P105 Lecture #20 visuals

25 Feburary 2013

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Acoustic Pressure is measured in decibels (dB)

• 1 atm = 100,000 pascals = 1011 micropascals• Threshold: the softest sound detectable is 20 micropascals (at

1000 Hz). 2 parts in 10 billion of an atmosphere • We hear sounds 1-10 million times more intense than threshold• dB are logarithmic units with 0 dB at threshold• adding 20 dB = factor of 10 increase in pressure

– 6 dB approximately doubles the pressure

• 40 dB SPL = 20 x 100 = 2,000 micropascals

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Hearing thresholdof a profoundlydeaf person (ex: Shipsey)Hearing

thresholdof a severelydeaf person

soft

loud

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The Ear Has Three Distinct Regions

ca. 550 B.C. Pythagoras & successors

ca. 175 A.D. Galen

Nerve transmits sound to the brain

It has taken until the present to unravel the rest

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

Auditory System Physiology

Illustration from E.J. Heller, “Why you hear what you hear”

3D Rendering of Auditory Transduction System

• Show video “Auditory Transduction”, by Brandon Pletsch. (This video was awarded 1st prize in the 2003 NSF/AAAS Science & Engineering Visualization Challenge)

http://www.youtube.com/watch?v=46aNGGNPm7s

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1543

Anatomist Andreas Vesalius describes the structure of the middle ear.

The tympanic membrane & ossicles

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

8

Why is our “sound sensor” not on the outside of our head?

Impedance mismatch overcome by ratio of areas and lever action

Hermann Ludwig von Helmholtz first to understand the role of the ossicles ( 1860’s)

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

Pressure Amplification in middle ear

Lever action of ossicles (gives 1.5x amplification of force)

Ratio of areas of oval window to tympanum (20x amplf’n of pressure

Illustration from E.J. Heller, “Why you hear what you hear”

Inner Ear

Illustrations from E.J. Heller, “Why you hear what you hear”

11

The cochlea and its chambers

1561 Gabriello Fallopio discovers the snail-shaped cochlea of the inner ear.

The cochlea is about the size of a pea

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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The Cochlea houses the Organ of Corti

AuditoryNerve

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Organ of Corti

1st detailed study of Organ of Cortiby Alfonso CortiOriginal figures (scanned) from: Zeitschrift für wissenschaftliche Zoologie (1851)

Hair Cells are mechano-electric transduction devices

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Georg von Békésy (Nobel 1961)

The Middle Ages

Experimentally measuredtraveling wave profiles published by von Békésy in Experiment in Hearing, McGraw-Hill Inc., 1960.

End of Early History

Hermann Ludwig von Helmholtz first theory of the role of BM as a spectrum analyzer providing a frequency-position map of sound Fourier components.

base apexSlide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Tonotopic Organization

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

Critical Bands & Pitch Determination• Can think of the 3.5-cm long Basilar Membrane as being divided

into 10 regions of 3.5 mm each providing sensitivity to ~10 octaves.

• The region of the basilar membrane excited by a pure tone of given frequency is wide: ~ 1.5 mm – “Critical Band”; region corresponds to just under 3 semitones (frequency range of about 18%), where 12 semitones = 1 octave.

• “just-noticeable difference” = ~ 1/10th of a semitone (i.e., ~ 0.6% difference in frequency)

• Interplay between physiological effects of signal sent to brain and signal processing by the brain are complicated and important!

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The Copernican Revolution

Von Békésy's findings stimulated the production of numerous cochlear models that reproduced the observed wave shapes, but were in contrast with psychophysical data on the frequency selectivity of the cochlea.

Davies (1983): a revolutionary new hypothesis there exists an active process within the organ of Corti that increases the vibration of the basilar membrane.

displacement

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Active amplification

Careful measurements on living animal cochlea

Same animal post mortem

Johnstone et al (1986)

What causes the amplification?

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Inner hair cells 10,000 afferent (signals go the brain)

Outer Hair Cells 30,000 Sparsely innervated

Rows of Hair Cells in the healthy cochlea

Hair cell30m

5m

Hair

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants

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Hair cells are mechano-electrical transducers

Both inner and outer hair cells work this way

500 nm

2nm diameter

1980’s

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The inner hair cells send signals to thebrain that are interpreted as sound. What do the outer hair cells do?Outer hair cells exhibit electro motilitythey are also electro-mechanical transducers

1987-2003

Slide from Ian Shipsey, Purdue U., presentation on cochlear implants