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Bionic ear could harness brain's 'octopus cells' to improve sound

Hearing tech People who use cochlear implants may one day be able to better understand speech against a noisy background thanks to a new Australian discovery.

Biomedical engineer Professor David Grayden, of the University of Melbourne, and colleagues, have identified how the brain uses neurones called 'octopus cells' to pick up the unique rhythm of someone's speech.

The findings are published in a recent issue of PLOS ONE.

One of the biggest challenges for people who use a cochlear implant (also called a bionic ear) is picking out particular speech in the presence of background noise, especially in a room full of other people talking.

"The worst situation is what's called the 'cocktail party situation'," says Grayden.

Current cochlear implants deal with this problem by using directional microphones pointing to the front of the head.

Even then, however, the target speech needs to be 10 to 15 decibels louder than the background sounds in order to be heard properly, says Grayden.

Grayden and colleagues figured that if they could work out how the brain manages this challenge, they could pave the way for improving the performance of bionic ears.

Computer model

Building on previous research, the researchers created a computer model of key parts of the brain's auditory system and looked at what happened when it processed speech-like sounds and random background noise.

The model included 'octopus-shaped' neurones that previous research had found are good at detecting speech.

In particular, they respond to the series of sound onsets, called "glottal pulses", which are the loudest part of speech, created by puffs of air passing through our vocal cords.

The pattern of these pulses creates a unique signal that a normal hearing system can identify as being speech.

"The key is that the 'rhythms' are different for different voices," says Grayden.

"The glottal pulses for different speakers will rarely occur at the same time, and the rate (frequency) of these pulses will be different (people speak with different pitches)."

But the questions is how exactly normal hearing people use these subtle cues to pick out one person's speech compared to another.

In their computer model, Grayden and colleagues found that a part of the brain called the 'ventral nucleus of the lateral lemniscus' (VNLL), integrates signals from octopus cells with other auditory neurones that respond to the volume of each frequency but don't distinguish at all between speech and noise.

"The brain uses the actual timing of glottal pulses as a cue to decide the most important time to listen," says Grayden.

The researchers tested their model using a 4000 Hertz sound, which was best heard when the VNLL played this role.

"When we ran it through the VNLL model this helped the 4000 Hertz sound really stood out," says Grayden.

This is the first time that this particular mechanism used by the brain to identify speech rhythms has been revealed, he says.

It explains how the brain is able to distinguish speech against background noise that is at the same volume.

Grayden says the findings could one day help refine the bionic ear speech processor.

And this could make cocktail parties a more pleasant experience for those wearing the devices.