A new look at the mechanisms of hearing.

This has certain implications for current research into hearing – and cochlear implant research and development.

http://www.sciencedaily.com/releases/2007/10/071011140215.htm

New Hearing Mechanism Discovered

MIT researchers have discovered a hearing mechanism that fundamentally changes the current understanding of inner ear function. This new mechanism could help explain the ear’s remarkable ability to sense and discriminate sounds. Its discovery could eventually lead to improved systems for restoring hearing.

MIT Professor Dennis Freeman, left, graduate student Roozbeh Ghaffari and research scientist Alexander J. Aranyosi have found that the tectorial membrane, a gelatinous structure inside the cochlea of the ear, is much more important to hearing than previously thought.

MIT Professor Dennis M. Freeman, working with graduate student Roozbeh Ghaffari and research scientist Alexander J. Aranyosi, found that the tectorial membrane, a gelatinous structure inside the cochlea of the ear, is much more important to hearing than previously thought. It can selectively pick up and transmit energy to different parts of the cochlea via a kind of wave that is different from that commonly associated with hearing.
Ghaffari, the lead author of the paper, is in the Harvard-MIT Division of Health Sciences and Technology, as is Freeman. All three researchers are in MIT’s Research Laboratory of Electronics. Freeman is also in MIT’s Department of Electrical Engineering and Computer Science and the Massachusetts Eye and Ear Infirmary.

It has been known for over half a century that inside the cochlea sound waves are translated into up-and-down waves that travel along a structure called the basilar membrane. But the team has now found that a different kind of wave, a traveling wave that moves from side to side, can also carry sound energy. This wave moves along the tectorial membrane, which is situated directly above the sensory hair cells that transmit sounds to the brain. This second wave mechanism is poised to play a crucial role in delivering sound signals to these hair cells.

In short, the ear can mechanically translate sounds into two different kinds of wave motion at once. These waves can interact to excite the hair cells and enhance their sensitivity, “which may help explain how we hear sounds as quiet as whispers,” says Aranyosi. The interactions between these two wave mechanisms may be a key part of how we are able to hear with such fidelity – for example, knowing when a single instrument in an orchestra is out of tune.

“We know the ear is enormously sensitive” in its ability to discriminate between different kinds of sound, Freeman says. “We don’t know the mechanism that lets it do that.” The new work has revealed “a whole new mechanism that nobody had thought of. It’s really a very different way of looking at things.”

The tectorial membrane is difficult to study because it is small (the entire length could fit inside a one-inch piece of human hair), fragile (it is 97 percent water, with a consistency similar to that of a jellyfish), and nearly transparent. In addition, sound vibrations cause nanometer-scale displacements of cochlear structures at audio frequencies. “We had to develop an entirely new class of measurement tools for the nano-scale regime,” Ghaffari says.

The team learned about the new wave mechanism by suspending an isolated piece of tectorial membrane between two supports, one fixed and one moveable. They launched waves at audio frequencies along the membrane and watched how it responded by using a stroboscopic imaging system developed in Freeman’s lab. That system can measure nanometer-scale displacements at frequencies up to a million cycles per second.
The team’s discovery has implications for how we model cochlear mechanisms. “In the long run, this could affect the design of hearing aids and cochlear implants,” says Ghaffari. The research also has implications for inherited forms of hearing loss that affect the tectorial membrane. Previous measurements of cochlear function in mouse models of these diseases “are consistent with disruptions of this second wave,” Aranyosi adds.

Because the tectorial membrane is so tiny and so fragile, people “tend to think of it as something that’s wimpy and not important,” Freeman says. “Well, it’s not wimpy at all.” The new discovery “that it can transport energy throughout the cochlea is very significant, and it’s not something that’s intuitive.”

The research is described in the advance online issue of the Proceedings of the National Academy of Sciences the week of October 8.

This research was funded by the National Institutes of Health.

Study Shows Isolation of Cochlear Stem Cells may Treat Hearing Loss

Cochlear regeneration is well on its way to be developed within our lifetimes, if the following news article is anything to go by (emphasis mine):

National Center for Regenerative Medicine researchers say that studies have shown noise-induced hearing loss is going to become the next big epidemic affecting our younger generation, though the effects won’t show until it is too late to treat. In addition to loud noise, certain cancer drugs or genetic factors can cause hearing loss in humans due to loss or faulty development of the sensory ‘microphones’ (hair cells) inside the ear – the cochlea.

Lost hair cells are not replaced and people exposed to these conditions face permanent hearing loss. Identification of the stem cells from the adult cochlea would be a major step forward to develop new therapeutic approaches to hearing loss.

Members of the National Center for Regenerative Medicine research team, Dr. Robert Miller and Dr. Kumar Alagramam, both of Case Western Reserve University School of Medicine, recently published research findings in Developmental Neuroscience which suggest new ways of treating hearing loss. These researchers have isolated “cochlear stem cells” located in the inner ear and already primed for development into ear-related tissue due to their proximity to the ear and expression of certain genes necessary for the development of hearing.

“Previous work in our lab with young-adult mouse cochlear tissue showed expression of genes normally found in stem cells and neural progenitors. This led us to hypothesize that cochlea harbors stem cells and neural precursor cells. Our work in collaboration with Miller’s lab supports our hypothesis,” Dr. Alagramam said.

They say that in early life, these precursor cells may be able to regenerate hair cells, but their capacity to do so becomes limited as the ear develops and ages. The team’s research is a major step in devising a therapy to reverse permanent hearing loss because it may lead to the activation of cochlear stem cells in the inner ear to regenerate new hair cells.

“Clearly we have miles to go before we reach our end goal, but the exciting part is now we can test compounds that could promote regeneration of hair cells from these precursor cells in vitro, we can study the genes expressed during the transition from stem cells to hair cells, and we can think of developing strategies for cell replacement, i.e. transplanting these cochlear stem cells into the adult cochlea to affect hair cell replacement in the mouse, by extension, in humans,” remarked Dr. Alagramam.

In this paper, Drs. Miller and Alagramam offer further evidence for the existence of cochlear stem cells in the mouse cochlea by confirming the ability to form ‘stem cell’ spheres in culture and by characterizing these cells in terms of neural and hair cell development using a panel of stem cell development and hair cell markers.

The formation of spheres from early postnatal cochlear tissues and their expression of a wide range of developmental markers unique to hair cells confirm the possibility that self-supporting hair cell precursors exist in or can be derived from the postnatal mammalian cochlea.

While further research is necessary, the researchers believe these precursor cells have the potential to regenerate the damaged hair cells and restore normal hearing.

The team has already begun animal studies to explore the use of cochlear stem cells in well-established hair cell ablation models and in deaf mouse mutants with predictable patterns of early hair cell loss. This line of research will evaluate the in vivo survival and differentiation of self-renewing cochlear cell populations and potentially lead to new therapies for the numerous individuals that are going to suffer from noise-induced hearing loss in the near future.

Cochlear Clinical Night

I have just come back from a Cochlear Clinical Night hosted in Singapore – the speaker was Brendan Murray, a cochlear implant specialist from Sydney. Some of you might be aware that he’s Lina Lim’s predecessor (Lina Lim is the area manager for South-East Asia), and was the first one to help set up Cochlear’s operations here in Singapore.

The talk was primarily for medical professionals and clinicians – but I found out and requested admission which was granted readily (thank you, you know who you are!) There were two main parts of the talk – clinical outcomes for Freedom, and the future of their implantable devices. Please take note scores provided below might not be accurate – they are from memory.

Cochlear did a Freedom trial on 71 postlingually deafened adults in North America. I think you should be able to know that results were fantastic according to them. Let’s go straight to the more exciting part- the implantable devices.

There are now apparently 3 new or upcoming implantable devices by Cochlear other than Freedom on the market – the Hybrid S, Hybrid L, and Auditory Brainstem Implant (ABI). The Hybrid implant is an alternative option for those who are worried about losing their residual hearing - it combines the benefits of both acoustic and electrical processing – in other words, it tries to combine the functions of a hearing aid and a cochlear implant. 

The Hybrid S measures about 10mm, and is jointly developed by the University of Iowa and Cochlear. Research results on Hybrid S have shown that pitch perception scores of trial recipients can be quite close to that of normal hearing (90% in some cases, as compared to 60-70% for Long Electrode/the traditional CI).

The Hybrid L measures about 16mm, and word recognition scores have apparently created a new industry benchmark – word recognition scores hit above 50% for 3 months postoperative implantees from just around 2-6% preoperative.

The Auditory Brainstem Implant (ABI) section was very interesting. It’s an alternative for people with cochlear implant failures, or are found to be unsuitable for cochlear implants – NF2 (Neurofibromatosis Type 2), cochlear asplasia, cochlear nerve agenesis, ossified cochleas etc. It’s implanted directly at the cochlear nucleus at the right side of the brain at the bottom (if I’m not wrong), allowing direct stimulation. However, trials found that there was stimulation of other parts of the brain that was not auditory in nature, causing side effects – dizziness, aches and pain in other parts of the body ie. eyes, chest, legs.

Personally, I felt that Cochlear needs to integrate pitch perception tests as part of their testing. Not only that, I was wondering about how a bilateral and a Hybrid would compare against each other on word recognition scores (although I’m sure a Hybrid wins hands down on pitch perception). I also felt that the ABI was a kind of dead end – it had many reported side effects, and there was no complete open set recognition of words for the postlingually deafened individuals – which says a lot. Not many benefited, and those who benefited still had to use lip-reading cues to aid listening. Nevertheless, some form of listening is better than none at all – only without side effects, however!

Nevertheless, the Hybrid represents a new advancement in cochlear implants for individuals with severe hearing loss. It is much better than the long-electrode traditional cochlear implant, judging from independent research and anecdotal experiences. Individuals with such hearing loss, and are worried about their residual hearing not being utilised need not worry further with this new CI.

Hopefully, for the rest of us who are already implanted, a fully implantable device or cochlear regeneration is not too far off in the future.