All ears

26 April 2011

 BIO 120 Lab Inner Ear 001
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BIO 120 Lab Inner Ear 001 by djneight, on Flickr

You’re listening to this Packed Lunch podcast, but how are the sound waves converted into what you hear? And how could understanding their amplification help those who can’t hear? UCL’s Ifat Yasin has been researching the inner ear, and Benjamin Thompson was there to hear what she said…

The final Packed Lunch event of March saw Dr Ifat Yasin, Lecturer in Auditory Anatomy and Physiology at the UCL Ear Institute, come and discuss her research and the science behind hearing to a room full of interested guests.

A new addition trialled during the talk were two large speech-to-text screens allowing those with lower levels of hearing to follow all the details. Dr Yasin talked us through her work, and explained what can go wrong within the ear and how it can lead to hearing difficulties.

Yasin described the current focus of her work. Picture yourself sitting in a sound-proofed room, in front of a computer monitor showing two lit squares. With a set of headphones on you’d have to listen carefully and press a button when you think you hear a ‘peep’ sound between two ‘shhh’ sounds.

These experiments can be very long: an hour, ten hours, 60 hours. That’s a lot of listening time. Thankfully the volunteers get lots of breaks! Yasin explained that many of the volunteers find the downtime really relaxing. These volunteers have hearing levels within normal limits – important given the subtle differences in volume and duration between the ‘peeps’ and the ‘shhh’s. By manipulating these parameters, Yasin can map out how the ear amplifies sound.

The talk moved on to discuss how amplification occurs. This happens inside an organ found within the inner ear called the cochlea. This is a snail-shell-shaped structure filled with liquid. Sound waves picked up by the ear cause the liquid to move. In turn this causes a membrane within the cochlea to vibrate. Very fine ‘hair’ cells detect these vibrations and turn the movement energy into electrical energy, sent to the brain via nerves attached to the cochlea. These hair cells only look like microscopic hair, but it turns out they’re not made of the same stuff as those on the top of your head.

Dr Yasin is investigating how the cochlea is able to act as an amplifier. Cleverly, it can amplify quieter sounds more than mid-level sounds. It turns out that low-level sounds make the membrane within the cochlea vibrate more than mid-level sounds. The response to rising noise levels is known as ‘nonlinear’ and is important for normal ear functioning.

‘Auditory illusions’ were also discussed. These aren’t quite illusions in the sense of hearing things that aren’t there, but are very useful for us day-to-day. When we’re in a noisy environment it can be difficult to pick out individual sounds. Thankfully our brains help to collate sounds of a similar pitch or loudness – helping us pick out relevant sounds within noise. This is known as the ‘cocktail party effect’.

Dr Yasin’s current round of experiments will take another three years. She hopes by understanding how a healthy ear is able to amplify sounds, the tests (when drastically reduced in length) can be used to diagnose those with cochlear problems.

Benjamin Thompson is a writer at the Wellcome Trust.