Neuroscience: Exploring the
Brain, 3e
Chapter 11: The Auditory and Vestibular Systems
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Introduction
• Sensory Systems
– Sense of hearing, audition
• Detect & localize sound
• Perceive and discriminate
– Sense of balance, vestibular system
• Head and body location
• Head and body movements
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The Nature of Sound
• Sound
– Audible variations in air pressure
– Sound frequency: Number of cycles per second
expressed in units called hertz (Hz)
– Cycle: Distance between successive compressed patches
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The Nature of Sound
• Sound
– Range: 20 Hz to 20,000 Hz
– Pitch: High pitch = high frequency; low frequency = low
pitch
– Intensity: High intensity louder than low intensity
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The Structure of the Auditory System
• Auditory System
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The Structure of the Auditory System
• Auditory pathway stages
– Sound waves
– Tympanic membrane
– Ossicles
– Oval window
– Cochlear fluid
– Sensory receptor and neuron response
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The Middle Ear
• Sound Force Amplification by the Ossicles
– Pressure: Force per surface area e.g. dynes/cm 2
– Greater pressure at oval window than tympanic
membrane, moves fluids
• The Attenuation Reflex
– Response where onset of loud sound causes tensor
tympani and stapedius muscle contraction
– Function: Adapt ear to loud sounds, understand
speech better
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The Inner Ear
• Anatomy of the Cochlea
• Perilymph: Fluid in scala vestibuli and scala tympani
• Endolymph: Fluid in scala media
• Endocochlear potential: Endolymph electric potential 80 mV
more positive than perilymph
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The Inner Ear
• Physiology of the Cochlea
– Pressure at oval window, pushes perilymph into
scala vestibuli, round window membrane bulges out
• The Response of Basilar Membrane to Sound
– Structural properties: Wider at apex, stiffness
decreases from base to apex
• Research: Georg von Békésy
– Endolymph movement bends basilar membrane
near base, wave moves towards apex
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The Inner Ear
• Travelling wave in the Basilar Membrane
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The Inner Ear
• The Organ of Corti and Associated Structures
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The Inner Ear
• Transduction by Hair
Cells
– Research: A.J.
Hudspeth.
– Sound: Basilar
membrane
upward, reticular
lamina up and
stereocilia bends
outward
(towards largest)
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The Inner Ear
• The Innervation of Hair Cells
– One spiral ganglion fiber: One inner hair cell, numerous
outer hair cells
• Amplification by Outer Hair Cells
– Function: Sound transduction
– Motor proteins: Change length of outer hair cells
– Prestin: Required for outer hair cell movements
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Central Auditory Processes
• Auditory Pathway
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Central Auditory Processes
• Response Properties of Neurons in Auditory Pathway
– Characteristic frequency: Frequency at which neuron
is most responsive - from cochlea to cortex
– Response Properties more complex and diverse
beyond the brain stem
– Binaural neurons are present in the superior olive
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Encoding Sound Intensity and Frequency
• Encoding Information About Sound Intensity
– Firing rates of neurons
– Number of active neurons
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Encoding Sound Intensity and Frequency
• Stimulus Frequency
– Tonotopic maps on the basilar membrane, spiral
ganglion & cochlear nucleus
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Encoding Sound Intensity and Frequency
• Phase Locking
– Low frequencies: phase-locking on every cycle or some
fraction of cycles
– High frequencies: not fixed
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Mechanisms of Sound Localization
• Techniques for Sound Localization
– Horizontal: Left-right, Vertical: Up-down
• Localization of Sound in Horizontal Plane
– Interaural time delay: Time taken for sound to reach
from ear to ear
– Interaural intensity difference: Sound at high
frequency from one side of ear
– Duplex theory of sound localization:
• Interaural time delay: 20-2000 Hz
• Interaural intensity difference: 2000-20000 Hz
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Mechanisms of Sound Localization
• Interaural time delay and interaural intensity difference
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Mechanisms of Sound Localization
The Sensitivity of Binaural Neurons to Sound Location
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Mechanisms of Sound Localization
• Delay Lines and Neuronal Sensitivity to Interaural Delay
– Sound from left side, activity in left cochlear nucleus,
sent to superior olive
– Sound reaches right ear later; delayed activity in
right cochlear nucleus.
– Impulses reach olivary neuron at the same time
summation action potential
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Barn Owl : Space Map in Inf. Colliculus
This is a
computational
map!
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Frog calls (Hyla regilla)
5
4
3
2
1
1
Ch an n el 0
Vo lts
Advertisement
call
0
-1
-2
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-4
-5
Encounter
call
0
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0.20
s
Model of short-pass duration selectivity
Excitation is
delayed and
inhibition
increases in
duration with
longer-duration
stimuli.
Leary et al (2008)
Intracellular recordings from a short-pass
duration-selective neuron
Made in the auditory midbrain of a frog
Frog calls (Hyla regilla)
5
4
3
2
1
1
Ch an n el 0
Vo lts
Advertisement
call
0
-1
-2
-3
-4
-5
Encounter
call
0
0.02
0.04
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0.18
0.20
s
Recordings from neurons selective for short
and long intervals
Neuron A –
selective for short
intervals
Neuron B –
selective for long
intervals
Models of long-interval selectivity
Model A – Each pulse
produces an EPSP
followed by an IPSP.
IPSP and EPSP from
following pulse
overlap at fast rates
(short intervals).
Model B – There is
depression of
excitation at fast
pulse repetition rates.
Edwards et al (2008)
Human speech also has periodic AM
flatounet.net
Auditory Cortex
• Primary Auditory Cortex
– Axons leaving MGN project to auditory cortex via
internal capsule in an array
– Structure of A1 and secondary auditory areas:
Similar to corresponding visual cortex areas
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Target distance and relative velocity
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Auditory Cortex
• Principles of Auditory Cortex
–
Tonotopy, columnar organization of cells with similar binaural interaction
–
Unilateral lesion in auditory cortex: No deficit in understanding speech.
But, Localization deficit.
–
Lesion in striate cortex: Complete blindness in one visual hemifield
–
Different frequency band information: Parallel processing e.g.
frequency-specific localization deficit assoc. with small lesion.
• Neuronal Response Properties
–
Frequency tuning: Similar characteristic frequency
–
Isofrequency bands: Similar characteristic frequency, diversity among
cells
–
Multiple computational maps- Bat auditory cortex.
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The Vestibular System
• Importance of Vestibular
System
– Balance, equilibrium,
posture, head, body,
eye movement
• Vestibular Labyrinth
– Otolith organs gravity and tilt
– Semicircular canals head rotation
– Use hair cells, like
auditory system, to
detect changes
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The Vestibular System
• The Otolith Organs: Detect changes in head angle, linear
acceleration
– Macular hair cells responding to tilt
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The Vestibular System
• The Semicircular Canal
• Structure
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The Vestibular System
• Push-Pull Activation of
Semicircular Canals
– Three semicircular canals
on one side
• Helps sense all
possible head-rotation
angles
– Each paired with another
on opposite side of head
– Push-pull arrangement of
vestibular axons:
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The Vestibular System
• Central Vestibular Pathways
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The Vestibular System
• The Vestibulo-Ocular Reflex (VOR)
– Function: Line of sight fixed on visual target
– Mechanism: Senses rotations of head, commands
compensatory movement of eyes in opposite
direction
– Connections from semicircular canals, to vestibular
nucleus, to cranial nerve nuclei  excite extraocular
muscles
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The Vestibular System
• The Vestibulo-Ocular Reflex (VOR)
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Concluding Remarks
• Hearing and Balance
– Nearly identical sensory receptors (hair cells)
– Movement detectors: Periodic waves, rotational, and
linear force
– Auditory system: Senses external environment
– Vestibular system: Senses movements of itself
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Concluding Remarks
• Hearing and Balance (Cont’d)
– Auditory Parallels Visual System
• Tonotopy (auditory) and Retinotopy (visual)
preserved from sensory cells to cortex code
– Convergence of inputs from lower levels  Neurons
at higher levels have more complex responses
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End of Presentation
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The Middle Ear
• Components of the Middle Ear
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Mechanisms of Sound Localization
• Localization of Sound in Vertical Plane
– Vertical sound localization based on reflections from
the pinna
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