Audition Outline
•
Perceptual dimensions
• Ear Anatomy
• Auditory transduction
• Pitch Perception
– by Place Coding
– by Rate coding
•
Sound Localization
– by phase difference
– by intensity difference
Perceptual Dimensions
Stimulus
Vision
Audition
Frequency
Hue (nm)
(‘color’)
Brightness
Pitch (Hz)
Amplitude
Purity (vs.
complexity)
(e.g. 440Hz)
Loudness (dB)
Saturation
Timbre
complexity
Sound:
Variation of pressure over time
Ear Anatomy
• Peripheral Structures
–
–
–
–
Outer ear
Middle ear
Inner ear
Auditory nerve
• Central Structures
– Brainstem
– Midbrain
– Cerebral
Ear Anatomy
Air
Bones
Liquid
Eardrum >> oval window
Tympanic Membrane
(ear drum)
semi-transparent
cone shaped
http://icarus.med.utoronto.ca/NeuroExam/
Pearly gray
1=Attic (pars flaccida)
2= Lateral process of malleus
3=Handle of malleus
4=End of the malleus
5=Light reflex
http://www.qub.ac.uk/cskills/Ears.htm
How to use an otoscope
http://medweb.uwcm.ac.uk/otoscopy/Default.htm
http://medweb.uwcm.ac.uk/otoscopy/common.htm
Virtual otoscope & common conditions
normal
Acute otitis media with effusion. There is:
- distortion of the drum,
- prominent blood vessels in the upper half
- dullness of the lower half.
- bulging of the upper half of the drum
- the outline of the malleus is obscured.
Normal
Membrane
Opaque with Inflammation
Chronic
Inflammation
Resolving Infection
Bulging Membra
Middle Ear
• Eustachian Tube: connects to
pharynx
• Ossicles: 3 bones, which
transmit acoustic energy from
tympanic membrane to inner ear
Ossicles’ functions
• To amplify sound waves, by a
reduction in the area of force
distribution (Pressure = Force/Area)
• To protect the inner ear from
excessively loud noise. Muscles
attached to the ossicles control
their movements, and dampen
their vibration to extreme noise.
• to give better frequency resolution
at higher frequencies by reducing
the transmission of low
frequencies (again, the muscles
play a role here)
Inner ear
Middle ear
www.iurc.montp.inserm.fr/cric/audition/english/cochlea/fcochlea.htm
Transduction
of sound
-
Basilar membrane oscillates
Outer Hair cell cilia bends
Cations inflow
Depolarization
Increased firing rate
• Bend on opposite direction
• Reduced firing rate
Pitch Perception:
Place vs. Rate Coding
Hz
0
20
Volley Code
language 500
HUMAN
RANGE
2000
4000
Place Code
Place Coding: Tonotopic representation
• Base
• High Freq
–
Apex
–
Low Freq.
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.
Pitch perception:
Place coding
• The cochlea has a
tonotopic organization
• For high frequencies
Pitch Perception: Rate code
• Used for low frequency sounds ( <1500 Hz )
• Mechanism: The rate of neural firing matches
the sound's frequency. For example,
– 50 Hz tone (50 cycles per sec) -> 50 spikes/sec,
– 100 hz -> 100 spikes/sec
• Problem: even at the low frequency range,
some frequencies exceed neurons’ highest
firing rate (200 times per sec)
• Solution: large numbers of neurons that are
phased locked (volley principle).
Sound Localization
Interaural Intensity Difference
(high frequency)
Interaural Time Difference
(low frequency)
Delay Lines –
Interaural Time
Difference (ITD)
Deafness
• Conduction deafness
– outer or middle ear deficit
– E.g. fused ossicles. No nerve damage
• Sensori-neural
– Genetic, infections, loud noises (guns & roses),
toxins (e.g. streptomicin)
– Potential Solution: Cochlear implants
• Central
– E.g. strokes
Central
Auditory
Mechanism
• Bilateral projection to auditory cortex (stronger contralateral).
• Also, efferent fibers from inferior colliculus back to ears:
•they attenuate motion of the middle ear bones (dampen loud sounds)
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
• Primary Auditory cortex:
– Tonotopic Organization
– Columnar Organization
– Cells with preferred
frequency, and
– cells with preferred interaural time difference
Anatomy (part 3) source : Palmer & Hall, 2002
Right
hemisphere
• Primary & non-primary
auditory
Sylvian cortex
Fissure
Medial
Temporal
Gyrus
planum polare
(nonprimary AC)
Superior
Temporal Gyrus
Superior Temporal Sulcus
Heschl’s gyrus
(primary AC)
planum temporale
(nonprimary AC)
Spare slides
Steps to Hearing: A summary
• Sound waves enter the external ear
• Air molecules cause the tympanic membrane to vibrate, which in
turn makes vibrate the ossicles on the other side
• The vibrating ossicles make the oval window vibrate. Due to small
size of oval window relative to the tympanic membrane, the force
per unit area is increased 15-20 times
• The sound waves that reach the inner ear through the oval window
set up pressure changes that vibrate the perilymph in the scala
vestibuli
• Vibrations in the perilymph are transmitted across Reissner’s
membrane to the endolymph of the cochlear duct
• The vibrations are transmitted to the basilar membrane which in
turn vibrates at a particular frequency, depending upon the position
along its length (High frequencies vibrate the window end and low
frequencies vibrate the apical end where the membrane is wide)
• The cilia of the hair cells, which contact the overlying tectorial
membrane, bend as the basilar membrane vibrates Displacement
of the stereocilia in the direction of the tallest stereocilia is
excitatory and in the opposite direction is inhibitory
• The actions are transmitted along the cochlear branch of the
Auditory Nerve Tuning Curves
(receptive fields)
Inner Ear - Labyrinth
Inner Ear – Organ of Corti