How do we hear and what is hearing impairment?

14 October 2012

Interview with

Owen Brimijoin, MRC Institute of Hearing Research, Glasgow

Chris -   First up, how do we actually hear?  What's happening in our ears and in our brains to give us a sense of sound and why are so many - in fact, 1 in 6 people in the UK - affected by problems with hearing?  With us to discuss these issues is Dr. Owen Brimijoin from the MRC Institute of Hearing Research in Glasgow.  Hello, Owen.

Owen -   Hello, hello.

Chris -   So, tell us a bit then about how the ear actually works.  What is it doing?

A left human earOwen -   Right.  Well, you have to consider that sound consists of these waves of pressure that are radiating outwards from any vibrating object.  So you need a way to sense those changes in ear pressure over time.  Deep inside our ears, inside a structure known as the cochlea, we have these little hair cells.  We have ones that like to respond to low frequency sounds, slow vibrations, and we have ones that respond better to high frequency sounds, and entire range in-between.  So, these little hair cells are dancing back and forth, and if you've got a complex sound like speech for example, it's vibrating in lots of different places at once.  And the brain is paying attention to what parts of the cochlea are most active.  And it's that pattern of activation across that allows you to detect your mother's voice and tell it from other people's voices.

Chris -   So the cochlea in the inner part of the ear is quite literally converting sound waves coming in from the outside world into brainwaves.  It's producing neurological information that's fired off into the brain that we then decode with higher brain processing into speech and music, and that kind of thing.

Owen -   Right and it's kept compartmentalised in the very beginning into low frequencies and high frequencies.  And so, the brain already has a lot of its task accomplished.

Chris -   So, when someone loses their hearing, what actually has gone wrong because people who complain of being hard of hearing often said they can hear some sounds, but not others.

Owen -   Right.  Well, one of the problems that the ear has is that the cochlea is full of fluid and as anyone has ever tried to shout at someone who's under the water, say, in a swimming pool, you know intuitively that sound doesn't like to travel from air into water.  So, we have these structures where the eardrum and the middle ear bones whose purpose is to kind of convert these sound pressure changes so that they can effectively vibrate the fluids of the cochlea.  Now that can go wrong.  You can have earwax on your eardrum that prevents it from vibrating properly.  Maybe you've got a middle ear infection and that prevents those little bones from vibrating properly.  But most devastating and distressingly common the death or destruction of these little hair cells themselves and some of that is attributable to wear and tear, some to noise damage.  There's any number of reasons why these die off and they don't come back.  Once they're gone, they're gone.  And so, the brain doesn't have any sensors in that area and can't detect those sounds.

Chris -   So some sounds, you'll become less sensitive to because you've lost these hair cells in regions of the cochlea that correspond to those particular sound frequencies, but you could equally have other bits of the cochlea that would decode other sound frequencies that would be working normally.  So you have gaps in your ability to hear certain types of sound.

Owen -   That's right and if you think about it, these hair cells have been banging away back and forth in response to sounds, sometimes a thousand of times every second since you were born.  So, it's perhaps no surprise that when people lose their hearing, it tends to be the high pitch sounds, the high frequency sounds that they lose first.

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