An introduction to cochlear dead zones—or the absence of the inner hair cells in a region of the cochlea where the basilar membrane vibration can’t be detected—by perhaps the world’s foremost authority on the subject, Dr Brian Moore. Dr Moore also describes his Threshold Equalizing Noise (TEN) Test for identifying cochlear dead zone regions. He details how these regions might be created, provides possible tell-tale signs and tip-offs for presenting in a patient, and outlines their implications for treatment of hearing loss and hearing aid fittings. Interviewed by Hearing Review Editor Karl Strom. Originally broadcast December 8, 2008.
Bryce Lochmann (00:21):
Welcome to the MEDQOR Podcast Network. The MEDQOR Podcast Network provides insights, reporting and analysis on med tech innovation across all of healthcare. Thanks for listening as we discuss technology and treatment trends ranging from audiology to respiratory care to cosmetics and more.We’re proud to be supported by 10 leading brands in healthcare whose chief editors will join us on a recurring basis to talk with key leaders in their industries about what’s happening across healthcare now.
Bryce Lochmann (00:54):
My name is Bryce Lochmann, the producer of this podcast, and today we will revisit a previous podcast from Karl Strom, Chief Editor of the Hearing Review. Karl talks with Dr. Brian C.J. Moore about diagnosing and managing dead zones in the cochlea. In 2000, Dr. Moore developed a test called the TEN Test, short for Threshold Equalizing Noise Test. This has proven quite helpful in the audiology field as the TEN Test provides clinicians with a quick and easy way to identify cochlear dead regions by measuring pure tone thresholds in the special masking noise. Having knowledge of the presence or absence of a dead region can have important implications for fitting hearing aids and for predicting the likely benefit of those hearing aids, ultimately improving the patient outcomes. It is this type of innovation by leading healthcare professionals that we aim to feature on the MEDQOR Podcast Network. I hope you enjoy.
Karl Strom (02:05):
Thanks for joining us today on the Hearing Review’s Science and Technology Thursday podcast. I’m Karl Strom, Editor of the Hearing Review and today’s podcast is part one of a two part series about dead zones of the cochlea. Today and next Thursday, we’ll be talking with Brian Moore, PhD of the University of Cambridge in England, who has been researching what happens to people’s hearing when hair cells are virtually wiped out of a particular frequency region. The term dead zone might provide the imagination with wild ideas about traveling to another dimension of not only sound and time, but of mind. I might have gotten a little carried away there. My apologies to both you and the brilliant Rod Serling. But dead zones also pose some very unique challenges to hearing care diagnosticians, audiologists and dispensing professionals.
Karl Strom (02:48):
What exactly is a dead zone? How do we know when we’re confronted with one and what do they look like on an audiogram? And what kind of tests are available for delineating the parameters of a dead zone? Dr. Brian C.J. Moore is Professor of Auditory Perception at the University of Cambridge and he’s written and edited 13 books and over 490 scientific papers and book chapters. Dr. Moore has been awarded the Silver Medal of the Acoustical Society of America, the first International Award in Hearing from AAA and the Award of Merit from ARO. He’s also the recipient to the Hugh Knowles Prize for Distinguished Achievement from Northwestern University, and is a two time recipient to The Littler Prize of the British Society of audiology. He joined us from his office at the University of Cambridge. Dr. Moore, thank you for joining us today on the HR podcast.
Brian C.J. Moore (03:30):
Oh, it’s a great pleasure to join you.
Karl Strom (03:34):
I guess I’d like to start out by asking what’s the technical definition of cochlear dead zones?
Brian C.J. Moore (03:39):
Okay. Well, I have a definition in terms of the survival of inner hair cells and neurons within the cochlea. Now the inner hair cells are the transducers of the cochlea. They detect vibrations on the basilar membrane and transform them into neural activity. And if those inner hair cells are completely not working, or if the neurons have died off or are not working for some reason, then I call that a dead zone or a dead region. So basically, it’s a region within the cochlea where the basilar membrane vibration isn’t detected at all.
Karl Strom (04:16):
What kind of identifying ideological features are there for dead zones? In other words, where are dead zones usually located? How are they usually created? And for that matter, what would they look like to a hearing care professional?
Brian C.J. Moore (04:28):
Okay. Well, you can get some idea from looking at the audiogram. And what we’ve found is that a dead region is quite common when the hearing loss at a particular frequency is 70 decibels or more. So basically, dead zones are commonly associated with large hearing losses, 70 dB or more. But you can’t be sure from the audiogram because a person with a hearing loss of let’s say 60 dB might have a dead region at frequency and a person with a hearing loss as big as 85 or 90 dB might not have a hearing loss. So it’s impossible to be sure from looking at the audiogram. Now, the commonest kind of dead zone is one that occurs at high frequencies. And I think many elderly people who have large hearing losses at high frequencies, have a dead zone sort of for the very high frequencies, but not necessarily extending down to medium frequencies.
Brian C.J. Moore (05:32):
The other kind of pattern where you get a dead region at low frequencies does occur sometimes, for example, in association with Meniere’s disease, but it’s much less common. Now when a person has a low frequency dead region, it can show up as a relatively flat audiogram because the vibration on the basilar membrane actually spreads up a lot towards the higher frequency regions. So these low frequency dead regions are quite hard to spot. One thing that you can use as a hint is if the person tells you that when you present a tone, when you’re doing the audiogram, and they say, “It sounds very distorted,” or “It sounds noise like,” that’s often a hint that a dead region is there. Again, it’s not completely reliable, but it gives you a hint and alerts you that something may be going on that this person may have a dead region.
Karl Strom (06:27):
And is that because it’s stimulating adjacent areas of the cochlea?
Brian C.J. Moore (06:33):
Yes. We think that in order to hear a clear pitch for a sound and to hear a tone sounding like a tone, you need to have a correspondence between the place in the cochlea that’s excited and the temporal information that you get from that tone in terms of the patterns of phase locking in the cochlea. And when there’s a discrepancy between the place information and the temporal information, that can give rise to a perception of a distorted sound and a tone that sounds noise-like or crackly or just highly distorted.
Karl Strom (07:10):
So it sounds like the actual audiogram could take on several different forms. Obviously, it could be a ski slope kind of loss, but it could also be a flat loss or maybe even a barn shaped loss.
Brian C.J. Moore (07:23):
Yeah, and that’s right. I mean, some people have mid-frequency dead regions, so they just show a dip in the audiogram. And we’ve found a number of cases of mid-frequency dead regions, which usually are genetic in origin. You often find that it runs in the family. But intense noise exposure, particularly impact sounds, gunshots, or being near explosions will often produce a high frequency dead region. And that’s often associated with a steeply sloping audiogram, as you said.
Karl Strom (07:57):
Now you’ve developed the TEN Test, a CD that can be played over a two-channel audiometer. Can you tell us a little bit about how this test works?
Brian C.J. Moore (08:04):
Okay. Well, TEN stands for Threshold Equalizing Noise, and it’s a special noise that we designed so that if you measure the threshold for detecting a tone in the noise for a person with normal hearing or a person with a hearing impairment, but without any dead region, you get approximately the same threshold for the tone, regardless of the frequency of the tone.
Brian C.J. Moore (08:30):
So you have a kind of uniform reference point to start with. If you measure the threshold for detecting a tone in the noise, you get the same result at all frequencies when there’s not a dead region present. Now, the idea behind the test is that if you present a tone whose frequency falls in a dead region, then the person will only detect that tone, if you turn its level up enough, so that it’s producing some vibration on the basilar membrane at a place in the cochlea that isn’t dead. But the cochlea always shows some degree of tuning, even in an ear with a hearing loss. And so the amount of vibration on the basilar membrane at the place where the tone is due will be less than normal and so the noise will be much more effective than normal in masking that tone.
Brian C.J. Moore (09:23):
And so basically, when you do the test, what you are looking out for is whether the signal threshold in the noise is higher than normal. And we’ve suggested a criteria that if the threshold for detecting the tone in the noise is 10 decibels higher than normal, then that may indicate a dead region. Now, there is a requirement in the test that the noise has to be intense enough to do some masking of the tone. So if the tone has a high threshold, just because the audiometric threshold is high at that frequencies, that’s not good enough. You have to make this TEN noise intense enough that you’re actually getting some masking. And when we first developed this test, we used a noise that covered a very wide frequency range and it sometimes got a bit too loud at high sound level when you were trying to get it intense enough to get enough masking.
Brian C.J. Moore (10:19):
So later on, we produced a new version that we call the 10(HL) Test, where everything’s calibrated in terms of dB hearing level or hearing threshold level. And that noise covers a slightly narrower range of frequencies, but that makes it less loud and makes it possible to conduct the test at higher sound levels. So that the test that’s being mainly used nowadays is the version that’s called the 10(HL) Test. And as you say, you need a CD player, but you play the sound from the CD through a two-channel audiometer and then use the controls on the audiometer to adjust the test tones. And we don’t recommend that this test is used all the time, but what we suggest is that if a person has a hearing loss of 70 dB or more at a frequency, then it might be worth checking for the presence of a dead region at those frequencies where the hearing loss is fairly big.
Karl Strom (11:21):
And where can dispensing professionals get the 10(HL) Test?
Brian C.J. Moore (11:25):
Well, they still need to buy it from me at the moment and there’s information about how to get it on my website. I’ve been trying to persuade manufacturers of audiometers to build the test into their audiometers, but they still think it’s too new for them. They’re all very cautious. But hopefully it will appear in the next few years.
Karl Strom (11:49):
Back to the physiological aspect of it, how many live hair cells are needed to not call a frequency region a dead zone or might this vary, or to use a stupid pun, are we splitting hair cells here?
Brian C.J. Moore (12:01):
Well, that’s an interesting question. I mean, I like to think of it in terms of the percentage of surviving hair cells. There were some anatomical studies done by Harold Schuknecht, who worked in Harvard at the Massachusetts Eye and Ear Infirmary. He did a lot of anatomical studies where he did ideological measurements and speech perception measurements in people, and then got them to sign something saying that when they died, they would donate their ears so that he could examine them. And what he found was that if you only had 5% of your inner hair cells surviving, you could still have almost a normal audiometric threshold.
Brian C.J. Moore (12:44):
So you have to look to really get a serious hearing loss from damage to the inner hair cells. You really need to have lost most of them, more than 95%. But by the time you’ve lost that 95%, the person may be having a lot of trouble in understanding speech. So of course there is an in between stage, what I’ve sometimes called a sick region, where a lot of the inner hair cells may be damaged, but they’re not entirely lost. And that still may cause a lot of problems to the person, but I would say that typically a dead region is associated with more than 95% damage to the inner hair cells.
Karl Strom (13:24):
Obviously, when virtually all the hair cells are lost, a patients hearing is lost in that region. Are there other physiological and neural processing aspects to consider as well? For example, what do we know about the other working parts of the cochlea in relation to the dead zone, like acclimatization or neural wiring or brain coating in response to near total loss of hair cells?
Brian C.J. Moore (13:46):
Okay, well taking the first part of that question first about inner and outer hair cells, my belief is that in many cases where the inner hairs cells are damaged, the outer hair cells are also damaged. And in our own work, when we’ve diagnosed people as having dead regions, we’ve tried to check whether the outer hair cells were functioning by measuring otoacoustic emissions. And we’ve never been able to measure otoacoustic emissions in people with dead regions. So that means in our sample of subjects, damage to the inner hair cells is always combined with damage to the outer hair cells. Now, I think there are some rare cases around where that’s not the case. There have been some cases of people with quite large hearing losses who do have otoacoustic emissions. And you could think of that maybe as a pure inner hair cell loss or pure neural damage. And those people usually have a very poor ability to understand speech.
Brian C.J. Moore (14:47):
And it may also be that what is labeled as auditory neuropathy may really be sort of patchy dead regions or substantial loss of inner hair cells, but with intact outer hair cells. Now going onto your other part of the question, I mean, one interesting thing is if a person has had a dead region for a long time, what happens to their ability to process sounds from the part of the cochlea that is still functioning? And in collaboration with a colleague, called Vinay from India, we’ve done a study of people where we’ve compared two groups, one who have high frequency dead regions, but reasonable low frequency hearing, and then another group who are matched for their low frequency hearing, but don’t have dead regions at the higher frequencies. And so these two groups are matched for their low frequency hearing, but they differ in whether or not they have a high frequency dead region.
Brian C.J. Moore (15:45):
And we’ve measured their ability to discriminate and process sounds that only contain low frequencies, so in the region where their hearing is matched. And what we found is that on some tasks, including the ability to understand speech that’s low past filtered to only have low frequencies, the people with dead regions actually do better. And we think that what’s happened is because they’ve had a dead region for a long time, they haven’t been getting any information sent from the cochlea to the brain about the high frequencies. And basically the auditory cortex has adapted to do a better analysis of the remaining low frequency hearing. So when you test them with sounds that are restricted to low frequencies, they actually do better than the people without dead regions. And so we think that’s an example of brain plasticity and how people have sort of learned to make best use of their residual hearing.
Karl Strom (16:44):
Relative to hearing aid audibility and distortion, haven’t there been researchers and amplification that have been hinting at the existence of dead zones or near dead zones without really specifically calling it that? For example, I’m wondering about some of the early work done by Margo Skinner in the seventies, as well as some of the folks at NAL, like Denis Byrne, who were working on research like [inaudible 00:17:04].
Brian C.J. Moore (17:04):
Yes. I mean, I don’t want to claim that it’s something that I invented. I maybe started using the name of dead zone or dead region, but I think the idea was around earlier. In fact, the earliest paper that I found on this was actually by a man called Troland, who worked mainly on lights and he has a unit of light named after him. But he published a paper in the 1920s where he talked about lacunae in hearing. And I think he was really referring to dead regions, but he didn’t call them that. And as you say, I’m sure that Margo Skinner and Denis Byrne had this concept in mind, but they just didn’t phrase it in quite the same way. So I guess it’s just the brutal frankness of the British in calling something a dead zone rather than skirting about it.
Karl Strom (17:59):
Could you tell us a little bit about some of the most frequent recommendations for applying amplification when faced with a cochlear dead zone?
Brian C.J. Moore (18:06):
Yeah. Well, we can consider two cases. If you have a person who’s got a high frequency dead region with reasonable hearing at low frequencies and let’s say we call the edge frequency of the dead region fe, that’s a symbol that I usually use to describe where the dead region starts, what we find is that it’s useful to apply amplification for frequencies up to about 1.7 times fe. So for example, if a person had reasonable hearing up to one kilohertz and then a dead region extending upwards from there, then we recommend applying amplification up to about 1.5 to two kilohertz, but not at much higher frequencies. And we found there’s no benefit in trying to amplify those higher frequencies, and it can just lead to problems with distortion and acoustic feedback and so on. Now an alternative for a person with a high frequency dead region is a hearing aid incorporating transposition. And there are a couple of manufacturers now that have such hearing aids on the market.
Brian C.J. Moore (19:16):
They haven’t really shown clear benefits yet, but I think it’s sort of worth trying for people with diagnosed dead regions and it may be worth trying a hearing aid with transposition. Now, the other case to consider are people with low frequency dead regions. And we’ve done some work recently showing that if you try to amplify the low frequencies for a person with a low frequency dead region, you actually make things worse. And again, you can describe this in terms of the edge frequency, if we call the edge frequency fe. It’s worth amplifying frequencies a little bit below the age frequency, but not more than an octave below. So if a person had say a low frequency dead region that started at one kilohertz and went down from one kilohertz, you shouldn’t amplify frequencies below about 500 hertz. In fact, it’s better to attenuate those to block the ear so that those lower frequencies don’t get through, but just to focus on giving reasonable amplification from a little bit below the value of fe up to the highest frequency that you can.
Brian C.J. Moore (20:28):
So it’s a very different case for the person with a low frequency dead region, where you really want to filter out the low frequencies and avoid amplifying those at all.
Karl Strom (20:38):
What’s been the most exciting part of this research for you, Dr. Moore? Are there any big questions left unanswered here? Can you tell us a little bit about the research that you might be currently working on or monitoring related to this topic?
Brian C.J. Moore (20:51):
Right. Well, I think one exciting thing for me is first of all, that this research did turn out to have some practical applications and to be relevant for hearing aid fitting. But it’s also raised a lot of interesting questions about the basic mechanisms of pitch perception and what makes a tone a tone, and what determines the perceived pitch of a tone. And those are questions that we are still exploring now.
Brian C.J. Moore (21:16):
One application that we are looking at the moment is connected with people who have some residual hearing at low frequencies, but pretty poor hearing at medium and high frequencies, who might be a candidate for a combination of a cochlear implant and a hearing aid. And we think for those people, it’s important to diagnose just where the dead region starts and both to guide the surgeon in deciding how deep the electrodes should be put into the cochlea, but also in setting up the signal processing and deciding which frequencies you’re going to try and transmit through the hearing aid and which frequencies you’re going to transmit through the implant. So that’s an active area of research at the moment where we think fundamental research on dead regions could pay off in better hearing for people using a combination of a hearing aid and an implant.
Karl Strom (22:10):
Well, I think that’s all the time we have today, Dr. Moore, but I want to thank you so much for joining us on the HR Podcast and been very fascinating.
Brian C.J. Moore (22:19):
Thanks a lot. It’s been a pleasure talking to you.
Karl Strom (22:20):
Dr. Brian Moore is Professor of Auditory Perception at the University of Cambridge, and he joined us from his office in Cambridge, England on October 21st, 2008. Dr. Moore is a fellow of the Royal Society, the Academy of Medical Sciences and the Acoustical Society of America and an honorary fellow of the Belgian Society of Audiology, as well as the British Society of Hearing Aid Audiologists. He is also President of the Association of Independent Hearing Healthcare Professionals in the UK, and serves as an Associate Editor of the International Journal of Audiology and the Journal of the Acoustical Society of America and he is a member of the Editorial Boards on Hearing Research and Audiology and Neurotology.
Karl Strom (22:55):
Among many excellent publications, he’s written two books related to auditory perception, Cochlear Hearing Loss, and An Introduction to the Psychology of Hearing, now on its fifth edition. Both can be purchased at amazon.com. Parts one and two of this podcast series, as well as all HR Podcasts, can be accessed by choosing the podcast tab of the media center at wwwhearingreview.com. Today’s podcast was produced by Robert Elmquist, Fabian Chung and featured music by Rob Meany and his Minneapolis based band, Terramara, available at wwwterramara.com. For the Hearing Review, I’m Karl Strom. Thanks for listening.