Otoacoustic Emissions

dr. Ted venema talks audiology the educational whiteboard series brought to you by next-gen hearing hi I'm Ted venema here to talk to you today about Auto acoustic emissions this can be construed as a counterpart to the tests of acoustic reflexes that we spoke about last time you'll recall last time when we talked about acoustic reflexes we said that was a non behavioral test part of tympanometry and it was used to test the function of the acoustic reflex a non behavioral type of phenomenon that occurs the tightening of the little ossicles of the middle ear ossicles now the acoustic reflex arc we said was was an afferent thing the inner hair cells speak to the brain they send information to the brain the outer hair cells take info from the low brainstem and serve in a more affair n't motoric type of function well out otoacoustic emissions is specifically a test of outer hair cell function whereas the acoustic reflex can be thought of as a test of inner hair cell function at any rate the acoustic reflex and tympanometry arose or became popular in clinical practice in the 1970s otoacoustic emissions on the other hand occurred about 20 years later or the clinical testing thereby their off became popular in the 90s so the 90s is actually quite an exciting decade in the hearing health field and here's why that was the time when the different roles of the inner versus outer hair cells became popularized research had looked at it earlier but it only became merged really into clinical practice and it's fingers were felt all the way through rippling through ripples of audiology and it even had implications for hearing aid design which is really quite exciting at any rate the inner hair cells a ferrant the outer hair cells efferent now figure 1 shows you this you can look at that figure and you can see the inner hair cells within the scale of media of the cochlea and you'll see the outer hair cells in rows of three shown in that figure also housed within the scale of media at any rate when we take a look at basically what the ear is doing sound hits the tympanic membrane and then the middle ear changes or transduces the sound waves into mechanical piston-like motion which wiggles the whole asik you lurch Ain and that vibration of the ossicular chain creates a tiny traveling wave inside the cochlea inside the fluids and the basilar membrane which is the floor upon which the hair cells all stand at any rate it's that hydraulic motion that excites hair cells which one specifically inner hair cells and that hydraulic motion is changed or transduced by the inner hair cells into electrical impulses which are sent up the eighth nerve to the brain well the the cochlea is what we call tonal topic in English this means specific frequencies are represented in specific places in the cochlea to be specific at the wide base of the cochlea treble or high frequencies are represented at the narrow apex it's the lows that are represented this little diagram I drew here is showing you the role of the outer hair cells in specifically at any rate if the traveling wave excites the hair cells at the base of the cochlea you hear high pitch if it excites the hair cells at the apex you hear a low you hear low pitches so that's what we call the frequency representation inside the cochlea it's tonotopic but the fun begins when we look at how intensity is represented or neroli coded in the cochlea you'll see this traveling wave I drew here it has like a little peak and that's stimulating some low frequency portion of the cochlea well it'll have its little peak but the outer hair cells do two things here first they amplify the peak of that travelling wave and secondly they sharpen the peak of the traveling wave why does this happen what's this for well inner hair cells by themselves cannot sense soft sounds below conversational speech loudness they can't pick it up there's the fluid motion isn't enough to excite them they need the mechanical action of the outer hair cells to help them sense soft sounds now the outer hair cells do that that the cochlea is an amplifier in that way and like all amplifiers you've got to pay the piper because all amplifiers distort and guess what the distortion created by the outer hair cells amplifying the peak of the travelling wave that distortion is auto acoustic emissions sometimes they're called distortion product Auto acoustic emissions they are the the the byproduct of the outer hair cells working their tails off you can think of it like you can have a light light bulb which passes off a lot of light but it also emits a bit of heat as a by-product well otoacoustic emissions are like that a byproduct now I said the travelling wave is also sharpened its peak is sharpened as well by the outer hair cells and this has big implications as well because if you didn't have that sharpening you'd have many different hair cells stimulated across a wide frequency range in the cochlea the outer hair cells sharpening serves to increase the frequency resolution inside the cochlea making it easier for the person to separate or distinguish among frequencies that are close together if you don't have the outer hair cells it's like you're combing your hair with about 4 teeth right when you've got outer hair cells your comb has way more teeth and you have way better frequency resolution inside the cochlea it's amazing what takes place inside that little snail shaped cochlea and hearing aids today they can't sharpen the peak of a travelling wave but they can amplify and focus their amplification on soft sound specifically to imitate the role of the outer hair cells because guess what the outer hair cells because they're the moving part in the cochlea they tend to die first and any in any system the moving part tends to go first just like in a CD player it's the thing turning the CD that's going to go first that's the mechanical part well with outer hair cell damage you have presbycusis the inability to hear soft sounds below 50 dB that's why presbycusis results in a mild to moderate sensory neural loss its damage to the outer hair cells of the cochlea not really the inner hair cells so much mostly the outers so now let's cut to the chase as as to how we measure Auto acoustic emissions all amplifiers we said distort so you've got otoacoustic emissions how do we measure them well like tympanometry we put a probe in the ear canal it doesn't have to be airtight like in tympanometry because you're not changing air pressure inside the ear canal the room just has to be generally quiet now like tympanometry figure 2 will show you it's got three holes two of the holes are tiny speakers and they emit each of them a separate tone the third hole is actually a microphone that picks up the resultant distortion product ohto acoustic emission here's how it works the client is seated the probe is put into the ear canal and the two frequencies are put out now it's very interesting the two tones research has discovered the tones need to be separated from each other in terms of their frequency or pitch they have to be separated by a one to one point to two ratio so for example if one tone is a thousand Hertz the second tones got to be twelve hundred and twenty Hertz these two tones are put into the ear out of the two little holes in the probe at about 62 d DB SPL they're put out at the end of the ear and a third tone is picked up by the microphone that's the otoacoustic emission and in humans the strongest otoacoustic emissions happens to be at 2 F 1 minus F 2 well in English again that means two times a thousand Hertz the frequency of the first tone which is going to be 2,000 minus the frequency of the second tone and that's at that particular frequency then that the otoacoustic emission will be if the two tones are twelve hundred and twenty and a thousand Hertz what we do in audiology is we emit pairs of tones surrounding 250 Hertz pairs of tones around 500 Hertz pairs at around a thousand pairs at two pairs at four and in that way we test the auto acoustic emissions resulting across the frequency range of human hearing it's quite amazing so figure 3 shows you then how the distortion product otoacoustic emission is actually lower than frequency 1 and frequency 2 you can see on this figure that they'll have a frequency of f1 and a frequency of f2 and then lower in frequency is the distortion product otoacoustic emission and I stress the word lower so that people won't think all it that maybe that emission is just a harmonic of the two tones ah uh it's not a harmonic it's a intermodulation type of distortion and it's unique to the cochlea but not all that unique in the fact that the cochlea is an amplifier and all amplifiers distort and that distortion is done because of the amplification properties of the outer hair cells not the inners otoacoustic emissions you can think of them as like the ear in Reverse their sounds are actually coming back out of the cochlea wiggling the ocular chain and then making the eardrum act like a speaker it's the ear in Reverse and one might think well how come we can't hear them well thank God we can't here's why sound coming when when sound comes into the ear the eardrum is large and then the footplate of the stay piece and into the oval window of the cochlea is small pressure over a big area is converged onto pressure onto a small area or a force over a big area is converged on to force onto a small area which increases pressure when that's what the purpose of having a middle ear is in the first place well when sounds go out of the middle ear the reverse takes place the solo acoustic emission becomes softer because sound over a small area becomes spread over a big area so that's incidentally why we don't hear our old acoustic emissions however another thing to be concerned or to realize is never confuse otoacoustic emissions with tinnitus they're not otoacoustic emissions are an actual measurable sound that's picked up by the probe ok tinnitus is a symptom report to by a client and only him or her that this only that person hears it and it's driving them bonkers but it's not like a physician is just able to sit there and listen to someone's tinnitus unless of course it's pulsatile and then of course we know that it's more vascular in origin but that's just something to take note of applications of otoacoustic emissions fantastic they are used mainly for infant hearing screening an infant can't tell you what he or she hears otoacoustic emissions are tested along with tympanometry both of them are non behavioral tests why we're whereby to assess the the hair cell integrity otoacoustic emissions for outer hair cells the acoustic reflex for inner hair cells and here's why they are both good cross tests both of them are obliterated by middle ear pathology together the middle ear is going to prevent the acoustic reflex but it's also going to vent the otoacoustic emission from exiting out through its space applications again infant hearing screening and also they can be used as a way to test people who cannot or will not respond behaviorally to pure tones when they're represented so they are wonderful as a screening test they're not very good at testing the degree of hearing loss when you've once you've got about a 30 40 DB hearing loss euro e's are gone remember they're only picking up soft sound so you don't have to have much of a hearing loss in order to have okay acoustically I mean otoacoustic emissions gone but they are wonderful as a screening test I close with this that we had the in the very these are the very same two boxes and I left them alone on the proof from the previous whiteboard on the acoustic reflexes I want to underline this point the word sensory neural in sensory neural hearing loss has two parts the sensory and the neural the sensory can be construed of as the outer hair cells of the cochlea good speech recognition you'll still have that you'll have a hearing loss but you'll have pretty good speech recognition and present acoustic reflexes albeit at reduced sensation levels but still you'll have them inner hair cell damage isn't as common but some unfortunate few do have it an inner hair cell damage will result in poor speech recognition ability and absent acoustic reflexes so you've got the aku's the otoacoustic emissions whereby to assess the integrity of the outer hair cells the neural a ferrant going portion of sensory neural loss is the inner hair cells when once someone has a severe sensory neural hearing loss more than a mile to moderate it gets down into severe then outer hair cells are damages accompanied by some inner hair cell damage outer hair cell damage results in up to about a 50 60 decibel sensory neural loss if someone presents with an 80 decibel sensory neural loss chances are or it's highly likely that not only our outer hair cells damage but now also some inner czar as well and that goes a long way to explaining why people with severe sensory neural hearing loss have poorer speech recognition abilities it all fits together like a bigger pizza pie anyway thanks for listening it's been a slice you

Channel: Dr. Ted V

Views: 41,095

Rating: 4.9658117 out of 5

Keywords: Otoacoustic Emission, Health (Industry)

Id: 3wy0DE03l8I

Length: 16min 20sec (980 seconds)

Published: Tue May 12 2015