Cocktails and hearing aids

It’s a Saturday night and you are going out. You can hear the bass beat a block before you get to the party. As you walk into the house, you are surrounded by people talking, the clinking of glasses and the ever-present music. Waving at you, the host appears and asks “What would you like to drink?”

It’s a common enough opening, and the interesting part is not the drink of choice, but how you could understand the question.

Every day, in buses, trains, crowded restaurants and open plan offices, most people easily understand other people talking. It hardly seems exciting and yet it has fascinated scientists for over half a century – ever since Cherry first described “the cocktail party problem”. The mechanisms underlying this talent are still not fully understood.

Why is it such a mystery?

The ear hears by collecting soundwaves and passing them along the ear canal, though a series of small bones, to the inner ear, where the soundwaves vibrate the basilar membrane (a tiny organ curled inside the spiral cochlear). The basilar membrane tapers along its length and different parts vibrate depending on the frequency of the sound waves. The narrow end vibrates in response to high frequencies (high pitch sounds), and the wide end to low frequencies (low pitch sounds). Hair cells pick up the vibrations and pass them along into the brain.

Since sounds are comprised of a number of frequencies (the average human voice ranges from the buzz of a mosquito to the highest note on a piccolo), all sounds sum to create the vibrations. Thus your brain knows only which frequencies are present in a room of noise, not which frequencies belong to the person you are trying to hear.

So how can we isolate just one person?

The tempo of their speech is different from that of the music, the conversation flows unlike the sporadic clink of glasses; they may have a lovely accent or a deep voice. Similarly hand gestures, facial expressions and lip movements also add to the experience. All of these speech cues can help you work out which sounds belong to the person you are listening to and which to unwanted intrusions.

Expectation is a factor. When someone is speaking about the latest football results, you can safely ignore any words and phrases relating to politics.

Having two ears also helps. Speech coming from a person on your left side will hit your left ear first and be louder in that ear than in the right ear. The brain can then use that time and volume difference to separate frequencies belonging to that person from those belonging to someone standing on your right.

But all this is done without thought. Can you consciously help?

How often have you missed a sentence because you “weren’t paying attention”? Concentrating on a person makes it easier to follow them. But when you concentrate on understanding a person, are you attending to their accent? their location? the cadence of their speech? or a mix of everything? Experiments are currently trying to work out which factors you can attend to and how this attention can make understanding easier. For example, do you increase your brain responses to speech when you are paying attention or do you just put more resources into interpreting the responses you subconsciously receive?

Who cares HOW you understand since it’s so easy?

If you don’t already, you will. As you age you will slowly lose the ability to listen to a friend in a noisy environment. This is the greatest hearing-related complaint from the aged and from hearing aid users.

Until we understand how a person with normal hearing can solve the cocktail party problem, we cannot work out what is going wrong as people age, nor can we design better hearing aids or assist those with impairments.

So next time you are offered a drink at a noisy party, spare a thought for your auditory system. Chances are its miracles are being wasted on acquiring luke-warm chardonnay.

References

This is the topic of my thesis, so to date I have well over 200 references. They would not all fit here, so I shall limit the list to the original paper that started the scientific debate:

Cherry EC (1953) Some experiments on the recognition of speech, with one and with two ears. J. Acoust. Soc. Am. 25: 975-979

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7 Comments

Filed under Curious Science, Science

7 responses to “Cocktails and hearing aids

  1. “… you can safely ignore any words and phrases relating to politics.”

    I thought most sensible people had that filter running by default – or maybe that’s only people who know me 😉

    Seriously, this is an interesting topic … does anyone have a handle on how much of the inability to distinguish between sound sources is due to deterioration of the sensory organs with age, & how much is due to neurological deterioration elsewhere?

  2. That’s all part of the mystery. The main structures which convert vibrations of the basilar membrane to electrical signals (which pass along the neurons to the brain) are the hair cells. These hair cells die over time and are not replaced.

    This is probably the cause of high frequency hearing loss – from the mid to late twenties everyone starts to lose the ability to hear high pitch sounds (hence the supersonic ringtones for teenagers). People who listen to loud music will lose this faster – perhaps the hair cells get tired of bending too much.

    However, it is not known if this natural loss of high frequencies is responsible for the problems of hearing speech in a noisy background. As the brain ages, other functions are also lost. Perhaps the aging brain cannot function as quickly to recognise and isolate a target sound. Perhaps attentional resources are less targeted or more limiting.

    Since no-one is quite sure how the brain can do any of this in normal hearing listeners, we are a long way from solving all the mysteries of aging.

    Since hearing aids at present basically amplify frequencies which a listener can’t hear as well, they can’t solve any underlying problems. Hence why people with hearing aids complain about, and often avoid situations with a lot of background noise.

  3. BTW I was speaking with people here in America about cochlear implants.

    Having no national health, profoundly deaf people often cannot afford cochlear implants. New charitable foundations have been set up to give some deaf people the opportunity. Thus there are populations who are only able to hear for the first time in their 40s and 50s.

    eg. One girl had an aunt recently fitted. She finally heard a bird but of course could not identify it. She also found the rustling of plastic bags extremely disconcerting.

    While this is a fascinating population scientifically (can the mature brain restructure to deal with a new sense?), I couldn’t think past how sad it was that these people had to wait so long 😦

  4. Liz Merceret

    I tried on a hearing aid and could bear to wear
    it for only a couple of minutes. It helped me hear
    paper crackling really well, and all I could think it
    now I have to learn how to ignore paper crackling.

    Are we sure that hair cells don’t occasionally regrow?

    After all people believed that neurons didn’t multply and that pancreatic islet cells didn’t regenerate, and both of these things were found false.

    I find gingko biloba helps, but not enough.

  5. Ed Pollitt

    I wear a fairly new & advanced hearing aid in my left ear which is about 70% deaf, due to a radical mastoid cavity operation when I was about 10 (I hope I spelt that correctly – apologies to Dr. Serfontein).

    My aid has four settings: omni-directional, forward enhanced, music (!) and of course T-loop (which is *really* great in lectures). In all these settings, the aid is designed to enhance ‘speech’ sound waves above other sounds.

    I’ve found the omni setting near useless, because when someone else besides who I’m trying to listen to, talks, even if they’re several metres away, I can hear them at the same volume. The music setting intrigues me. I can switch over to that in, say, a pub, and speech is suddenly drowned out by the music – wonderful, and very useful sometimes.

    Anyway, I guess the point of my post is just to share some interesting info about where I reckon bionic implants and cool stuff like that might lead in the future. At least, I hope so 🙂

  6. Reply to Liz

    There is some early research showing that it may be possible to regrow hair cells (working with mice) but nothing yet in humans. Ginko will almost certainly not help cells to regrow but could help you concentrate more on the signals you are getting – making things sound clearer.

  7. Reply to Ed

    It must be very restful to be able o turn off speech and listen to music!

    In terms of the settings:

    Omni-directional amplifies all sounds, forward is a directional mike enhancing only sounds from directly in front of the aid. I’m not sure of the other settings but as a guess, I think it will work on frequencies.

    Speech has most of it’s energy in frequencies between 500 Hz to 3400 Hz. Music covers a larger range. By either amplifying only those speech frequencies, or by suppressing them, I imagine the hearing aid can make speech appear out of a noisy background or disappear.

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