EVER wondered why listening to loud music or singing at the top of your voice is such fun? Scientists at the University of Manchester say a pleasure-inducing hearing mechanism we've inherited from our fishy ancestors could be to blame.

A team led by psychologist Neil Todd, an expert in music perception, has discovered that the sacculus, an organ forming part of the balance-regulating vestibular system in our inner ear, is tuned in to respond to sound frequencies that predominate in music--despite the fact that the sacculus is not thought to have any hearing function. Even more curious, says Todd, our saccular frequency sensitivity appears to mimic that of fish--the only type of creature known to use its sacculus for hearing. "This primitive hearing mechanism from our vertebrate ancestors appears to have been conserved as a vestigial sense in humans," says Todd.

Because the vestibular system has a connection to the hypothalamus, the part of the brain responsible for drives like hunger, sex and hedonistic responses, Todd believes that people might be getting a pleasurable buzz when they listen to music--which could explain why music has developed into such a cultural force. This buzz may mimic the thrills people get from swings and bungee jumping, where motion stimulates the balance centre.

But there is a proviso: the sacculus only appears to be sensitive to loud volumes--above 90 decibels. Despite this, crooners could also love their own singing because sound levels in the larynx have been estimated to be as high as 130 decibels. "It's bloody loud in there," Todd says.

Because the sacculus is buried deep within the ear (see Diagram), Todd and his colleagues Frederick Cody and Jon Banks could not measure its reaction to sound directly. But in regulating balance--particularly head posture--the sacculus evokes electrical signals in certain neck muscles. So the team exploited this by asking students to tense their neck muscles, and using surface electrodes, they measured the extra signals produced when the sacculus responds to sound rather than balance.
 
In tests, 11 students listened to tone pips of varying frequencies. Their saccular sensitivity ranged from 50 hertz up to 1000 hertz, with a peak between 300 and 350 hertz. On the musical scale, middle C is 261 hertz; male and female voices have frequency ranges up to 200 and 400 hertz respectively. The researchers will publish their results this spring in a forthcoming edition of the journal Hearing Research.

"The distribution of frequencies that are typical in rock concerts and at dance clubs almost seem designed to stimulate the sacculus. They are absolutely smack bang in this range of sensitivity," Todd says. Large groups of people singing or chanting together, such as a choir or a crowd at a sporting event, could also trigger the mechanism, he adds.

Paul Marks

From New Scientist magazine, 19 February 2000.