SETI: The Early Years

Tinnitus discovery could lead to new ways to stop the ringing

People with tinnitus -- a constant ringing or buzzing in the ears -- can take heart from a new study by neuroscientists that points to several new strategies for alleviating the problem. 

In experiments on rats, researchers have shown that tinnitus results from decreased inhibition in the auditory cortex. Thus, training that boosts inhibition or drugs that increase the levels of inhibitory neurotransmitter may alleviate the symptoms. 

(Credit: © Taffi / Fotolia)

Sustained loud noises, as from machinery or music, as well as some drugs can damage the hair cells in the inner ear that detect sounds. Because each hair cell is tuned to a different frequency, damaged or lost cells leave a gap in hearing, typically a specific frequency and anything higher in pitch.

Experiments in the past few years have shown that the ringing doesn't originate in the inner ear, though, but rather in regions of the brain -- including the auditory cortex -- that receives input from the ear.

Bao's experiments in rats with induced hearing loss explain why the neurons in the auditory cortex generate these phantom perceptions. They showed that neurons that have lost sensory input from the ear become more excitable and fire spontaneously, primarily because these nerves have "homeostatic" mechanisms to keep their overall firing rate constant no matter what.

"With the loss of hearing, you have phantom sounds," said Bao, who himself has tinnitus. In this respect, tinnitus resembles phantom limb pain experienced by many amputees,

One treatment strategy, then, is to retrain patients so that these brain cells get new input, which should reduce spontaneous firing. This can be done by enhancing the response to frequencies near the lost frequencies.

Experiments over the past 30 years, including important research by Merzenich, have shown that the brain is plastic enough to reorganize in this way when it loses sensory input. When a finger is amputated, for example, the region of the brain receiving input from that finger may start handling input from neighbouring fingers.

Bao noted that retraining the ear has been tried before, but with limited success. Most such attempts have taken patients with some residual hearing and trained their ears to be more sensitive to the affected frequencies. This wouldn't work for patients with profound hearing loss, however.

Most retraining is also based on the assumption that reorganization of the brain -- that is, changing how frequencies "map" to regions of the auditory cortex -- is a cause of the tinnitus. This is the opposite of Bao's conclusion.

"We argue that reorganizing the cortical map should be the goal, so that the nerves get some input and stop their tinnitus activity," he said. "You don't want to leave these cells without sensory input."

"We changed our (brain training) strategy from one where we completely avoided the tinnitus domain to one where we directly engage it and try to redifferentiate or reactivate it, and we seem to be seeing improvement," Merzenich said.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Berkeley. The original article was written by Robert Sanders, Media Relations.

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Journal Reference:

1. S. Yang, B. D. Weiner, L. S. Zhang, S.-J. Cho, S. Bao. Homeostatic plasticity drives tinnitus perception in an animal model. Proceedings of the National Academy of Sciences, 2011; 108 (36): 14974 DOI: 10.1073/pnas.1107998108

Why 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.

Brain activity in speakers and listeners

When two people experience a deep connection, they’re informally described as being on the same wavelength. There may be neurological truth to that.

Brain scans of a speaker and listener showed their neural activity synchronising during storytelling. The stronger their reported connection, the closer the coupling.

The experiment was the first to use fMRI, which measures blood flow changes in the brain, on two people as they talked. Different brain regions have been linked to both speaking and listening, but “the ongoing interaction between the two systems during everyday communication remains largely unknown,” wrote Princeton University neuroscientists Greg Stephens and Uri Hasson in the July 27 Proceedings of the National Academy of Sciences.

They found that speaking and listening used common rather than separate neural subsystems inside each brain. Even more striking was an overlap between the brains of speaker and listener. When post-scan interviews found that stories had resonated, scans showed a complex interplay of neural call and response, as if language were a wire between test subjects’ brains.

The findings don’t explain why any two people “click,” as synchronisation is a result of that connection, not its cause and while the brain regions involved are linked to language, their precise functions are not clear.

But even if the findings are general, they support what psychologists call the “theory of interactive linguistic alignment” — a fancy way of saying that talking brings people closer by making them share a common conceptual ground.

“If I say, ‘Do you want a coffee?’ you say, ‘Yes please, two sugars.’ You don’t say, ‘Yes, please put two sugars in the cup of coffee that is between us,’” said Hasson. “You’re sharing the same lexical items, grammatical constructs and contextual framework. And this is happening not just abstractly, but literally in the brain.”

The researchers didn’t test brain synchronization during phone calls or video conferencing, but Hasson speculates that “coupling would be stronger face-to-face.” He also thinks dialogue will produce especially strong forms of synchronization, and plans to run scans of people engaged in deep conversation, rather than telling or listening to long stories.

“But first, we’ll look at cases where there’s a failure to communicate,” said Hasson.