Download Visuals (powerpoint) for Lecture #20, 02/25/13

P105 Lecture #20 visuals
25 Feburary 2013
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.,
2
presentation on cochlear implants
loud
Hearing
threshold
of a severely
deaf person
Hearing threshold
of a profoundly
deaf person
(ex: Shipsey)
soft
3
The Ear Has Three Distinct Regions
Slide from Ian Shipsey, Purdue U.,
presentation on cochlear implants
ca. 175 A.D. Galen
ca. 550 B.C.
Pythagoras &
successors
Nerve transmits
sound to the brain
It has taken until the present
to unravel the rest
4
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
The tympanic membrane & ossicles
1543
Anatomist
Andreas Vesalius
describes the
structure of the
middle ear.
Slide from Ian Shipsey, Purdue U.,
7
presentation on cochlear implants
Why is our “sound
sensor” not on the
outside of our head?
Hermann Ludwig von
Helmholtz first to
understand the role of the
ossicles ( 1860’s)
Impedance mismatch overcome
by ratio of areas and lever action
Slide from Ian Shipsey, Purdue U.,
8
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”
The cochlea and its chambers
Slide from Ian Shipsey, Purdue U.,
presentation on cochlear implants
The cochlea is about the size of a pea
1561 Gabriello Fallopio
discovers the snail-shaped
11
cochlea of the inner ear.
The Cochlea houses the Organ of Corti
Auditory
Nerve
Slide from Ian Shipsey, Purdue U.,
12
presentation on cochlear implants
Organ of Corti
Slide from Ian Shipsey, Purdue U.,
presentation on cochlear implants
Hair Cells are mechano-electric
transduction devices
1st detailed study of
Organ of Corti
by Alfonso Corti
Original figures (scanned) from:
Zeitschrift für wissenschaftliche
Zoologie (1851)
13
End of Early History
The Middle Ages
Georg von Békésy
(Nobel 1961)
Hermann Ludwig von
Helmholtz first theory of the
role of BM as a spectrum
analyzer providing a
frequency-position map of
sound Fourier components.
Experimentally measured
traveling wave profiles
published by von Békésy
in Experiment in Hearing,
McGraw-Hill Inc., 1960.
Slide from Ian Shipsey, Purdue U.,
presentation on cochlear implants
base
apex
14
Tonotopic Organization
Slide from Ian Shipsey, Purdue U.,
15
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!
The Copernican Revolution
Slide from Ian Shipsey, Purdue U.,
presentation on cochlear implants
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.
displacement
Davies (1983): a revolutionary new hypothesis
there exists an active process within the
organ of Corti that increases the vibration
of the basilar membrane.
17
Active amplification
Careful measurements on living animal cochlea
Same animal post mortem
What causes the
amplification?
Johnstone et al (1986)
Slide from Ian Shipsey, Purdue U.,
18
presentation on cochlear implants
Rows of Hair Cells in the healthy cochlea
Inner hair cells 10,000 afferent
(signals go the brain)
Outer Hair Cells 30,000 Sparsely
innervated
Hair
5 m
30 mHair cell
Slide from Ian Shipsey, Purdue U.,
19
presentation on cochlear implants
Hair cells are mechano-electrical transducers
1980’s
500 nm
Both inner and outer hair cells work this way
2nm diameter
20
The inner hair cells send signals to the
brain that are interpreted as sound. What
do the outer hair cells do?
Outer hair cells exhibit electro motility
they are also electro-mechanical
transducers
1987-2003
Slide from Ian Shipsey, Purdue U.,
21
presentation on cochlear implants