the cochlea coiled to provide sound localization?” EPL, 98 (2012) 58002. DOI: 10.1209/0295-5075/98/58002

4.6 /5 (16 votes)

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Miah28
2.3 / 5 (3) Jun 14, 2012
Just off the top of my head I imagine its coiled due to the physics of audible vibrations, as the sounds (vibrations) bounce (echo) off the walls traveling from the larg entrance to the smaller center. The (echo) bounce expends energy and the echo loses power as it travels down the spiral, the collapse of the spiral compensates for the loss of energy required to continue the echo. Just a guess I may be full of crap...
Terriva
1 / 5 (2) Jun 14, 2012
IMO the saving of place for long resonator is the sufficient reason for to have cochlea coiled.
MrVibrating
3 / 5 (2) Jun 14, 2012
Fascinating finding, and makes sense.

Incidentally, it's just occurred to me that a narrowing spiral also helps to significantly amplify the power of the waves as they reach the terminus, as they trade progressivley more wavelength for amplitude. Such simple mechanical amplification is of course already known to be employed via the ossicles (the hammer, anvil and stirrup bones form a lever which likewise provide a passive signal boost)... so perhaps spirals are also more efficient, as well as more functional...

Edit: oops, props to Miah28 for beating me to it...
MrVibrating
not rated yet Jun 14, 2012
IMO the http://watermarke...ound.jpg for long resonator is the sufficient reason for to have cochlea coiled.
..also a very good reason..!
Skepticus
3 / 5 (4) Jun 14, 2012
It'd be fun to hear how the Creationists explain this hearing mechanism. Am I hearing a deafening silence?
A2G
2 / 5 (4) Jun 14, 2012
Skepticus...there weren't any religious comments until you wrote yours. You want people to just discuss science here, then why don't you just do it?
verkle
1.4 / 5 (9) Jun 15, 2012
Hi Skepticus,

It is so interesting to see scientists behold God's creation and ask these kind of questions. "Why did God make us this way?" "Why did God make our ear coiled?" "Why are we here seemingly alone in the midst of this vast universe?" These are very good questions, and should be asked.

The more we seek, the more we will find out how wonderfully we have been created. Chance and time don't make things beautiful.

I think you are familiar with the exclamations that David made 3000 years ago.

"Many, O Lord my God, are thy wonderful works which thou hast done." (Psalm 40:5)

and

"I will praise thee; for I am fearfully and wonderfully made: marvellous are thy works; and that my soul knoweth right well." (Psalm 139:14)

Thanks for asking.

roboferret
5 / 5 (3) Jun 15, 2012
He also said
"Happy those who seize your children and smash them against a rock."
So you shouldn't pay much attention to the ramblings of a bronze age tribal warlord.

Here's a quote for you -
"Beauty, like supreme dominion
Is but supported by opinion"

Benjamin Franklin.
TkClick
not rated yet Jun 15, 2012
..also a very good reason..!
Nevertheless it was downwoted by verkle and average_guy already. But the trivial reasons tend to be most reliable. The idea, that the spiral cochlea maintains upright position can be tested easily with comparison of its orientation for animals, which are moving in vertical or horizontal direction. I'd expect no significant dependence of the cochlea orientation to the posture, which would invalidate this theory.

This is how the science is supposed to work: until we test our theory by REAL examples, it remains a hypothesis - no matter how perfect simulations and computing we actually did and it has no meaning to spend a money for future experiments.

But it's difficult to explain it to people, who are getting employed just from organization of another and another experiments and simulations.
TkClick
not rated yet Jun 15, 2012
In addition, the people who were learned the complex math and way of thinking often refuse to see the things simply and clearly - it's their professional bias, which is typical for all scientists and so-called experts. The stupid but complex idea counts more than the clever but simple explanation for these people, because it provides better justification of their existence and qualification. A relevant reading: Quantum gravitist Carlo Rovelli: Science is not about certainty. Science is about finding the most reliable way of thinking.
MrVibrating
not rated yet Jun 15, 2012
@TKClick yes i had the same reservations with regards to orientation - but i remain open to the possibility it may work as described in any plane.. dunno...

Come to think of it, another backup method of fixing height may arise from delays sensed by other parts of the ear not normally associated with audition, such as the saccules or vestibular apparatus, which nonetheless do resolve some sound, and are situated above the cochlea. Then there's the possibility of sound carried through the body and heard 'internally' such as by bone conduction.. this could provide yet another way to reference vertical delays.. maybe...
Skepticus
3 / 5 (2) Jun 16, 2012
Hi Skepticus,

It is so interesting to see scientists behold God's creation and ask these kind of questions. "Why did God make us this way?"....These are very good questions, and should be asked.

Thanks for your opinion. But I have no proof that A Being On High Made me (and the rest of 3/4 of humanity) this way. . Show me your proof that did not come from a book, thank you.