Audition 2: Auditory Pathways

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Audition 2 Auditory Pathways

• Neurophysiology for Prelims
• Dr. Jennifer Bizley
• Jennifer.bizley_at_dpag.ox.ac.uk

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Objectives of This Lecture

• Encoding of sounds in the auditory nerve (AN)
• AN fibre tuning curves
• Phase locking
• Structure and Function of the Auditory Brainstem
• Divisions of the Cochlear Nucleus
• Properties of the Superior Olivary Nuclei
• Stations of the Auditory Midbrain
• Inferior Colliculus and Medial Geniculate Nucleus
• Auditory Cortex
• Organisation of primary auditory cortex (A1)
• Higher order cortical fields

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The Auditory Pathway
AUDITORY CORTEX
MEDIAL GENICULATE BODY
INFERIOR COLLICULUS
SPIRAL GANGLION
LATERAL LEMNISCUS
COCHLEAR NUCLEUS
COCHLEA
SUPERIOR OLIVARY COMPLEX
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The Audiogram

• Discrimination thresholds
• Loudness 1dB
• Frequency 2Hz

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Afferent Innervation of the Hair Cells
type 1
type 2
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Response of an Auditory Nerve Fibre
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Sound Intensity
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Neural Coding of Sound Intensity

• AN fibres increase their firing rate as a
  function of sound intensity (rate code).
• AN fibres differ in sensitivity and dynamic
  range.
• Low spontaneous rate fibres (stippled lines) have
  the highest thresholds and saturate only at high
  sound levels (above 90 dB).
• Intermediate SR fibres (continuous line) have
  intermediate thresholds and saturate by about 60
  dB SPL.
• High SR fibres (not shown) are the most
  sensitive, and may saturate by 40 dB.
• Differences in sensitivity allow for a population
  code for intensity.

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Tonotopicity in the Auditory Nerve
Tonotopicity provides for a population code for
sound frequency
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Phase Locking - provides a spike timing code for
sound frequency
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Phase locking single neurons and populations
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Phase Locking How Does it Work?
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Phase locking single neurons and populations
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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).
• The fact that there is virtually no AC response
  for frequencies above 3kHz sets an upper limit to
  the temporal resolution of temporal patterns that
  can be encoded by phase locking.
• AN fibres with CF gt 3 kHz can use phase locking
  to encode other (slower) temporal patterns, like
  amplitude modulation.

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The Auditory (Vestibulo-Cochlear or VIII) Nerve
a Summary

• The VIII nerve contains approximately 10,000
  auditory afferents (AN fibres).
• Ca. 95 of these are type1, fast myelinated
  fibres receiving divergent input from IHC. The
  rest are type 2, small unmyelinated fibres
  receiving convergent input from OHC.
• Type 1 fibres can be subdivided into high, medium
  and low spontaneous rate (SR) fibres. High SR
  fibres are the most, low SR the least sensitive.
  (Rate and population codes for sound intensity)
• AN fibres are frequency tuned their discharge
  rate depends on the amount of acoustic energy at
  or near the neurons characteristic frequency.
  This tuning is almost entirely due to basilar
  membrane mechanics. (Place code for frequency)
• AN fibres can encode the temporal structure of
  sounds through phase locking. (Temporal pattern
  code for frequency/periodicity)

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Tonotopicity in the Cochlear Nucleus
The base of the BM projects to medial CN, the
apex to lateral CN
Anteroventral CN
Posteroventral CN
Anteroventral CN
CochlearNerve
Dorsal CN
Posteroventral CN
Cochlea
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Cell Types in the Cochlear Nucleus
DCN
PVCN
AVCN
PVCN
AVCN
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Lateral Inhibition in the Cochlear Nucleus
Bushy
Multipolar (Stellate)
Pyramidal
Octopus
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The Cochlear Nucleus a Summary

• ANs branch and contact a variety of different
  cell types within the CN. Each of these cell
  types appears to extract different aspects of the
  incoming acoustic information, and passes this on
  to different points in the auditory pathway
• Primarylike (bushy) cells in the AVCN preserve
  temporal information contained in phase locking
  and project to the superior olivary nuclei.
• Chopper (stellate / multipolar) cells in the
  PVCN, as well as pauser and build-up cells in
  DCN, may use lateral inhibition to extract
  spectral contrast and project to the inferior
  colliculus.
• Onset cells are an in-homogenous class, some are
  stellate, some octopus. Among other things they
  may encode temporal pattern information across
  many AN fibres or encode sound intenstity. They
  project to inferior colliculus or lateral
  lemniscus.

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Superior Olivary Nuclei Binaural Convergence

• Medial superior olive
• excitatory input
• from each side (EE)

• Lateral superior olive
• inhibitory input
• from the contralateral
• side (EI)

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Binaural Localization Cues
Sounds arrive earlier in the near ear (interaural
time difference cues ITDs),and they are louder
in the near ear (interaural level difference cues
ILDs).
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Measuring auditory localization accuracy

• 10-20 µsec

• 1-2 degrees

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Binaural cues are frequency dependent
The head casts an acoustic shadow
High frequency sound ? Inter-aural intensity
difference
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Interaural phase differences

• Phase ambiguity as wavelength is reduced
  (frequency rises)

• Low frequency sound ? Inter-aural time difference

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Processing of Interaural Level Differences
Lateral superior olive Rate code for sound source
direction
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Processing of Interaural Time Differences
Contra- lateral side
Sound on the ipsilateral side
Medial superior olive Originally thought to use
place code for sound source direction.This is
now disputed.
MSO neuron response
Interaural time difference
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How Does the MSO Detect Interaural Time
Differences?

• Jeffress Delay Line and Coincidence Detector
  Model.
• MSO neurons are thought to fire maximally only if
  they receive simultaneous input from both ears.
  (Place code).
• If the input from one or the other ear is delayed
  by some amount (e.g. because the afferent axons
  are longer or slower) then the MSO neuron will
  fire maximally only if an interaural delay in the
  arrival time at the ears exactly compensates for
  the transmission delay.
• In this way MSO neurons become tuned to
  characteristic interaural delays.
• The delay tuning must be extremely sharp ITDs of
  only 0.01-0.03 ms must be resolved to account for
  sound localisation performance.
• Recent work by scientists like McAlpine, Palmer
  and Grothe suggests that the Jeffress Model is
  not a good description for what goes on in the
  mammalian MSO.

From contralateral AVCN
From ipsilateral AVCN
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Processing of Interaural Time Differences
Contra- lateral side
Sound on the ipsilateral side
Medial superior olive Originally thought to use
place code for sound source direction.This is
now disputed.
MSO neuron response
Interaural time difference
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The Superior Olivary Nuclei a Summary
Excitatory Connection

• Most neurons in the LSO receive inhibitory input
  from the contralateral ear and excitatory input
  from the ipsilateral ear (IE). Consequently they
  are sensitive to ILDs, responding best to sounds
  that are louder in the ipsilateral ear.
• Neurons in the MSO receive direct excitatory
  input from both ears and fire strongly only when
  the inputs are temporally co-incident. This makes
  them sensitive to ITDs.

Inhibitory Connection
Midline
MNTB
LSO
MSO
CN
CN
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The Auditory Pathway
AUDITORY CORTEX
MEDIAL GENICULATE BODY
INFERIOR COLLICULUS
SPIRAL GANGLION
LATERAL LEMNISCUS
COCHLEAR NUCLEUS
COCHLEA
SUPERIOR OLIVARY COMPLEX
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The Inferior Colliculus
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The Role of the Midbrain

• The midbrain contains two major, obligatory
  auditory relays, the inferior colliculus (IC) of
  the tectum, and the medial geniculate nucleus
  (MGB) of the thalamus.
• While the MGB, like the LGN in vision, is
  probably a true relay (predominantly gating,
  rather than processing information) the IC is
  probably a major neural processing centre.
• Although a great deal is known about properties
  of IC neurons, no coherent unifying theory of
  IC function has as yet emerged.
• The IC has a number of anatomical subdivisions.
  Some of these like the central nucleus (ICc), are
  tonotopically organised, while others, like the
  external nucleus (ICx) or the nucleus of the
  brachium (BIN), are not.
• On the basis of current evidence it appears
  possible that the various sub-nuclei of the IC
  subserve different functional roles (e.g.
  periodicity processing in ICc, spatial hearing
  in ICx and BIN).

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The Auditory Cortex
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Are There Columns in Primary Auditory Cortex?

• Some models put forward in introductory textbooks
  are oversimplified to the point where they
  become factually wrong.(You still need to know
  them though!)

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Is there a map of auditory space?
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Multisensory convergence onto individual neurons
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Higher Order Cortical Areas

• In the macaque, primary auditory cortex(A1) is
  surrounded by rostral (R), lateral (L),
  caudo-medial (CM) and medial belt areas.
• L can be further subdivided into anterior, medial
  and caudal subfields (AL, ML, CL)

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Are there What and Where Streams in Auditory
Cortex?
AnterolateralBelt

• Some reports suggest that anterior cortical belt
  areas may more selective for sound identity and
  less for sound source location, while caudal belt
  areas are more location specific.
• It has been hypothesized that these may be the
  starting positions for a ventral what stream
  heading for inferotemporal cortex and a dorsal
  where stream which heads for postero-parietal
  cortex.

CaudolateralBelt
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Brain activation by music
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Summary of Summaries

• Acoustic information is carried to the brain by
  type 1 afferent nerves in the AN. The AN encodes
  stimulus intensity by discharge rate and uses
  tonotopicity and phase locking to encode stimulus
  frequency and temporal patterns.
• The acoustic information is then analysed in the
  cochlear nucleus, where a variety of different
  cell types are selectively responsive to
  particular aspects of the stimulus (temporal
  pattern, spectral contrast, intensity ...).
• Binaural interactions first occur at the level of
  the superior olive. These are important for sound
  localisation and related phenomena.
• The functional organisation of the auditory
  midbrain and cortex is still not well understood
  (or easily summarised) even though a lot is known
  about them. Current theories which attempt to
  assign specific functions to particular areas
  (like what and where processing streams) are
  not without controversy.

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Copies of the handout

• www.physiol.ox.ac.uk/jkb/teaching.shtml

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