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The Auditory System (Lectures 7 and 8)

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Title: The Auditory System (Lectures 7 and 8)


1
The Auditory System (Lectures 7 and 8)
  • Harry R. Erwin, PhD
  • COMM2E
  • University of Sunderland

2
Background
  • This is my speciality.
  • I currently have three relevant grant proposals
    active, in development or in review.
  • I supervise research students in this area.

3
Organization of the Lecture
  • Outside to inside.
  • Issues associated with specific nuclei will be
    discussed.
  • How the auditory system works will be addressed,
    to the extent that it is currently known.
  • The goal is to give you insight into how
    biologically-inspired robots might hear.
  • Perhaps you might want to build a system to
    localize gunshots. Heres an approach.

4
Resources
  • Webster, Popper, and Fay, 1992, The Mammalian
    Auditory Pathway Neuroanatomy, Springer Handbook
    of Auditory Research, volume 1.
  • Popper and Fay, 1992, The Mammalian Auditory
    Pathway Neurophysiology, Springer Handbook of
    Auditory Research, volume 2.
  • Popper and Fay, 1995, Hearing by Bats, Springer
    Handbook of Auditory Research, volume 5.
  • Hawkins, McMullen, Popper and Fay, 1996, Auditory
    Computation, Springer Handbook of Auditory
    Research, volume 6.
  • Blauert, 1997, Spatial Hearing, revised edition,
    MIT Press.
  • Nolte, 1993, The Human Brain, 3rd edition, Mosby
    Yearbook.
  • Oertel, Fay, and Popper, 2002, Integrative
    Functions in the Mammalian Auditory Pathway,
    Springer Handbook of Auditory Research, volume 15.

5
The auditory system is a typical mammalian
sensory system
  • The auditory signal is processed by brainstem
    modules before the information arrives at the
    cortex.
  • Extensive cortical and somatic reafference is
    used to tune the brainstem processing.
  • Supports a series of functions
  • Reflexive movements (e.g., startle reflex)
  • Orientation towards stimuli (attention)
  • Localization (where is it?)
  • Classification (what is it?)
  • Multisensory integration (especially with vision
    and touch)

6
Illusions are a basic tool in understanding
sensory processing
  • Illusions occur when the perceived stimulus does
    not accurately reflect the actual stimulus.
  • Usually reflect specific implementation in the
    sensory processing.
  • Examples of auditory illusions
  • The precedence effect
  • Elevation illusions produced by filtered sounds
  • (based on the discussion in Middlebrooks, et
    al., in Oertel, Fay and Popper, 2002)

7
The Precedence Effect
  • You can usually localize clicks accurately in the
    horizontal dimension. However, when the clicks
    are separated by a brief delay, you experience an
    illusion.
  • If the interval is lt 5 milliseconds (msec), you
    experience a single sound.
  • If the interval is 1-5 msec, the perceived
    location is determined by the leading click.
  • If the interval is lt 1 msec, the perceived
    location is intermediate between the actual
    locations.
  • Earlier and louder clicks influence the perceived
    location.

8
Elevation Illusions
  • Your external ear (pinna) filters broadband
    sounds to produce peaks and notches in the
    spectrum.
  • These serve as cues to location, particularly in
    elevation and resolving the front/back dimension.
  • You can apply filters to a sound to confuse this
    localization.
  • The types of confusion that occur give insight
    into how the cues are processed.

9
Implications
  • The auditory system seems to have a minimum
    resolution of 1-5 msec.
  • There seems to be a trade-off between sound
    intensity and timing.
  • Different cues play different roles in
    localization.
  • Learning is probably important in calibrating
    (and recalibrating) the auditory system.

10
Possible Localization Cues
  • Azimuth (Jeffress, 1948)
  • Interaural intensity difference (IID)
  • Interaural phase difference
  • Interaural (onset) time difference (ITD)
  • Elevation
  • Spectral shape
  • Spectral notch movement
  • Interaural line rotation
  • Range
  • Echo delay (biosonar)
  • Target motion analysis (TMA)
  • Near-field stereophonic audition (triangulation)
  • Motion
  • Doppler shift
  • Phase shift
  • Intensity shift

11
Components of the auditory system
  • Neurotransmitters and receptors
  • Cell Types
  • Neural Circuits
  • Overall organization

12
Neurotransmitters
  • Glutamate (Glu)
  • AMPA receptorsexcitatory, fast
  • NMDA receptorsexcitatory, learning, much slower
  • Aspartateexcitatory, fast, found in the cochlea.
  • GABAstandard inhibitory, very slow.
  • Glycineinhibitory, fast, common in audition
  • Acetylcholineexcitatory
  • Various neuromodulators
  • Remember the Cl- reversal potential!

13
Some basic cell types of the auditory brainstem
  • Primary-like (PL)
  • Primary-like, notch (PL-N)
  • Phase-lock (onset)
  • Onset, lock (O-L)
  • Chopper

14
Primary-like (PL)
  • Their output is similar to the output of auditory
    neurons, hence the name. Only a few afferents,
    resulting in some jitter.
  • Low threshold current (LTC) K channels open
    quickly and with a low threshold (10-15 mV
    depolarization from resting).
  • A second high threshold K current (HTC) then
    activates at 20 mV depolarization.
  • These cells do not spike repetitively.
  • Moderate time constant unless previously
    depolarized.
  • Function as transducers

15
Primary-like, notch (PL-N)
  • PL with many more afferents, which must sum to
    threshold. The presence of the notch reflects the
    very accurate initial spike timing and the
    following refractory period. Very little jitter.
  • Principle cells of the MNTB are PL-N because they
    are tightly locked to their globular bushy cells.
  • Edge detectors

16
Phase-lock (Onset) and Onset, lock (O-L) cells
  • Octopus cells of the PVCN provide an initial
    well-timed spike (like PL) cells, followed by a
    low level of activity. However, they phase-lock
    to low frequency sounds (up to 800 Hz!, higher in
    some mammals and much higher in some birds).
    Thick axons short latencies.
  • May function as pitch or coherence detectors.
  • Sample many (gt60) auditory neurons over a 200?sec
    integration window. LTC and IH (hyperpolarization-
    activated) potassium channels. Extremely short
    membrane time constant (200?sec) near their
    resting potential. Very low input resistance, so
    they need lots of input to depolarise.

17
Chopper Cells
  • Stellate cells that spike repetitively.
  • Have a high-threshold potassium channel,
    producing a classical Hodgkin-Huxley-like cycle.
  • As long as the depolarizing current is sustained,
    will spike regularly.

18
Auditory Midbrain Rules of Organization
  • Many specialized nuclei, organized into parallel
    paths.
  • Convergence at the inferior colliculus (IC), much
    of it inhibitory or shunting. Left-to-right
    reversal at the IC (like vision). Does the IC
    function like the basal ganglia? We may know in 3
    yrs.
  • Glycine (Gly) is the most common inhibitory
    neurotransmitter, probably due to a faster time
    constant (1 msec) than GABA (5 msec).
    Inhibitory rebound is extensively exploited to
    produce delayed responsesa cell depolarizing
    enough to spike after being hyperpolarized.
  • Glutamate (Glu) is the usual excitatory
    neurotransmitter. AMPA receptors are fast
    subtypes, so a time constant of 200 ?sec
    (200x10-6 sec!) is typical. (Brand et al., 2002,
    in Nature indicate 100 ?sec for both Gly and Glu,
    which is probably too low.)

19
Duration Tuning Mechanisms
  • Duration selective neurons seem to use inhibition
    and inhibitory rebound.
  • Involves inhibitory circuits. Can be modulated.
  • Initially, a duration-selective neuron is
    inhibited from firing in response to a sound.
  • When the inhibition is released by the end of the
    sound, the neuron depolarizes for a short
    interval.
  • If delayed excitation arrives while the neuron is
    depolarizes, it spikes. Otherwise it remains
    silent.

20
Break
21
Stages in Mammalian Audition(Lecture 8)
  • External Ear (Pinnae)
  • Middle Ear
  • Inner Ear (Cochlea)
  • Inner Hair Cells
  • Type I Spiral Ganglion Cells
  • Cochlear Nucleus (dorsal CN and ventral CN)
  • Medial Nucleus of the Trapezoidal Body
  • Lateral Superior Olivary Nucleus
  • Medial Superior Olivary Nucleus
  • Lateral Lemniscus
  • Central Nucleus of the Inferior Colliculus
  • Medial Geniculate Nucleus
  • Auditory Cortex

22
The Principle Connections of the Mammalian
Auditory System
Planum temporale
Planum temporale
Corrected from http//earlab.bu.edu/
intro/auditorypathways.html
23
External Ears (Pinnae)
  • Directional receivers, steerable in many mammals.
  • The transfer function between the free-field
    sound (with head not present) and the sound at
    the ear drum is called the HRTF (head-related
    transfer function).
  • Multipath interference occurs and seems to play a
    role in generating elevation cues.
  • Intensity, onset time, and phase differences
    between the two ears seem to play a role in
    estimating azimuth

24
The Middle Ear
  • Contains the stapes, incus, and malleus.
  • Translates the motion of the ear drum into
    pressure waves in the cochlea (inner ear).
  • In the bat, muscles of the middle ear contract or
    relax to mute the sound of its cry and possibly
    to normalize the intensity of the echo based on
    distance to the target.
  • Figure from http//oto.wustl.edu/cochlea/
    intro1.htm

25
Inner Ear
  • The cochlea (so called from the snail-shell
    shape).
  • A spiral organ with about 1000-4000 inner hair
    cells. The tip is low-frequency.
  • The strongest response to each frequency is at a
    specific position, producing a tonotopic
    mapping throughout the auditory system. This is
    the only such mapping known in mammals.
  • Figure from http//hyperphysics.phy-astr.gsu.edu/h
    base/sound/ cochlea.htmlc2

26
Inner Hair Cell
  • Uses an excitatory neurotransmitter (Glu or Asp)
  • Vesicle release in response to movement of
    stereocilia on the apex. Logarithmic response to
    pressure.
  • Fast time constants. Bats can sense time
    intervals less than 100 nsec, probably by
    detecting interference.
  • Figure from http//www.neurophys.wisc.edu/www/aud/
    johc.html

27
How the Inner Hair Cell Works
  • Vesicle release appears to reflect Ca entry
    into the cell (Ray Meddis). Motion of the
    stereocilia modulates K influx, which causes
    Ca influx, but there is also background Ca
    leakage, so vesicles are released even without
    sound input. The release rate varies among
    synaptic terminals, resulting in variation in
    sensitivity.
  • The auditory neurons that synapse on the inner
    hair cell use AMPA receptors and have a very
    short time constant (200 ?sec).
  • The cochlea functions as a biological FFT.

28
Outer Hair Cells
  • May adjust the motion of the basilar membrane so
    that a specific 30 dB interval is chosen within
    the 120 dB range of sounds that can be detected.
  • An active cochlear amplifier is likely but not
    fully proven.
  • Would be controlled by reafference from the
    superior olivary complex (later).
  • Figure from http//www.neurophys.wisc.edu/www/aud/
    johc.html

29
Type I Spiral Ganglion Cells of the Eighth Nerve
  • The auditory neurons (ANs), forming the spiral
    ganglion.
  • 10 to 70 synapse on each inner hair cell.
    Bipolar, consisting of a dendritic element, a
    somatic compartment, and a usually myelinated
    (non-myelinated in humans, so slower) axonal
    element that divides in the cochlear nucleus. Can
    synapse on multiple inner hair cells. Excitatory
    (Glu). Extremely sharp best frequencies.
  • Cover a range of 30 dB in sensitivity.
  • Show spontaneous activity (up to 140 Hz, Gulick).

30
How the Spiral Ganglion Cells Work
  • Multiple vesicles are often released at the inner
    hair cell synapses, although one is enough to
    cause firing.
  • Variable vesicle release rates by synapse seem to
    produce the range of sensitivities seen. Vesicle
    release reflects Ca entry into the cell.
    Integrate and fire dynamics (Meddis), and
    spontaneous firing rates reflect this. Some new
    results.
  • Fast time constant 200-300 µsec.
  • Collectively can phase lock to a sinusoid up to
    3-4 KHz (9 KHz in owls, Carr).

31
The Cochlear Nucleus (CN)
  • The first stage of auditory processing after the
    cochlea.
  • At the CN, the auditory neurons divide into two
    branches, one dorsal and one ventral. Each branch
    may terminate on multiple neurons.
  • The cochlear nucleus is divided into the dorsal
    CN, anterior ventral CN, and posterior ventral
    CN, apparently with different functions.
    Attention plays a role in the AVCN (Covey).

32
Dorsal Cochlear Nucleus (DCN)
  • Laminar or layered structure. Cerebellar-like
    per Curtis Bell. Seems to play a role in
    estimating sound elevation. Lesions have subtle
    effects.
  • Somatosensory reafference is received from the
    thalamic reticular nucleus (TRN), reporting on
    pinna muscle activity. Issues here. Startle
    reflex.
  • Glycinergic primary cells in the DCN appear to
    respond to lines and notches centered on their
    best frequencies, reporting to the IC.
  • Complex inhibitory circuits in the DCN involving
    sensory profiles produce this response.

33
DCN Circuits
  • DCN cells participate in circuits that integrate
    somatosensory data with sound.
  • Also detect spectral notches in the signal with
    moderate width.
  • Output chopper, onset, and build-up patterns.

34
Ventral Cochlear Nucleus (VCN)
  • Bushy cells (in the AVCN) are primary-like
    cells that track the spiking of auditory nerve
    cells directly. These have a dendritic element, a
    soma, and a myelinated axon that passes to the
    superior olivary complex, the lateral lemniscus,
    and to the inferior colliculus, with excitatory
    signalling.
  • Multipolar or stellate cells (in the DCN, AVCN,
    and PVCN) project to the pontine tegmentum (SOC
    and LL). These are chopper cells that
    periodically modulate the input signal.
  • Octopus cells (PVCN) appear to be broadly tuned
    onset detectors. Insensitive to intensity.
    Project to the pontine tegmentum and then to the
    lateral lemniscus.

35
Superior Olivary Complex (SOC)
  • Consists of the
  • Lateral superior olivary nucleus (LSO)
  • Medial superior olivary nucleus (MSO)
  • Medial nucleus of the trapezoid body (MNTB)
  • Size of the complex varies greatly among species
    as do the sizes of the individual nuclei.
  • In bats, cell counts of about 20000 (Ellen Covey,
    personal communication)
  • Secondary nuclei present as well.
  • Plays a role in the stapedius reflex which
    protects the middle ear from loud sounds.

36
Trapezoidal Body
  • Large multipolar principal cells. Synaptic input
    is via very large calyceal endings (end-bodies of
    Held). PL-N dynamics.
  • Input from globular bushy cells in the
    contralateral AVCN.
  • Reverses the sign of the signal. Inhibitory
    output.
  • Projects to the ipsilateral LSO and LL.
    Glycinergic.
  • High-frequency sensitive.
  • Not considered important in humans.

37
Lateral Superior Olivary Nucleus
  • Ipsilateralexcitatory, spherical bushy cells
    (PL)
  • Contralateralinhibitory input via the
    trapezoidal bodyglobular bushy cells (PL-N)
  • Outputs bilaterally to the lateral lemnisci and
    to the IC. Glycine with glutamate or possibly
    aspartate. Mostly chopper cells.
  • Sensitive to high frequency sounds and used for
    comparing the signal intensities at each ear.
  • Small multipolar principal cells.
  • Codes for auditory localization in azimuth.

38
Medial Superior Olivary Nucleus
  • Excitatory input from both sides into separate
    dendrites. Source spherical bushy cells that
    track the afferent signal.
  • Feeds forward to the inferior colliculus, mostly
    ipsilateral.
  • Generates one or two spikes at sound onset. Other
    roles possibly present.
  • Important in large mammals with good
    low-frequency hearing (sounds are diffracted, so
    intensity is not a good cue for azimuth). Note
    that phase ambiguity disappears over multiple
    frequencies.

39
Nucleus of the Central Acoustic Tract
  • Small, importance unknown
  • Directly projects to SC and MGB, bypassing IC.
  • Large multipolar neurons
  • Bilateral input from the AVCN

40
Lateral Lemniscus
  • Major auditory tract. Contains 2nd, 3rd, and 4th
    order axons. Seems to perform spectral analysis
    (e.g., vowel detection, line spectra tracking)
    and detection of transients, and to have a role
    in measuring the timing of echoes. (Like the
    Basal Ganglia?)
  • Octopus cell axons end in the ventral nucleus of
    the lateral lemniscus (VNLL), with large calyceal
    endings. Part of the short latency acoustic
    startle reflex pathway to the reticular
    formation. Monaural. Transient detection.
  • Stellate, bushy (excitatory) and MNTB, DCN
    (minor, glycinergic) cells also project to the
    VNLL.
  • VNLL is glycinergic. Choppers and PL. On-going
    research area.
  • DNLL inputs binaural. Projects to the IC.
    Functional role unknown.

41
Central Nucleus of the Inferior Colliculus
(Mesencephalon)
  • Largest auditory structure of the brainstem on
    the roof of the midbrain. A tectal structure
    behind the superior colliculus (SC). There is a
    spatial mapping from the IC to the SC (that
    triggers visual orientation to sounds in barn owl
    and possibly in mammals).
  • Primary point of convergence in the auditory
    brainstem. Sounds arrive here 2-5 msec after the
    inner hair cells are activated.
  • Bidirectional connectivity with the auditory
    cortex. Excitatory inputs are received from the
    part of the AC (layer V) that then receives the
    outputs. This is fast enough to support
    cortically-controlled analysis of current sound
    afference.

42
IC components
  • Small multipolar fusiform cells with tufted
    dendrites. Cochleotopic tonotopic laminar
    organization, uniting inputs from all lower
    nuclei and the contralateral IC.
  • The anterior portion of the laminae receive
    cortical inputs, while the posterior portion
    receives brainstem and IC inputs.
  • Stellate cells also present that cross the
    laminae.
  • Recently it has been found that the signal at the
    IC is normalized in intensity. Several possible
    mechanisms.
  • Partly cerebellar-like (Curtis Bell).
  • Match/mismatch processing? Sparsification? Motion
    processing?
  • My current grant is in this area.

43
Medial Geniculate Nucleus (or Body)
  • AKA the auditory thalamus. Similar to the LGN
    (vision).
  • Transduces the output of the colliculi for the
    auditory cortex. Tonotopically organized. In
    bats, may encode distance.
  • Ventral, dorsal, and medial (or magnocellular)
    divisions.
  • Ventral divisionabout half the structure,
    projects to primary auditory cortex (A1).
    Excitatory output.
  • Dorsal divisionprojects to association auditory
    cortex (A2). Auditory attention? Both excitatory
    and inhibitory output.
  • Medial divisionlarge multipolar neurons.
    Multisensory arousal system? Both excitatory and
    inhibitory output.

44
Cerebellum
  • Receives auditory data from the auditory cortex
    and the pontine nucleus.
  • Possible roles include coordinate transformation,
    motor timing, and localization.

45
Primary Auditory Cortex (A1)
  • Transverse gyri of Heschl
  • True primary auditory cortex or koniocortex.
    Called Area 41, A1, TC, or Kam/Kat depending on
    the author.
  • Six-layered. Layer III functions differently from
    visual cortex. Strong contralateral connectivity
    from III, V and VI. Corticalfugal connectivity
    from V.
  • Tonotopically organized with alternating bands
    responding to a difference signal from the ears
    (/-). Sharp tuning and short latencies.
  • Some visual sensitivity (from SC and late visual
    areas)

46
Secondary Auditory Cortex (A2)
  • Parakoniocortex (Area 42, TB, or PB) in this
    area.
  • Visual sensitivity.
  • Multiple tonotopic maps, some complete. Longer
    latencies, broader tuning, less sensitive to
    tones.
  • In bats, the secondary tonotopic maps are quickly
    sensitive to complex sounds.
  • In mustached bat, there is a secondary area with
    a bicoordinate frequency representation over a
    very narrow frequency interval centered on the
    second harmonic of the cry.
  • Additional fields in bat are delay tuned.

47
Planum Temporale
  • Smoother portion of the superior surface of the
    temporal lobe (Area 22 or Tpt)
  • Area 22 tends to extend somewhat onto the
    parietal operculum and inferior parietal lobule
    in humans.
  • On the left side, this is Wernickes area.
  • Areas 39 and 40, the left inferior parietal
    lobule, is probably a higher association area.
  • Now suspected of being the point at which sounds
    are correlated to auditory streams. Complex
    auditory computation. Motion sensitivity? Visual
    sensitivity.

48
Other Language Cortices
  • An association pathway (arcuate fasciculus)
    connects Area 22, the inferior parietal lobule
    (Areas 39 and 40, a complex multimodal
    integration area), and the area triangularis of
    the inferior frontal gyrus (Areas 44 and 45,
    Brocas area).

49
Where do things happen?
  • Azimuthbinaural, measured in the SOC (MSO, LSO,
    and MNTB).
  • Elevationmonaural, probably based on DCN notch
    detection.
  • Range, timing, and intervalsmonaural, measured
    by the LL, using inhibitory mechanisms.
  • Line spectrummonaural, measured by the LL.
  • Sensory integrationfor individual sounds,
    binaurally in the IC, using evidence developed by
    lower nuclei.
  • Comparisons between soundsauditory cortex.

50
Reconstructing the acoustic scene
  • How separate sound sources are distinguished,
    assigned to sound streams, and localized is not
    understood.
  • Attention probably chooses sounds out of
    background. Otherwise, the first sound has
    preference. Ray Meddis thinks sounds are
    disambiguated by ignoring ambiguous cues.
  • Intervals between sounds are very important in
    disambiguating them. Auditory neuroscientists are
    dubious about the binding problem.
  • Distinct sound characteristics are also important
    in assignment to sound streams. Harmonics
    important as are spectral segments of about 1
    kHz.
  • There are a number of interesting auditory
    illusions that we can explore.

51
Some lessons to draw
  • Dense representations are found throughout the
    auditory brainstem. The sparse representations
    needed for associative learning and retrieval
    seem to be cortical.
  • The auditory brainstem has solved the problem of
    handling (and modulating) duration tuning. This
    is currently a hard problem in cortical modeling,
    probably because the role of inhibition and
    inhibitory rebound is not well-understood. Recent
    results on persistent activity are important.
  • There is no evidence for a spatial map anywhere
    in the auditory brainstem. This probably means
    space is represented in spectral form. (Think
    spatial Fourier transform and Gabor functions.)
  • Timing, not synchronization, probably solves the
    binding problem in the auditory system.
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