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Last Updated: Wednesday, 4 August, 2004, 23:02 GMT 00:02 UK
Hearing more complex than thought
Humans may adopt different strategies
Human hearing may be more complex than previously thought, say researchers.

It has widely been thought that humans pinpoint the location of sounds in a way similar to barn owls.

But an analysis, published in Nature, by University College London scientists suggests this only applies to the tracking of high frequency sounds.

For lower frequencies the UCL team believes that humans use a different system also used by small mammals like gerbils and guinea pigs.

For animals and humans, locating the source of a sound can mean the difference between life and death.
Dr David McAlpine
The UCL team hope their work could lead to more sophisticated hearing aids, and better ways to track sound sources in noisy environments.

Animals and humans use small differences in the arrival time of sound at each ear to locate its source.

In fact, the human brain can detect differences in the arrival time of a sound at the two ears of about 10 millionths of a second.

Barn owl theory

For decades it has been assumed that human hearing was wired up in the same way as barn owls - a species that has been widely studied since the 1970s.

Barn owl
Barn owls are expert at tracing high-pitched sounds
Barn owls are believed to have an array of brain cells, or neurons, each of which fire in response to different time difference intervals.

The birds are able to locate the source of a sound precisely depending on which neurons have fired.

This is a very highly tuned system, which works well because high-pitched sounds of the sort encountered by the birds have a short wave length.

However, it cannot work so well for low-pitched sounds of the sort encountered by small mammals because they have a much longer wavelength.

The UCL team carried out tests on small mammals, and found that these animals employ a different system.

Their neurons are not so finely tuned and can be activated at different levels by a range of sounds.

The animals are then able to pinpoint the location of the sound by measuring gradients of activity across different populations of neurons.

Human tests

The UCL team carried out an analysis of sounds typically encountered by humans in a city environment.

Gerbils have a less finely tuned system
The results suggest it is unlikely that humans rely on one system or the other.

Rather, the scientists believe human brains are able to 'pick and choose' from the different strategies, depending on sound frequency.

The UCL team believes that the human brain adopts a strategy similar to a barn owl's brain for sound pitches above G above middle-C.

But for lower frequency pitches - below middle-C - it is more likely to use the system employed by small mammals.

Researcher Dr David McAlpine hopes his findings will help engineers to develop technology to a similar standard to the human brain.

Current sound tracking devices work well in quiet places, but suffer considerably in the sort of noisy environments in which humans have little trouble in following a conversation.

Dr McAlpine said: "For animals and humans, locating the source of a sound can mean the difference between life and death, such as escaping a pursuer or crossing a busy street.

"Our study suggests that the brain adopts an efficient strategy for doing this, adapting to different frequencies, or pitches, of sound.

"Knowing how the brain creates a sense of sound space is the first step to recreating spatial hearing in the deaf.

"Recent advances in cochlear implants allow people to have implants in both ears, with the potential to restore spatial hearing."

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