Audition 1: Sound and the Ear

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Audition 1 Sound and the Ear

• 1st Year PPP/EP Neurophysiology Prelim
• Dr. Jennifer Bizley
• Jennifer.bizley_at_dpag.ox.ac.uk

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On the Menu for this Lecture

• Physical properties of sound (frequency,
  amplitude, bandwidth) and their psychophysical
  correlates.
• Transmission of sound through outer and middle
  ear
• Filtering of sound by the outer ear,
• Impedance matching in the middle ear.
• Structure and function of the inner ear
• Cochlear mechanics, travelling wave
• Transduction by hair cell receptors
• Active mechanisms, role of outer hair cells

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Sound As a Pressure Wave

• Vibrations of objects set up pressure waves in
  the surrounding air.
• The elastic property of air allows these
  pressure waves to propagate (spread).

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Pure Tones (Sine Waves)

• For pure tones, the tone frequency is directly
  related to the pitch of the sound, and the
  amplitude is related to perceived loudness.
• These relationships are logarithmic. Hence octave
  and decibel scales every doubling of frequency
  increases pitch by one octave, every doubling of
  amplitude increases the loudness by 6 dB.

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Harmonic Complexes (Complex Tones)

• Most natural oscillators do not just emit simple
  tones of a single frequency. For example, an
  ideal string in a string instrument would
  maintain the triangular shape set up when the
  string is plucked throughout the oscillation.
• Fourier analysis makes it possible to approximate
  the these vibrations as a sum (series) of sine
  wave vibrations. For large numbers of sine waves
  (Fourier components) the approximation becomes
  very good, for an infinite number of components
  it is exact.
• Here, as is often the case for natural sounds,
  all sine components are harmonically related i.e.
  their frequencies are all integer multiples of a
  fundamental frequency.

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Fourier Spectra

• Because every sound can be decomposed into sine
  (or equivalently, cosine) components, each sound
  has a frequency domain description, which,
  instead of describing the waveform as a function
  of time, states the amplitudes (and phases) of
  the cosine components of the sound. This
  frequency domain description is known as the
  (Fourier) spectrum.

• sequence

• 500 Hz

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Broad Band and Narrow Band Sounds

• Sounds in which a relatively small number of
  components contain most of the energy are called
  narrow band. The pure tone is an extreme
  example. Narrow band sounds are more or less
  periodic and may evoke an identifiable pitch.
• Broad band sounds contain very many components of
  similar amplitude and often do not evoke a strong
  pitch (although there are exceptions). Noises and
  clicks are typical examples of natural broadband
  sounds.

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The Spectrogram

• The spectrum is a complete description of a sound
  only if the frequency composition of the sound is
  constant over time. However, natural sounds
  usually do vary with time. To deal with this,
  sounds can be divided into short time segments,
  and spectra calculated for each time segment in
  turn. The result of this analysis is called a
  spectrogram.

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The Neurogram

• As a crude approximation, one might say that it
  is the job of the ear to produce a spectrogram of
  the incoming sounds, and that the brain
  interprets the spectrogram to identify sounds.
• This figure shows histograms of auditory nerve
  fibre discharges in response to a speech
  stimulus. Discharge rates depend on the amount of
  sound energy near the neurons characteristic
  frequency.

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The Long Road from Spectrogram to Auditory Scene
Analysis

• The Neurogram idea is deceptively simple, and
  does not capture some of the fine scale temporal
  encoding the auditory system is capable of.
• It is nevertheless clear that the job of the
  auditory system is to perform some sort of
  spectro-temporal analysis to identify and
  localise auditory objects.
• The auditory system is astonishingly good at this
  job. To appreciate how hard this is, try to guess
  what this sound to the left represents.

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The Human Ear
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What Does the External Ear Do?
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Spectral Cues For Sound Location
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The Middle Ear Is an Air-filled Cavity
Nasopharynx
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Impedance Matching by the Middle Ear Difference
in Membrane Area
Middle ear mechanics
AIR
FLUID
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Middle Ear Ossicles and Muscles

• Malleus
• Malleus ligament
• Incus
• Incus ligament
• Stapes muscle (stapedius)
• Stapes footplate
• Eardrum
• Eustachian tube
• Malleus muscle (tensor tympani)
• Nerve (chorda tympani) sectioned.

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Middle Ear (Stapedius) Reflex
Activated by intense sounds
INACTIVE STAPEDIUS
ACTIVE STAPEDIUS
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The Inner Ear ComprisesFluid-filled Chambers
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Cross Section of Cochlea
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Cross Section of Cochlea
PERILYMPH
ENDOLYMPH
PERILYMPH
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Cross Section of Cochlea
PERILYMPH
ENDOLYMPH
PERILYMPH
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Propagation of Mechanical Energy in the Cochlea
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Travelling Wave Along the Basilar Membrane
Von Békésy
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Travelling Wave Peaks at Different Locations As
the Frequency Changes
Base
Apex
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Frequency Tuning Along the Basilar Membrane
Base
Apex
Base
Apex
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Frequency Tuning Along the Basilar Membrane
LOW FREQUENCY
Stiffness decreases from base to apex
HIGH FREQUENCY
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Movement of theOrgan of Corti

• Up-down movement of the basilar membrane causes
  the tectorial membrane to slide sideways over the
  membrane, causing a sideways displacement of the
  hair cell bundles in the cochlear hair cells.

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Hair bundle movements

• Tip-link between
• adjacent stereocilia

• BASILAR
• MEMBRANE
• MOVES DOWN

• BASILAR
• MEMBRANE
• MOVES UP

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Transduction by Hair Cells

• The stereocilia of the hair cell bundle are
  connected by tip links.
• Movement of the bundle is thought to change the
  tension on the tip links, thereby opening or
  closing stretch sensitive K channels.
• K channel opening causes an influx of K from
  the endolymph, depolarisation of the hair cell
  membrane, opening of V-gated Ca channels and an
  increased probablility of transmitter release.

K
Endolymph
K
80 mV
Reticular lamina
Voltage-gated Calcium channel
-50 mV
Hair cell
Ca
Perilymph
Auditory Nerve fibre
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Stretch receptors associated with the tip links

• Source Ashmore (2004) Nature 432, 685 - 686

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Receptor Potentials

• At low frequencies the membrane potential of the
  hair cell follows every cycle of the stimulus (AC
  response, top).
• At high frequencies the membrane potential is
  unable to follow individual cycles, but instead
  remains depolarised throughout the duration of
  the stimulus (DC response, bottom).
• At intermediate frequencies the membrane
  potential exhibits a mixed AC DC response.

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Inner and Outer Hair Cells
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Organ of Corti after antibiotics

• NORMAL

• KANAMYCIN

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Active Mechanisms Contributing to Cochlear
Mechanics

• OHC change length when stimulated.
• Movement of the cilia causes depolarisation, as
  in IHCs.
• Depolarisation triggers a rapid change in length.

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The outer hair cells special trick
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Active Mechanisms Contributing to Cochlear
Mechanics

• The length change is mediated by an unusual motor
  protein (probably Prestin) in the OHC membrane.
• OHC motility produces a localised amplification
  of the basilar membrane motion, leading to higher
  sensitivity and sharper frequency tuning.
• It is also a source of non-linearity weak
  stimuli are amplified more effectively than
  strong ones.

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Summary 1

• Sound is a pressure wave set up by oscillations
  of a physical object.
• Every sound can be thought of as a sum of sine
  waves. The frequency composition of the sound
  (spectrum) can be highly informative about the
  sound source.
• The outer ear funnels sound to the ear drum and
  filters broad band sounds to generate spectral
  cues to sound location.
• The middle ear provides impedance matching,
  using piston and lever actions to facilitate the
  passage of the sound wave from air to fluid
  (endolymph) in the inner ear.

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Summary 2

• Oscillations of the oval window of the cochlea
  set up travelling waves in the basilar
  membrane.
• The position of the peak of the travelling wave
  depends on the sound frequency (the lower the
  sound, the closer the peak moves to the apex).
  The basilar membrane therefore acts as a
  mechanical frequency analyser.
• Basilar membrane motion causes displacement of
  hair cell bundles on sensory receptors cells in
  the organ of Corti. Stretch receptors on or near
  the hair cell bundles then translate the
  mechanical acoustic signal into an electrical
  signal. This electrical signal controls the
  probability of transmitter release onto auditory
  nerve fibre afferents.

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Transduction by hair cells

• Tectorial
• membrane

• Basilar membrane

• Depolarization

• Hyperpolarization

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Travelling waves peaks at different locations as
the frequency changes
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Frequency tuning along the basilar membrane

• LOW
• FREQUENCY

• Stiffness decreases from base to apex

• HIGH
• FREQUENCY

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Frequency tuning of basilar membrane is
physiologically vulnerable
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Frequency tuning of basilar membrane is
physiologically vulnerable

• KANAMYCIN

• Threshold (dB)

• NORMAL

• Frequency (kHz)

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Frequency selectivity

• Stiffness
• Active amplification

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Afferent innervation of the hair cells
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Listening to auditory nerve fibres
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Response of an auditory nerve fibre
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Firing rate increases with sound intensity
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Tonotopicity in the auditory nerve
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Frequency tuning at high sound levels

• Characteristic frequency

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Phase locking

• Period of cycle

• Time

• Pressure

• Hair cell
• response

• Auditory
• nerve

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Middle Ear Cavity
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Sine Waves and Linear Systems

• Many natural oscillators are linear systems
  because the physical laws governing them are
  linear Newtons 2nd law F m a Hookes
  law F k x
• Linear systems obey the superposition
  principle, i.e. the response to the sum of two
  inputs is the sum of the responses to each
  individual input g(a)x g(b)y ?
  g(ab)xy.
• Since Fouriers theorem states that any sound is
  the sum of sine waves. It is therefore possible
  to characterise linear systems one sine wave at
  a time to arrive at a complete description of
  the systems behaviour.
• This insight motivated the choice of sine wave
  stimuli in auditory research. Unfortunately, many
  aspects of the auditory system, including basilar
  membrane motion are fairly non-linear.