Otoacoustic Emissions, Tinnitus, Distortion Product OAEs, and Transient OAEs: Interview with David Kemp, PhD
Dr. Kemp talks with Douglas L. Beck, AuD, Web content editor about otoacoustic emissions and more. Dr. Kemp, discoverer of OAEs, is a featured presenter at the inaugural Academy Research Conference (ARC), held the first day of AudiologyNOW!, April 1, 2009, in Dallas, TX. This year’s theme: “Otoacoustic Emissions—Improving Practice Through Science.
Academy: Hi, David. Thanks for your time this morning. It's an honor to speak with you.
Kemp: Thanks, Doug. Good to be with you.
Academy: David, I wonder if we can start with your education?
Kemp: Sure. I'm a physicist by training. I received my degree in physics from King's College at London University. I earned my PhD in a particular area of study called "radio physics." I worked with electrical signals recorded from the atmosphere and studied them with regard to geophysics.
Academy: What was the topic of your dissertation?
Kemp: I studied and reported on the ongoing extremely low frequency electrical resonances of the space between the Earth and the ionosphere that is stimulated by thunderstorms. Those natural electrical signals are not unlike EEG signals.
Academy: Very interesting. So you actually heard the resonance of the whole Earth?
Kemp: Yes, speeding up the tapes by 10 times you could hear it as a kind of howl. It was quite fascinating.
Academy: I'll bet. How did you get from there to audiology and otoacoustic emissions (OAEs)?
Kemp: The link is through audio frequency signals. We have a scientific field here called "audiological physics," which grew out of the need for acoustics and calibration in audiology. It's not so much clinical as a scientific research field and I took a position in that role in the 1970s at the Royal National ENT Hospital in London. We worked on psycho-acoustic experiments, too, which was very interesting for me. I was actually brought in to develop psychoacoustic tests for children to help understand their speech development capacity but OAEs came about as I studied the detailed psychoacoustics of threshold hearing in healthy adults.
Academy: Did that relate at all to tinnitus at that time?
Kemp: Yes. In some ways it did. A number of people at that time had claimed to have been able to record tinnitus with a microphone put to the ear with tinnitus. One had found tones and another clicks. So the idea of listening to an ear with a microphone was not new even though the consensus was then (as it is today) that most tinnitus was not due to a physical vibration in the ear. But the idea of a sound coming out of a normal ear was indeed, novel.
Academy: Was there a particular moment when you thought, "Aha!"
Kemp: Well, yes. We were exploring and mapping the ear's fine structure as it related to threshold and low-level loudness perception and the issue became—what is responsible for that pattern of rapid variations with frequency? It wasn’t a new phenomenon, even then, but no one knew if we were measuring something in the auditory pathway or in the cochlea.
There had been very little interest in the topic. I discovered the phenomena didn’t occur in impaired ears with cochlear pathology—so I reasoned it must be a cochlear effect. If the cochlea behaved like a resonator then the psychoacoustic microstructure made mathematical and physical sense to me and I could model and study it. The important "Aha" moment came, when as a physicist, I realized if there were physical resonances occurring inside the cochlea it should be possible to detect these from outside, acoustically in the ear canal, because of the way the middle ear links the cochlea and the ear drum.
Academy: And that must've been about 1977 or so?
Kemp: Yes. That's right, spring of 1977 through 1978.
Academy: And so how did you go about measuring the sound from the cochlea?
Kemp: At first I used a pure tone stimulus and I sealed a microphone in the ear canal under a headphone. As the stimulus sounded louder and softer with frequency, the sounds in the ear canal varied exactly in time although the headphone signal was steady. This proved the effect wasn’t in the auditory pathway. Of course, that was the analog world and the measurements were more hands-on and delicate, but it got us going in the right direction. It was easy to improvise and experiment with different stimuli. With two tone stimuli we found distortion products and with click stimuli we got an echo from the cochlea. When there was no stimulation we could sometimes pick up a tone generated by the ear itself. It was very exciting.
Academy: Were you aware of the physiology of the cochlea, and of outer hair cells at this time?
Kemp: I knew a little of the anatomy and physiology of the cochlea –but at that time the role of outer hair cells was totally unclear to everyone and we were all convinced the cochlea was a physically passive structure that could not possibly resonate in the way I was observing. But, to me, OAEs required the kind of amplifier in the cochlea that was proposed by Thomas Gold back in 1948. OAEs really challenged established opinion and theories. The momentus discoveries of sharp mechanical cochlear tuning and of outer hair cell electromotility came several years after OAEs and made people take the cochlear amplifier idea much more seriously.
Academy: David, OAE testing is common place today and the cochlear amplifier is generally accepted. What are your thoughts regarding comparing and contrasting distortion product OAEs (DPOAEs) and transient OAEs (TEOAEs)?
Kemp: This question comes up quite a lot and I think there is sustained clinical confusion, too. Remember, these are technical signal processing terms, and although everyone would like to be really comfortable with these terms, the technical discussions of OAEs is very sophisticated and hard to grasp.
DPOAE and TEOAE tests use two quite different stimuli and processing techniques to observe the cochlea. Neither test records and analyzes the whole OAE response to the sound—only parts of it. Those parts are different for the two tests and as Chris Shera noted, they tend to come to us via two different routes too. But the key thing is that both DPOAEs and TEOAEs need outer hair cells to be working. In that sense, they test the same thing in different ways—but the two tests each have their own advantages and disadvantages that need to be understood.
Academy: So what are those advantages and disadvantages?
Kemp: Well, the distortion product method works well over a wider frequency range than the transient stimulation method, and so DPOAEs are essential if you need to confirm normal function above the main speech frequency range.
On one hand, you can also apply more intense stimulation using pure tones rather than clicks, so DPOAEs can be used to obtain hair cell responses when there is a mild hearing loss. This can be valuable clinically—but of course caution is required with the stimulus level used for screening or the test becomes insensitive.
On the other hand, transient OAEs are conveniently optimal over the primary speech discrimination frequencies and are highly sensitive to mild losses of function that makes them ideal for screening too. And because transient OAEs provide frequency specific information simultaneously across frequency they give a rapid “panorama” of functional activity that can accelerate testing.
Academy: And, if you could only measure one, as in screenings, which would you pick?
Kemp: Tricky question. I presume you mean newborn screening where the aim is to pick out infants whose development of speech and language may be at risk? Well, then both techniques have proved to be very effective indeed, and can be equally sensitive. I’d say the choice boils down to more practical issues.
In the final analysis, it's got to do with the equipment, how the tests are implemented, and the quality of the ongoing feedback helping you optimize testing. At the moment, I am more partial to the transient, but if you're using a totally automated system and your eyes are closed it probably doesn't matter very much—provided the instrument designers have done their job well that is!
Academy: Okay, that's fair. What can you tell me about the Academy's upcoming Academy Research Conference on otoacoustic emissions?
Kemp: I've been assigned the role of “grandfather”of OAEs. I’ll be starting the conference by talking briefly about the history of the subject and how important clues can often be overlooked in auditory research. The featured speakers are just fantastic and the conference is destined to be very interesting. You can see from the witty titles in the program that speakers are going to keep a cap on the math and the technical jargon so as to communicate their wonderful scientific research to all of us.
We will hear how OAEs arise, what they can and can’t do for us in the clinic and what they might be able to do for us in the future. I’m looking forward to an excellent event.
Academy: Great, David. Thanks so much for your time and knowledge.
Kemp: My pleasure, Doug. It's an honor to chat with you, too.
David Kemp, PhD, is a professor of Auditory Biophysics at the Centre for Auditory Research, The UCL Ear Institute, London, England.
Douglas L. Beck, AuD, Board Certified in Audiology, is the Web content editor for the American Academy of Audiology.
At ARC you will be provided with a continental breakfast, boxed lunch, and a wine and cheese reception. Poster sessions are included in the program. The call for poster submissions will be open through February 1, 2009.