Hearing, or audition, is one of the traditional five senses and refers to the ability to detect sound.In human beings, hearing is performed by the ears, which also perform the function of balance, a sense in itself but not one of the traditional list (due to Aristotle). This is in common with most mammals. Many other organisms also have some form of hearing, either by some sort of ear, or by other structures, or by a combination.

A common rule of thumb used to describe human hearing is that human hearing is sensitive in the range of frequency of 20 Hz to 20 kHz, though this varies significantly with age, occupational hearing damage, and gender; some individuals are able to hear up to 22 kHz and perhaps beyond, while others are limited to about 16 kHz. Frequencies capable of being heard by humans are called audio or referred to as sonic. Frequencies higher than audio are referred to as ultrasonic, while frequencies below audio are referred to as infrasonic.

Some organisms are able to hear ultrasound and/or infrasound. Some bats use ultrasound for echo location while in flight. Dogs are able to hear ultrasound, which is the principle of 'silent' dog whistles. Snakes sense infrasound through their bellies, and there is evidence that whales and elephants may use it for communication. The hearing can be tested using a device or computer program called audiometer.

Continued

Scientists Identify Molecular Cause For One Form Of Deafness Science Daily - February 6, 2007

Human Ears Evolved from Ancient Fish Gills Live Science - January 20, 2006

More than 28 million Americans have some degree of hearing loss Newsweek - June 6 Issue

New bionic ear uses smart plastic News in Science - April 2005

Hearing Loss Tied To Heart Disease Science Daily - March 2005

Cool ears give the brain away News in Science - March 2005
The temperature of your inner ear provides a new way of understanding whether you're using your left or right brain

Research Identifies Promising Route For Treating Age-Related Hearing Loss Science Daily - January 2005

Mechanical ear makes its debut PhysicsWeb - January 2005

Scientists 'find key to hearing' BBC - October 2004

Deaf people are making a profound contribution to the study of language Economist.com - March 2004

Just as biologists rarely see a new species arise, linguists rarely see a new language being born. You have to be in the right place at the right time, which usually you are not. But the past few decades have seen an exception. Linguists have been able to follow the formation of a new language in Nicaragua. The catch is that it is not a spoken language but, rather, a sign language which arose spontaneously in deaf children. Ann Senghas, of Columbia University, in New York, one of the linguists who has been studying this language, told the AAAS conference in Seattle about her discoveries. And Susan Goldin-Meadow, of the University of Chicago, who studies the spontaneous emergence of signing in deaf children, filled in the background by showing how such children use hand signals in a different way from everybody else.

The thing that makes language different from other means of communication is that it is made of units that can be combined in different orders to create different meanings. In a spoken language these units are words. In a sign language these units are gestures. Dr Senghas has been studying the way those gestures have evolved in Nicaraguan Sign Language (NSL).

The language emerged in the late 1970s, at a new school for deaf children. Initially the children were instructed by teachers who could hear. No one taught them how to sign; they simply worked it out for themselves. By conducting experiments on people who attended the school at various points in its history, Dr Senghas has shown how NSL has become more sophisticated over time. For example, concepts that an older signer uses a single sign for, such as rolling and falling, have been unpacked into separate signs by youngsters.

Early users, too, did not develop a way of distinguishing left from right. Dr Senghas showed this by asking signers of different ages to converse about a set of photographs that each could see. One signer had to pick a photograph and describe it. The other had to guess which photograph was being described.

When all the photographs contained the same elements, merely arranged differently, older people, who had learned the early form of the language, could neither signal which photo they meant, nor understand the signals of their younger partners. Nor could their younger partners teach them the signs that indicate left and right. The older people clearly understood the concept of left and right, they just could not converse about it‹a result that bears on the vexing question of how much language merely reflects the way the brain thinks about the world, and how much it actually shapes such thinking.

For a sign language to emerge spontaneously, though, deaf children must have some inherent tendency to tie gestures to meaning. Spoken language, of course, is frequently accompanied by gestures. But, as a young researcher, Dr Goldin-Meadow suspected that deaf children use gestures differently from those who can hear. In a 30-year-long project carried out on deaf children in America and Taiwan, whose parents can hear normally, she has shown that this is true.

Even deaf children who have no deaf acquaintances use signs as words. The order the signs come in is important. It is also different from the order of words in either English or Chinese. But it is the same, for a given set of signs and meanings, in both America and Taiwan.

Curiously enough, the signs produced by children in Spain and Turkey, whom Dr Goldin-Meadow is also studying, while similar to each other, differ from those that American and Taiwanese children produce. Dr Goldin-Meadow is not certain why that is. However, the key commonality is that their spontaneously created languages resemble fully-formed languages.

That result, if confirmed in other studies, could have profound implications. Another much-argued question in linguistics is whether there is some sort of grammatical template that acts as part of a language instinct and is wired into the brains of new-born children. Such a template would help a child to learn a language quickly. But the different grammars of actual languages suggest it cannot, assuming it does exist, be imposing itself too strongly. However, the sign languages of Dr Goldin-Meadow's children are entirely self-invented. If there is a deep structure, they are surely drawing on it directly. The order of their signs may thus be a direct reflection of what that structure is. For now, Dr Goldin-Meadow is cautious. But it may turn out that the truth comes not out of the mouths of babes and sucklings, but from their hands.

Ear cells offer hearing promise BBC - October 2003

New Hearing Test Simulates Noise Of Real World Science Daily - May 2002
Binaural hearing - developmental and learning disabilities

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