Why Current In-the-Ear Hearing Aids Fail

Hearing aids have improved greatly over recent years, but they continue to be a surprisingly frustrating experience for new wearers.

Clearly, today’s hearing aids are tiny, nearly invisible in fact, and they amplify sound and are able to present a higher range of frequencies, but they have not yet completely solved the problem of amplifying the peripheral sounds we just don’t want, or don't need to hear.

For new wearers the crumpling of a paper bag on the other side of a room can sound like a jackhammer.

This is a huge challenge for technology because it is dependent on how the brain perceives sound and how we have learned to filter peripheral sound out of normal hearing. Andrew J. Oxenham is a psychologist and hearing expert at the University of Minnesota and an expert in psychoacoustics.

Oxenham explains: The ear works by analysing sound and breaking it into different frequencies and with many forms of hearing impairment it’s this frequency selectivity that is impaired.

What that means is that the ear doesn’t filter as well as it did before. So instead of having very sharp tuning to filter out different frequencies the filtering becomes much broader and there is no real way of compensating for that.

You can’t sharpen the filters or you can’t pre-process sound so it’s sharp. It’s like a broken TV set. You can process the signal going into the TV as much as you like but you still won’t get a clear picture of the output.

Recent hearing aids have made a lot of progress, like being able to present frequencies of up to 6000 Hz as opposed to limited frequencies up to about 4000 Hz, by using digital signal processing, and a lot more computing power on a lot smaller chip.

Another big leap forward has been made with directional hearing. They can focus the microphones toward the front and filter out a lot of the sound coming from the side and back. And although that is a fairly simple technique, it involves signal processing that wasn’t possible with earlier hearing aids.

Ambient or peripheral sound is horribly distracting for hearing aid wearers. A paper bag being crumpled across a room sounds screechingly loud.

This is common complaint of people who recently start wearing a hearing aid. Their hearing has deteriorated, often without them being completely aware of it, over a period of time.

When they are suddenly fitted with a hearing aid, they hear sounds they’ve got used to not hearing. The sounds are suddenly annoying and distracting. It’s a contrast effect.

It’s more to do with perception i.e the brain’s ability to analyse and prioritise different sounds.

It’s a complex interaction between the ear and the brain. The ear sends signals up to the brain; the brain does an awful lot of processing on top of that; then sends signals back down to the ear. These signals change the way the ear accepts input.

This is partly why hearing aids are not perfect because the hearing aid is not part of that natural feedback loop. There’s no way with current aids that the brain can interface with a hearing aid directly to change its characteristics.

Hearing Loops
To deal with background noise there are things called “hearing loops.”

These are systems that are set up within places like concert halls and churches that interface directly with the hearing aid. It’s like sending a radio signal to the hearing device.

The idea is that this hearing loop picks up the sound directly from the microphone in front of a speaker.

If you are in a conference and the speaker is talking into a microphone. Normally we hear the sound acoustically through the airwaves.

If you are wearing a regular hearing aid the microphone will pick up the sounds on the airwaves but that is together with all the background noise and reverberation in the room.

With a hearing loop it sends the signal directly from the microphone to the ear and bypasses all the acoustics in the building itself. So the ear is getting a much better, clearer and cleaner signal of what’s coming into the microphone.

Two hearing aids better than One?
It’s only recently that people have routinely been fitted with two hearing aids. Often people only got one.

Directional hearing and the way we localise sound: To know where the sound is coming from the brain compares the signals coming into the two ears. So if it’s slightly louder on one side then the brain knows the sound is coming from that side.

More importantly it’s the time of arrival difference between the two ears. If you think about a sound coming from the right. The sound will reach your right ear a little bit before it reaches your left ear.

Although we are talking about millionths of seconds, your brain needs two ears to make a distinction. If you only have one you lose that ability to localise sound and tell which direction it is coming from.

It’s also an important part of filtering out sound and noise. The brain can determine if there is speech right in front and background noise in back of and to the side. The brain can use those differences in localisation to help to make the speech more intelligible.

So the biggest technical challenge is developing hearing aids that can focus on what we really need and want to listen to. This is the current problem.

The Solution
We are hoping through even more sophisticated signal processing schemes that we’ll be able to work on artificial source segregation; i.e. analysing the signal that is coming in and figuring out what is speech and what isn’t, and only presenting to the ear the wanted signal.

Distinguishing between speech and noise
The assumption is that what you really want to listen to is speech, and so there are certain acoustical aspects of speech that we can recognise and there are certain acoustical aspects of noise that are different from speech.

So, we need to establish a suitable algorithm to be able to distinguish between speech and noise that will help you towards filtering the unwanted signal.

A more complete solutiion could mean that brain-computer interface may be part of the hearing aid systems of the future. Where the hearing aid is tapping into brain responses to pick up the specific signal the person wants to pay attention to.

This is an ongoing process with incremental steps and we will continue to see improvements over the next 15 years.

Children with hearing loss in one ear fall behind in language skills

By the time they reach school age, one in 20 children have hearing loss in one ear. This one impediment can raise significant hurdles for these children, because the loss of hearing in one ear hurts their ability to comprehend and use language.

"For many years, paediatricians and educators thought that as long as children have one normal hearing ear, their speech and language would develop normally," says Judith E. C. Lieu, MD, a Washington University ear, nose and throat specialist at St. Louis Children's Hospital.

"But then a few studies began suggesting these children might have problems in school. Now our study has shown that on average, children with hearing loss in one ear have poorer oral language scores than children with hearing in both ears," Lieu says.

Hearing loss in one ear can stem from congenital abnormalities in the ear, head trauma or infections such as meningitis. Children with hearing loss in one ear may go undetected because they can appear to have normal hearing. Their difficulty hearing may be mistaken simply for lack of attention or selective hearing, says Lieu, assistant professor of otolaryngology.

Even children with recognised 'one-side hearing loss' often aren't fitted with hearing aids and often don't receive accommodations for their impediment.

The researchers studied 74 six- to 12-year-old children with hearing loss in one ear. Each was matched with a sibling with normal hearing so that the researchers could minimise the possible effects of environmental and genetic factors on the children's language skills. The children were tested with the Oral and Written Language Scales (OWLS), a widely used tool to assess language comprehension and expression.

An average OWLS score is 100, and hearing loss in one ear caused, on average, a 10-point drop in scores. The oral composite score, which reflects both children's ability to understand what is said to them and their ability to respond or express themselves, averaged 90 in children with hearing loss in one ear.

Lieu says that the study demonstrated the strongest effect from hearing loss in one ear in children who are living below the poverty level or with mothers who have little education. Poverty levels and maternal education levels are well-established influences on language skills, and hearing loss in one ear may increase that effect.

"This study should raise awareness that if children with hearing loss in one ear are having difficulties in speech or reading in school, their hearing may be part of the problem," Lieu says. "Parents, educators and paediatricians shouldn't assume that having hearing in one ear means children won't need additional assistance."

The study does not address which possible solutions will be most effective for overcoming the decrease in language skills seen in the children with hearing loss in one ear, but Lieu suggests that studies could be done to see if hearing aids or amplification systems in the classroom will help.

In addition, having an educational audiologist as part of an individualised educational plan also might be beneficial.

"The effect of hearing loss in one ear may be subtle," Lieu says. "These children may shun large group situations because the noise overwhelms them, and they have a hard time understanding speech. They could have difficulties playing team sports because they can't localise sound as well as others and therefore, can't tell who is calling to them or from which direction.

"For them, listening takes a lot more work, and they may have to put in extra effort," Lieu says. "We don't know yet if the hearing loss ultimately affects their overall educational achievement and eventually, even which occupations they choose."

Clearly there is still much to discover with this study but it does re-enforce the need to check the hearing of children who are having learning difficulties or are not socialising as well as they might.

Link: http://www.medicine.wustl.edu