Dalhousie University Otoacoustic Emissions: Bekesy & Gold

• Otoacoustic Emissions is being looked at in the context of hearing measurement at Dalhousie University in 2025.

Georg von Békésy

• Joined Harvard in 1947 under Stevens.
• He partially dissected the Basilar Membrane (BM) in cadavers.
• He used a microscope with a strobe light to measure BM movements, precise to 1/1000th of a mm.
• Stroboscopy created a visual 'beat' effect due to the difference between two frequencies.
• He won the Nobel prize in 1961.
• His work also suggested that tuning wasn't sharp enough to account for human frequency discrimination, it was a smooth travelling wave.
• He spent his career working on a model of neural lateral inhibition to explain these human abilities.
• This model proved to be incorrect, but inspired similar work in the visual system leading to a different Nobel prize winner.

Thomas Gold

• Thomas Gold argued in 1948 that the passive cochlea model was not tenable.
• He proposed that an active mechanism was vital to add energy to the vibration.
• Thomas believed that people think whatever is learned in college or in the first few years thereafter is all to be learned in the subject and they are practitioners not having to learn anymore.
• Thomas wrote that ears make clear, clean-pitched noises which run into self oscillation, which is clearly the symptom of an ill-controlled active receiver.
• David Kemp proved this theory in 1978.
• The sound energy appears to be emitted by the auditory system, into the external ear canal, for some tens of ms after impulsive acoustic excitation.
• Outer Hair Cells (OHCs) are motile, move in response to sound and add energy to the mechanical movement of the BM.
• OHC damage from Furosemide interferes with their motility.

Cochlear Amplifier

• Enhances vibration of the basilar membrane near the displacement peak for softer inputs of less than 60 dB.
• Accomplished in two ways:
  • OHC somatic motility - cell bodies elongate and plays an important role in the generation of Otoacoustic emissions (OAEs), and in the ability to get any OAEs at soft levels.
  • Stereociliary transduction - the stereocilia also elongate in response to sound, amplify its movement and play an importnat part in the generation of some OAEs.

Types of Otoacoustic Emissions

• Spontaneous OAEs
• Evoked OAEs
  • Transient Evoked
  • Stimulus Frequency
  • Distortion Product

Spontaneous OAEs

• Present in most ears.
• Tend to be larger in females than males.
• Vary along the gender continuum.
• Frequencies are stable in a given individual.
• Amplitudes may fluctuate, being related to hormone levels.
• Occur most often between 1 and 2 kHz.
• Decrease with aging/hearing loss.
• Very rare to hear.
• If heard then they cause Tinnitus.

Transient Evoked OAEs

• The probe has a tiny speaker and a tiny microphone.
• A short signal is played to the ear, generally an 80 s click.
• The microphone records the sound that comes from the cochlea after the click is over.
• The sound produced by the cochlea is very soft
• It cannot record unless using a very sensitive microphone, and many responses are averaged
• The frequency of the output decreases over time
• It can only be picked up when each cochlear echo must happen after sound has travelled to a place in the cochlea and the echo then needs to travel back out (and through the middle ear)
• A click provides a place-specific response.
• The health of different places on the cochlea (i.e. the OHCs) can be inferred by the amplitude of the response.
• On the lower right is the frequency transform of a click-evoked OAE with green being the response, and red signifies noise.
• Great for infant hearing screening.
• Similar to SOAEs being larger in females, and largest between 1 and 2 kHz.
• TEOAEs may be present for mild loss, but highly unlikely to be present for moderate or greater loss
  • A TEOAE is present if there is no more than a mild loss.

Stimulus Frequency OAEs

• Correlated sounds vary together and if the sounds are correlated that are in phase then the resulting sound has twice the pressure.
  • A sound is 6 dB higher if it is perfectly correlated and in phase with with equal pressure.
• If phase correlated sounds are out of phase, then the sound has zero pressure.
• If there is 2 uncorrelated sounds, then the pressure differences between them are constantly different.
• There is no constant phase or pressure relationship
• The intensities are simply added.
• There is a 3db higher sound as a result
• If there is a correlation between the input sound and the sound from the cochlea then it depends on whether they are in or out of phase.
• If the cochlear "echo" emission is delayed by 5ms it would result in:
  • 100 Hz tone perfectly out of phase (no sound)
  • 200 hz tone perfectly in phase (+ 6dB)
• As the frequencies are run though, they will add, then cancel, add then cancel, etc
• This is the spectral periodicity
• Phase only matters when the levels are similar.
• Close in level is when the tone sweep is played ton the ear canal at low levels, and the emission sound are close in level and spectral periodicity occurs.

Distortion Product OAEs

• Instead of a noise, two tones are being played
• Called F1 and F2
• The cochlea produces a third frequency as a product of the distortion
• It is the emission
• Two separate speakers are uses for each primary
• These tones have overlapping displacement patterns on the basilar membrane
• The frequency ratio is usually 1.2 or 1.22 which is a constant ratio or distance on the BM
• It is where if F1 is 1000 Hz, F2 is 1200 or 1222 Hz that distortion is created near the F2 place where the travelling waves overlap the most
• The frequency of the distortion is primarily 2F1-F2 which is called the cubic distortion product

Otoacoustic Emissions Clinical Value

• OAEs are used in screening which provides a sensitive measure of cochlear health
  • OAEs are generally absent fo moderate or greater loss for TEOAES
  • Transient OAEs (TEOAEs) and distortion-product OAEs (DPOAEs) are most useful.

In General Clinical Practice

• Assesses the sensory organ of the cochlea (outer hair cells), providing specifics on the diagnosis
  • Assesses dysfunction in the cochlea such as Méniere's, ototoxicity,s and noise damage
  • Plays a critical role in determining if neuropathies are caused from sensory loss
  • Must consider the implications of normal function of the cochlea for hearing loss
  • Testing is rapid and objective

Tympanometer

• Gain must increase to keep a constant level at the microphone when less is reflected back (i.e, when admittance is high)

Anatomy of a Tympanogram

• Admittance when lots of positive pressure in teh ear (ear drum stiff) = external ear canal admittance in mmhos (or volume, in cc)
• Peak admittance: the TM and ossicles are maximally set in motion and occur here with atmospheric pressure.
• The peak admittance less the ear canal admittance is the admittance (often called compliance) of the middle area.
• Two Types of Tympanograms are Non-compensated and Compensated
• Complete typanogram has:
• OE Y, is often smaller (and M.E. Y, is larger) when using 300 or -400 daPa compared to +200 daPa.

Standard Values in 90% range

• Tympanometric peak pressure (TPP) in daPA is -100 tp + 50 daPa
• Equivalent ear canal volume: VEC in cm² is .6 to 2 cm³
• Middle ear admittance, or peak-compensated static acoustic admittance (or compliance) is: YTM (mmho) or .3 to 1.7 mmho
• Tympanogram width: TW (daPa) is: 51 to 114 daPa

Types of tympanograms"

• Have a normal A, B and C type
• Types can have a sub type
• A-S - shallow
• A-D - deep

On Non-Compensated Typanogram

• These will look similar to compensated tymanograms except for the difference being the ECV

The Acoustic Reflex

• BIlateral
• Can have contra or ipsilateral pathways.
• Reflex thresholds using SNHL can be normative with Gelfand
• Acoustic reflex site can have a lesion that affects different parts like the stimulus or Right measurement
• One can use the acoustic reflex to identify a site of lesion with a related worksheet.