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December 9, 2002

Upgrade brings Mozart and Chinese to the deaf

From: ABC Online - 09 Dec 2002

The profoundly deaf may soon be able to understand the subtleties of tonal languages like Thai and Chinese, and even appreciate a Mozart concert, thanks to new software developed for cochlear hearing implants.

The new cochlear technology was developed by Dr Robert Fearn of the University of New South Wales in Sydney, as part of a doctoral thesis.

"I wanted to use my engineering skills to try and help people more directly," Fearn told ABC Science Online. "I didn't just want to be some number-crunching engineer."

People who are profoundly deaf can currently hear speech using a conventional cochlear implant, a miniaturised hearing aid that was originally developed by Melbourne researchers and is now used by more than 30,000 people around the world.

"Now people can hear speech, they say they want to hear music," said Fearn, who has a background in electrical engineering and has played trumpet and guitar from a young age.

The problem with conventional implants is they cannot distinguish between the tonal subtleties necessary for appreciating music or understanding languages in which tonal variation can change the meaning of words.

For example, the Cantonese word 'fu' has six different meanings depending on its intonation: it can mean 'husband', 'tiger', 'trousers', 'to hold', 'woman' or 'father'. The inability to distinguish such different meanings could result in social disaster.

Fearn likened wearing a conventional implant to "having a piano with 22 keys" instead of the 88 used - making it difficult to play or hear "Für Elise",for example.

An implant consists of an array of miniature electrodes surgically inserted into the cochlear, an area of the internal ear where hearing is conducted. These electrodes carry out the function of hair cells which normally stimulate auditory nerves in response to sound, but which do not function in the profoundly deaf.

Sound collected by an external microphone and a signal processor is converted to electrical impulses which are then delivered by radio link to the cochlear. Depending on which frequency band the sound falls into, particular electrodes will stimulate particular nerves, enabling the wearer to hear high or low tones.

A conventional implant only has 22 frequency bands - which is why pitch discrimination is so poor and the device is not suitable for music or tonal speech.

Rather than increasing the number of 'keys' on the cochlear implant's 'piano', Fearn just changed the way they are played. He wanted to stimulate the nerves in a way that is more akin to the way a normal ear works.

In a standard implant, electrodes are stimulated at a constant rate per second. However, in a normal ear, nerves can be stimulated at different rates. Not only does the pitch you hear depend on the placement of the nerve that is stimulated, but it is also affected by the rate at which that nerve is stimulated.

As part of his research, Fearn discovered that the stimulation of nerves in different locations of the cochlear could give the same pitch, as long as the nerves were stimulated at different rates. Not only this, but different combinations of the nerve being stimulated and the rate of stimulation could give a different quality of sound for the implant wearer.

In this way, Fearn's study of people wearing cochlear implants has not only developed more sensitive implants - it has contributed to the fundamental understanding of how the human ear works.

By stimulating specific nerves at specific rates to an implant wearer, and getting their subjective feedback, Fearn was not only able to increase the number of tones they heard but also to optimise the quality of the sound.

Fear estimates that it will be a year before there is a commercially available signal processor that will be able to process data fast enough to cope with his new software. Meanwhile trials of the prototype are set to continue in Sydney and in Iowa City, USA.

"I've had some great comments and feedback … some people didn't want to give the processor back at the end of the trial," Fearn said. "One thing about the cochlear community is they keep pushing. It's not like the technology driving the people, it's the people driving the technology."

Fearn's research was jointly funded by the Australian Research Council and Sydney-based Cochlear Ltd, which holds rights to the patents. He was recently awarded the Young Investors Award by the Hong Kong-based Far Eastern Economic Review magazine.

Anna Salleh - ABC Science Online
© 2002 ABC