A simple change to the design of 'bionic ear' implants dramatically improves the quality of sound they provide, say researchers in the US who have tested a prototype on cats.

Around 100,000 people around the world have been fitted with conventional cochlear implants. These are built into the skull and tune into the signal produced by a wireless external microphone. The implant sits below the skin behind the ear and sends electrical signals to a set of electrodes coiled into the cochlea – the spiral organ in the ear that senses sound.

This process is far from perfect, however. These implants have a limited range of tones because it is difficult to insert the electrode array beyond the outer turns of the cochlea. The outer turns pick up the high frequencies, so people using today's implants are sensitive only to the highest tones.

In addition, the signals must travel through liquid in the cochlea and then through the organ’s bony wall to the nerves outside. "It is like talking to someone through a closed door – the signal gets muffled," says John Middlebrooks of the University of Michigan, US, who developed the new implant with Russell Snyder of the University of California, San Francisco, also US.

Increased range

The new device bypasses the cochlea and instead connects directly to the nerves that carry information to the brain.

Experiments in cats that compared the brain's response to the established designs with its response to the prototype show the new device dramatically improves the range of tones that can be heard.

"Current implant users do very well with speech in a quiet environment but struggle with background noise," Middlebrooks explains, "they also have very poor pitch perception and cannot appreciate music – in an environment like a crowded room, you use pitch to tune into a person's voice."

Experiments on 10 cats involved first recording the response of the brain to a range of tones. The cats were then deafened and the same tones were played with a conventional, and then experimental, implant installed.

Fully implantable?

Conventional implants only allow detection of tones as low as around 7 kHz, but the new implant allowed frequencies as low as 0.6kHz to be detected.

"We saw that the brain could detect a much greater range of frequencies," says Middlebrooks.

The new design also produced a cleaner/more specific response in the brain to a particular frequency, and used much less power to get results. "If we develop this for humans it would have much lower power demands, so it could be much smaller and perhaps fully implanted," says Middlebrooks.

Tests in which animals are fitted with the implants for periods of months are now planned.

"Cochlear implants themselves have provided a revolution," says Brian Lamb, director of the Royal National Institute for the Deaf. "These implants – if successfully transferred to people – could offer further, major benefits."

Journal reference: Journal of the Association for Research in Otolaryngology (DOI: 10.1007/s10162-007-0070-2)