Transcript Temporal Aspects of Visual Extinction

Chapters 9,10 Auditory and Vestibular Systems
 Chris Rorden
University of South Carolina
Norman J. Arnold School of Public Health
Department of Communication Sciences and Disorders
University of South Carolina
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Audition
The ear converts sound energy into patterns of
neural firing: transduction
Outer ear: collect and amplify sound, aid in
localization
Middle ear: impedance matching
Cochlea: frequency and intensity analysis
Auditory pathway: complex signal processing
Ear structures
 Peripheral
–
–
–
–
Outer ear
Middle ear
Inner ear
Auditory nerve
 Central
– Brainstem
– Midbrain
– Cerebral
Anatomy and function
Pinna: the projecting part of the ear lying outside of
the head (also called auricle, or just ear lobe)
Reflection of sound in pinna provides spectral cues
about elevation of a sound source
Outer Ear – Auditory Canal
 External auditory meatus
– Provides communication between
middle and inner ears by
conducting sound to the ear drum
– S-shaped tube @ 2.5cm long and
@ 7mm wide
– Lining of the lateral 1/3rd of canal
has cilia and glands
– Cerumen (ear wax): protects ear
canal from drying out and
prevents intrusion of insects
Outer Ear - Ear drum
Tympanic membrane
– Separates outer and middle
ear
– Compliant
– Thin, three-layered sheet
– Epithelium of EAM: outer
layer
– Middle layer: fibrous
(strong) tissue
– Inner layer of middle ear:
mucous membrane
Outer Ear - Ear drum
 Slightly concave to EAM, cone-shaped
 Most depressed and thinnest point is called
the umbo
 End of the attachment of malleus
 ‘Cone of light’ from umbo to periphery
reflects light when viewed with otoscope
 Slightly oval, taller than wide
 Otoscope: if you pull the pinna up and back
the tympanic membrane is visible
Middle Ear – Tympanic Membrane
Role of outer ear
To augment the sound shadow
Ear canal protects delicate parts of middle and
inner ear from impact.
To heighten our sensitivity to sounds
– Ear canal boosts sounds 15 to 16 dB
between 1.5 and 8 kHz (in the area of
speech)
– This is due to resonance of ear canal
– Just like vocal tract this tube amplifies and
dampens certain frequencies based on its
length and composition
Localization and shadowing
Intensity differences: louder if nearer, less shaded
– Inter-aural timing differences
– Frequencies influenced by location relative to
pinna.
–
Middle Ear – Eustachian Tube
Establishes communication between middle ear
and nasopharynx
~ 35 to 38 mm long, typically closed
Biological functions:
– To permit middle ear pressure to equalize with
external air pressure
On the air plane, change in atmospheric pressure but not
pressure in middle ear
Yawning or swallowing opens pharyngeal orifice of tube to
equalize pressures
– To permit drainage of normal and diseased middle
ear secretions into the nasopharynx
Middle Ear - Ossicles
 3 of the smallest bones
– Malleus (hammer)
– Incus (anvil)
– Stapes (stirrup)
 Ossicular chain: Transmits acoustic energy
from tympanic membrane to inner ear
– Acts as lever: large weak motion of TM causes
small forceful movement of stapes.
 Takes force from gas (air) and matches impedance to
liquid (inner ear).
– Muscles allow movement to be attenuated:
Prevents the inner ear from being overwhelmed
by excessively strong vibrations
Middle Ear – Ossicles
Middle Ear – Ossicles - Malleus
Malleus (hammer) 9 mm long
Manubrium (handle): attaches to
tympanic membrane; pulls the
drum medially
Caput (head): jointed (quite
inflexibly) to Incus
Middle Ear – Ossicles - Incus


The ossicles give the eardrum mechanical
advantage via lever action and a reduction in the
area of force distribution
– Pressure = Force/Area; so less area = more
pressure
– the resulting vibrations would be much
smaller if the sound waves were transmitted
directly from the outer ear to the oval window.
The movements of the ossicles is controlled
muscles attached to them (the tensor tympani
and the stapedius).These muscles can dampen
the vibration of the ossicles, in order to protect
the inner ear from excessively loud noise and
that they give better frequency resolution at
higher frequencies by reducing the transmission
of low frequencies
Middle Ear – Ossicles - Stapes
Head (caput) jointed to incus
Anterior and posterior crura (legs)
Footplate: joins oval window of inner ear (opening in
temporal bone) via annular ligament
Cochlea and neighbors
Inner Ear - Cochlear
Osseous cochlea
• Oval window
– Connects scala vestibuli and middle ear
• Round window
– Connects scala tympani and middle ear
Cochlear Structures
Cochleus from 5mo fetus:
– Oval window (blue arrow)
– Round window (yellow arrow)
Tonotopic
Base
 High Freq
–
Apex
–
Low Freq.
Travelling wave
Always starts at the base of the cochlea and
moves toward the apex
Its amplitude changes as it traverses the length
of the cochlea
The position along the basilar membrane at
which its amplitude is highest depends on the
frequency of the stimulus
Traveling wave
High frequencies have peak influence near
base and stapes
Low frequencies travel further, have peak near
apex
A short movie:
– www.neurophys.wisc.edu/~ychen/auditory/animation/animationmain.html
–
Green line shows
'envelope' of travelling
wave: at this frequency
most oscillation occurs
28mm from stapes.
Cochlear structure
Cross-section shows the coiling
of the cochlear duct The red
arrow is from the oval window,
the blue arrow points to the
round window.
1)Scala media – filled w Endolymph
2)scala vestibuli filled w Perilymph
3)scala tympani filled w Perilymph
4)spiral ganglion
5)nerve fibres
www.iurc.montp.inserm.fr/cric/audition/english/cochlea/fcochlea.htm
Inner Ear - Labyrinth
Endolymph
K+ ~100 mV
Reissner’s
Membrane
Perilymph
Na+ ~20mV
Basilar Membrane
Inner Ear – Organ of Corti
Both types of hair cells protrude into endolymph of scala media,
Inner Ear – OHC & IHC
 Inner Hair Cells
– Non-motile
 Outer Hair Cells
– Vibrates when
triggered – acts as
preamplifier.
Hair cells are mechanically gated
ion channels: deflection of hairs
depolarizes the cell, resulting in a
receptor potential – causing
calcium ions to enter, which in turn
stimulates the release of
neuroreceptors.
Neural connections
–
–
Inner hair cells: many nerve fibers
for each cell (many-to-one
innervation) 3500
Outer hair cells: each nerve fiber
connected to many hair cells (oneto-many innervation). 12000
Function of the cochlea
• First stage of auditory processing
1. Spectral analysis
– Extracts frequency and amplitude
information from sound waves
2. Temporal analysis
– Basic temporal characteristics of sounds
The ear codes frequency in two ways:
1) Position of neural responses along basilar
membrane changes with frequency - tonotopic
organization or the place coding
2)Timing of neural responses follows the time
waveform of sound – phase-locking
Place coding
 Place coding: Auditory frequency
coded by location of stimulation.

Base

High Freq
–
Apex
–
Low Freq.
Phase locking
The rate of neural firing
matches the sound's
frequency.
Problem: some auditory
frequencies much faster
than neurons can fire
– Each neuron can only fire
around 200 times per sec.
– Solution: volley principle:
large numbers of neurons
that are phased locked can
code high frequencies.
Afferent and efferent innervation
• Afferent: signals from sense organ to brain
– Auditory signals
• Efferent: signals from brain to sense organ
– Inhibits auditory signals
Both cochlear and ossicles
– Improves signal-to-noise ratio by suppressing
noise
Primary auditory cortex
Medial geniculate
Body (thalamus)
Inferior colliculus
(in midbrain)
Auditory radiation
Cochlear and Superior olivary
Complex in the Medulla
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Central Auditory Mechanism
• Auditory input projects to the
cortex bilaterally, with stronger
contralateral connections.
• The superior olive and the
inferior colliculus send efferent
fibers back to attenuate motion
of the middle ear bones
(dampen loud sounds)
 Cochlear Nucleus
– Evidence of signal processing
(monaural)
 Superior Olivary Complex (SOC)
– Binaural processing
– Localization of sound source
– Low frequency sounds: arrival
time compared
– High frequency sounds: intensity
level compared
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Auditory Pathway
• Lateral Lemniscus
– Fiber tract within CNS
– From SOC to IC
• Inferior Colliculus (IC)
– Bilateral innervation
– Frequency, intensity and temporal processing
• Medial Geniculate Body (MGB)
– Tonotopic mapping
– Complex responses to contralateral signals
Cerebral cortex
• Signal comes primarily from
contralateral ear via ipsilateral
MGB
• Heschl’s gyrus
• Tonotopic mapping in columns
• Each column has one
characteristic frequency
• Neurons in column responsive
to different stimulus
parameters, like frequency
and intensity
Anatomy and function
Many sound features are encoded before the
signal reaches the cortex
- Cochlear nucleus segregates sound
information
- Signals from each ear converge on the
superior olivary complex - important for
sound localization
- Inferior colliculus is sensitive to
location, absolute intensity, rates of
intensity change, frequency - important
for pattern categorization
- Descending cortical influences modify
the input from the medial geniculate
nucleus - important as an adaptive ‘filter’
cortex
medial geniculate
body
inferior colliculus
cochlear nucleus
complex
cochlea
superior olivary complex
Clinical Notes
 Conductive
– Cerumen in canal, Otitis Media of Middle Ear (ear infection)
 Sensorineural
– Meniere’s disease (abnormality in the fluids of the inner ear
= vertigo), Presbycusis (age related hearing loss)
 Central
– Pathology in cortex
– Bilateral auditory cortex lesions result in:
 Profound loss of auditory discriminative skills
 Impaired speech perception
 Hearing loss
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