In the study published in this week's Nature, Dr David McAlpine and Nicol Harper asked volunteers to wander the streets of London wearing microphones in their ears. The microphones measured the time difference between sound arriving at each ear for a range of noises that people typically encounter in the city.
While it was already known that animals and humans use small differences in the arrival time of sound at each ear to locate its source, the UCL study found that the human brain adopts a strategy similar to a barn owl's brain for sound pitches above middle-C, and a gerbil's below middle-C.
David McAlpine says: "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."
For over 50 years a single model has been used to explain how brain cells represent the time difference between the ears. The 'classic' model assumes that specific brain cells are allocated to specific time differences, where the relevant cells fire depending on which direction a sound is coming from.
Because different animals need to detect sounds relevant to their own environment, their brain cells shift their tuning until they code most accurately for sounds the animal is likely to encounter.
Recordings from the brain of barn owls – a species that hunts at night using only sound – appear to confirm this. However, the classic model could not account for recent evidence that the brain cells of small mammals appear to respond most to time differences that the animal is never likely to hear.
The alternative model, developed by Nicol Harper in Dr McAlpine's lab, explains this anomaly. Small mammals such as gerbils or guinea pigs can follow low-pitched sounds. Surprisingly, to enhance this ability at low frequencies, the brain cells organise to respond most to time differences outside the range the animal naturally encounters.
This strategy does not suit higher frequencies i.e. higher-pitched sounds. Thus, barn owls' brains follow the classic model of brain cells firing most for time differences within the animal's range. Human brains appear to 'pick and chose' from the different strategies, depending on sound frequency.
Dr 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.
Notes for Editors
Ten things you might not know about your ears and hearing
1. The outer ear, known as the pinna, helps determine the position of a sound source. It distinguishes whether a sound is coming from above or below or from in front or behind.
2. The three ossicles - the bones of each middle ear that transmit sound from your eardrum – are the smallest bones in the body.
3. The inner ear consists of the spiral-shaped hearing organ – the cochlea – and the semi-circular canals required for the sense of balance.
4. A thin membrane called the basilar membrane runs along the length of the cochlea. Sound waves of different frequencies (pitches) cause different parts of the basilar membrane to vibrate.
5. The cochlea contains thousands of tiny 'hair cells' sitting on the basilar membrane that respond to the vibrations generated by sound waves.
6.Loud noise, certain anti-cancer drugs and anti-biotics damage the hair cells making them lose their 'hairs'. This is irreversible. Most deafness is associated with loss of hair cells.
7. If the average nightclub noise was played in a factory, it would be illegal to make workers listen to it for longer than 3 minutes.
8. The human brain can detect differences in the arrival time of a sound at the two ears of about 10 millionths of a second. This is 100 times shorter than the electrical impulses that transmit information in the brain.
9. Two ears are better than one. In noisy environments, the brain is able to compare the sound at the two ears and cancel some of the noise, making speech easier to hear. People who are deaf in one ear are doubly disadvantaged at a noisy party.
10. Tinnitus – ringing in the ears - is one of the most common forms of hearing problem. The cause of, and the cure for, tinnitus are yet to be discovered.
About University College London
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About the Medical Research Council
MRC is a national organisation funded by the UK tax-payer. Its business is medical research aimed at improving human health; everyone stands to benefit from the outputs. The research it supports and the scientists it trains meet the needs of the health services, the pharmaceutical and other health-related industries and the academic world. MRC has funded work which has led to some of the most significant discoveries and achievements in medicine in the UK. About half of the MRC's expenditure of £430 million is invested in its 40 Institutes, Units and Centres. The remaining half goes in the form of grant support and training awards to individuals and teams in universities and medical schools.
Journal
Nature