Scottish Sensory Centre

Audiology Update

Presented on Thursday, 25 November 2010

Audiology - A Curriculum for Excellence

Brian Shannan
Educational Audiologist Sensory Support Service Fife
brian.shannan@fife.gov.uk

Curriculum for Excellence – Issues

• Less direct teaching
• Group work
• Active learning
• Independent learning

Issues for the Deaf

• Sensori-neural hearing loss
• Unilateral hearing loss
• Room acoustics
• Language level/deficit of children
• Hearing aid user
• Cochlear implant user

Some options – BUT not solutions!

•  Hearing aid programming
• FM systems & assistive devices
• Soundfield
• Classroom management/strategies
• How do we know what is best?
• A more comprehensive survey/questionnaire?

Hearing Mechanism Very Quick Overview

Anatomy of the Ear

Stages of the Basic Auditory Pathway

• Sound waves move the tympanic membrane
• Tympanic membrane moves the ossicles
• Ossicles move the membrane at the oval window
• Movement at the oval window moves the fluid in the cochlea
• Movement of the fluid in the cochlea causes a response in sensory neurons

Function of the Outer Ear

• Aids in localization
  noticeable differences in transfer function based on orientation of the pinna (Outer Ear)
• Aids in sound amplification
  resonance in concha
  resonance in external auditory meatus

Function of the Middle Ear

Middle ear cavity

Function of ossicles

• 99.9% sound is reflected due to high impedance of fluid in the cochlea (0.1% sound is only passed = - 30 dB sound loss from air - fluid impedance mismatch)
• Middle ear bones overcome the loss of sound by increasing sound pressure (+34dB)
• => Impedance matching

Notes: Here we have three middle bones again. The arm of the malleus is attached to the eardrum, and the footplate of the stapes is attached to the oval window of the cochlea (inner ear). What they are doing is they transmit vibrations of the eardrum into the cochlea. But here we have a problem. Because the impedance of air and impedance of liquid is really different. Which one is bigger? Impedance in the liquid is much bigger than that of air.

We can think of an example. Let's say we are in a swimming pool. And we are under water in a swimming pool. And then we cannot hear well the speech of outside even though the voice is loud. Try it later at the gym. That is because the impedance of liquid is so high, most of sound is reflected when the sound hits the water. And 99.9%, most of sound is lost. In other words, only 0.1% of power is passed.

That sound loss gives us -30dB sound level loss just because of impedance mismatch between air and liquid. But fortunately our middle ear bones overcome that sound loss. The process is called impedance matching because they are matching, making up that loss.

Then how does it happen?

Three mechanisms for impedance matching

1. Area ratio of the ear drum to the stapes footplate (20:1) => 20 log (20/1) = +26dB SPL * Basic concept: p = f/a
2. Lever action of the ossicles (1.3:1) => 20 log(1.3/1) = +2 dB SPL
3. Buckling of ear drum ( x 2 pressure increase => 20 log(2/1) = +6dB SPL

Notes: What they are doing is they are amplifying sound level to overcome mismatched impedance. It can work due to their physical structure.

1) First of all, we have a really big ear drum relative bones. Especially, ear drum is really big and stapes footplate is really small. Here as we can see, the area of ear drum is twenty times bigger than the area of stapes footplate. (Ear drum 60 mm2, stapes 3m2).

Using equation of decibel, we calcualte how much gain it boosts.

Area ratio is 20, so area of ear drum to area of stapes footplate 20/1 = 20 log (20/1)=26dB gain is boosted by this area ratio between ear drum and stapes footplate. (The same concept=> If we think about hitting a nail with a hammer, we put the force to the head of the nail. But the force is gonna be bigger at the point of nail. Why is it? Because when the same force is applied, then the pressure is gonna be incrased from larger to smaller area. p=f/a)

2) They work as like lever. Because as we can see in this picture, the arm of malleus is longer than that of incus. So different distance makes lever ratio. So this lever action gives another increase about 1.3 times which is equal pressure increase by 2dB. (What that means is that the stapes is displaced much less than TM. TM is displaced up to 2mm, but stapes is displaced by 0.1mm.)

3) buckling of the ear drum. As we saw before ear drum changes its shape in a complicate way when the sound hits ear drum. Each part of ear drum response to different frequency in a different way. So ear drum itself can increase force when ear drum moves. This buckling effects increase pressure by 6 dB (by a factor of 2). All together, these three factors provide 26dB+2dB+6dB = more 34dB gain (Or linearly, 20*1.3*2=by a factor of 52)

Cochlear Functions

• Transduction - Converting acoustical-mechanical energy into electro-chemical energy.
• Transduction occurs in the Organ of Corti
  The Organ of Corti resides on the basilar membrane between the scala media and scala vestibuli
• Frequency Analysis - Breaking sound up into its component frequencies

Organ of Corti

• The organ of Corti sits on the basilar membrane
• The basilar membrane and the tectorial membrane are fixed at different locations to the modiolus
• It is made up of:

1. The tectorial membrane
2. Three rows of outer hair cells
3. One row of inner hair cells
4. The basilar membrane
5. Supporting cells

• An analogy of a house has been used in which the basilar membrane is the base, tectorial membrane is the roof and the reticular is the structure holding the hair cells

Perceptual effects of sensori-neural hearing loss

• Raised threshold for hearing
• Reduced dynamic range
• Reduced frequency discrimination
• Increased susceptibility to noise

IHCs, OHCs And Their Stereocilia

• OHCs (at top)
  3, 4 or 5 rows
  Approx 12,000 cells
  V- or W-shaped ranks of stereocilia
  50 to 150 stereocilia per cell
• IHC (at bottom)
  1 or 2 rows
  Approx 3,500 cells
  straight line ranks of stereocilia
  50 to 70 stereocilia per cell

Outer hair cells

• Have a role in achieving high sensitivity and sharp tuning
• Improves Sensitivity for soft sounds
• Improves frequency resolution
• Most of the efferent neurons synapse directly with the outer hair cells
• Efferent neurons carry information from higher auditory system to cochlea
• Afferent neurons carry information from the cochlea to the higher auditory system

Inner hair cells

• Causes the release of neurotransmitter and the initiation of action potentials in the neurons of the audtiory nerve.
• Action potential – 'firing'of a neuron. Propagation is in one direction only down the length of the axon
• Most of the afferent neurons make contact with the inner hair cells
• Possibly all information about the input sound is conveyed via the inner hair cells

How The Cochlea Functions

• When sound enters the cochlea a travelling wave moves along the basilar membane
• The amplitude gradually rises before reaching a maximum at its point of resonance (characteristic frequency) beyond which it collapses abruptly

Basilar Membrane (BM)

• The BM has two structural properties that determine the way it responds to sound.
• BM is narrow and stiff at its base and becomes broader and more flexible towards the apex
• BM a mechanical frequency analysis - separating the incoming sound signal into its frequency components, - processed at different locations along the length of the cochlea
• High frequency sounds are processed at the base. Low frequency at the apex

Bekesy's Theory describes Passive Mechanics

• Based on work in 'dead' cochleae
• Highly damped - not sharply tuned
• Active Undamping occurs in live and healthy cochleae
• Like pumping on a swing - adds amplitude

Transduction by Hair Cells

• When the basilar membrane moves in response to a motion at the stapes, the entire foundation supporting the hair cells move, because the basilar membrane, rods of Corti, reticular lamina and hair cells are all rigidly connected
• These structures move as a unit pivoting up or away from the modiolus
• This set up a shearing motion of the hair cell sterocilia

'Transduction process': mechanical energy into electrical energy

• Depolarization Decreases Length OHC
• Hyperpolarisation increases length OHC

Implications

• Damage is it OHC/IHC?
• DSP hearing aids – gain from OHC
• What if significant IHC damage?

Amplification

• OHC constitute a cochlear amplifier
• When the outer hair cells amplify the response of the basilar membrane, the stereocilia on the inner hair cells bend more, and the increased transduction process in the inner hair cells produce a greater response in the auditory nerve.
• Gain from OHC may be 50 dB at low and medium sound levels

Frequency Tuning Curves Show these Effects

• They have a characteristic shape
• sharp tip (shows best sensitivity at one freq)
• steep high frequency tail
• shallow low frequency tail

OHC and Frequency Slectivity

• shape of the tuning curve changes drastically when the sensory hair cells are damaged. Instead of a sharp tip region, the frequency selectivity is broadened
• Reduced frequency discrimination
• Increased susceptibility to noise

Dynamic Range Compression

• Normal hearing dynamic range is about 120 dB
  The loudest sound has an amplitude 1 million times as great as the quietest sound we can hear!
• Nerves have about a 30 dB range
• OHC feedback compresses the auditory signal
• Amplification is known to be highly nonlinear

OHC and OAE

• The cochlear amplifier generates its own sounds
• These sounds can be detected and form the basis for OAE assessment

Implications

• CI different to hearing aid user
• OHC/IHC bypassed
• Noise a different issue

Sound Localisation Two Ears are better than one

Outer Ear - Auditory Localisation

• Auditory space - surrounds an observer and exists wherever there is sound
• Sounds are localized in space by using
  • Azimuth coordinates - position left to right (binaural cues: ITD and IID)
  • Elevation coordinates - position up and down (pinna spectral cues and head movements)
  • Distance coordinates - position from observer - (inverse-square law, reverberation)

Azimuth, elevation, and distance coordinates for localization. Two elevation coordinates are shown, one (M) in which the vertical coordinate is positioned on the person's midline, and the other (S), which is off to the side.

Auditory Localisation

• Location cues are not contained in the receptor cells like on the retina in vision; thus, location for sounds must be calculated using cues
• The direct sound carries information about the location of the source relative to the listener.
• Indirect sound informs the listener about the space, and the relation of the source to that space.

Interaural Time Difference

The principle behind interaural time difference (ITD).

• The tone directly in front of the listener, reaches the left and the right ears at the same time (A).
• However, when the tone is off to the side (B) it reaches the listener’s right before it reaches the left ear.

Schematic illustration of interaural differences

Interaural Time Difference (ITD)

• interaural time delay applies to low frequency localisation – less than approximately 1500Hz.
• The average distance between the ears is 20cm resulting in a 600 microsecond delay between hearing the incident sound in one ear and hearing in the other
• Interaural time difference - difference between the times sounds reach the two ears
  When distance to each ear is the same, there are no differences in time
  When the source is to the side of the observer, the times will differ

Interaural Intensity Delay

• IID is dependent on frequency
• If the wavelength is equal to or greater than the width of the head, the sound will bend or diffract around the head and be heard with almost equal intensity in the opposite ear

Interaural Intensity Difference

• However, for higher frequencies there is absorption of sound energy by a solid medium (head)
• Resulting in sound shadow, region of effectively zero energy

Sound Localization

Interaural Level Difference

Schematic illustration of interaural differences

Interaural Intensity difference

• Head Shadow Effect (Interaural Intensity difference) - Localisation of high frequencies (above approximately 1.5 kHz) is dependent upon the head shadow effect.
• The head casts a sound shadow, which in turn attenuates sound by at least 6dB between the two ears. However, can reach 20dB at higher frequencies
• If a high frequency sound is perceived in one ear at a significantly higher intensity than the other ear, the brain concludes that the sound has originated from the higher intensity side

Outer Ear Vertical Localization

Vertical localization - based on reflections from the pinna

Vertical Localisation

• Vertical localisation is achieved using pinna echoes.
• Sound from below will produce a slightly more delayed echo (about 300 microseconds) than if the sound came from above (echo after about 100 usecs)
• Such echoes are involved in the frequency range 3.3 to 10kHz.
• The bumps and ridges of the outer ear apparently produce reflections of the entering sound. The delays between the direct path and the reflected path make vertical localization possible (Bear et al 1996)
• Vertical localisation deteriorates markedly with hearing loss
• High frequencies are especially affected by vertical localisation

Significance of Sound Localisation

Localisation is important in:

• Speech perception in background noise
• Communication in background noise
• Safety

Binaural Squelch

• Ability to suppress background noise and attend to a specific auditory signal.
• Fortunately, the auditory nervous system is wired to help in noisy situations as long as there is functional input from both ears, that is, the auditory system and brain can combine information from both ears so that there is a better central representation than would be had with only information from one ear
• The squelch effect takes advantage of the spatial separation of the signal source and the noise source(s) and the differences in time and intensity that these create at each ear.
• Speech recognition in such noisy environments is even harder for a person with sensorineural hearing loss both because of the inherent distortion and loss of normal nonlinearities introduced by cochlear damage

References

Bess & Humes (2003) Audiology: The Fundamentals (3rd Ed).

Lippincott Williams and Wilkins Bear M F, Connor B W & Paradiso M A (1996) Neuroscience – Exploring the Brain.

William & Wilkins Dallos, P, Zheng, J, Cheatham, MA, (2006) Prestin and the cochlear amplifier. Journal of Physiology vol 576, pp 37–42.

Durrant & Lovrinic (1995) Bases of Hearing Science (3rd Ed)

Lippincott Williams and Wilkins Frolenkov G I (2006) Regulation of electromotility in the cochlear outer hair cell, Journal of Physiology 538 pp 43-48

Fettiplace R & Ricci A J (2003) Adaptation in auditory hair cells Current Opinion in Neurobiology vol. 13 pp 446–451

Fettiplace R (2006) Active hair bundle movements in auditory hair cells, Journal of Physiology 576 pp 29-36.

Hibino H & Kurachi Y (2006) Molecular and Physiological Bases of the K+ Circulation in the Mammalian Inner Ear, Physiology vol 21 pp 336-345

Hudspeth A J (1997) Mechanical amplification of stimuli by hair cells, Current Opinion in Neurobiology 1997, vol 7 pp 480–486

Li Z, Anvari B, Takashima M, Brecht P, Torres JH, Brownell WE (2002) Membrane tether formation from outer hair cells with optical tweezers. Biophysical Journal vol 82 pp 1386-1395.

Kennedy H J, Evans M G,Crawford A C & Fettiplace R (2006) Depolarization of Cochlear Outer Hair Cells Evokes Active Hair Bundle Motion by Two Mechanisms The Journal of Neuroscience, Vol 26 pp 2757–2766

Liberman M C, Gao J, He D Z, Wu X, J S & Zuo J (2002). Prestin is required for electromotility of the outer hair cell and for the cochlear amplifier. Nature vol 419, pp 300–304.

Moore B C J (1997) An Introduction to the Psychology of Hearing (4th Ed) Academic Press

Oghalai J S (2004) The cochlear amplifier: augmentation of the traveling wave within the inner ear Current Opinion in Otolaryngology & Head and Neck Surgery vol. 12 pp. 431–43

Pickles J O (1987) An Introduction to the Physiology of Hearing Academic Press

Ren T & Gillespie P G (2007) A mechanism for active hearing Current Opinion in Neurobiology vol 17 pp 498–503

Robinson D (2002) Audio coding: 3-dimensional stereo and presence The human auditory system University of Essex

Santos-Sacchi J (2003) New tunes from Corti's organ: the outer hair cell boogie rules. Current Opinion in Neurobiology 13 pp 459-468.

Ulfendahl M & Flock A (1998) Outer Hair Cells Provide Active Tuning in the Organ of Corti News In Physiological Sciences vol 13 pp 107-111

Yates G K & Kirk D L (1998) Cochlear Electrically Evoked Emissions Modulated by Mechanical Transduction Channels The Journal of Neuroscience vol 18 pp 1996–2003

• www.yaleearlab.org/
• www.neurophys.wisc.edu/animations/
• www.neuroreille.com/promenade/english/corti/hcells/fhcells.htm

Where next

• Audiology update