Transcript Ear Structure - Auditory Processes

Hearing (a.k.a. Audition)
Our auditory sense
If a tree falls in the forest
and no one is there to hear
it, is there any sound?
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The answer to this philosophical question is quite
simple when it comes to science…
Yes, because sound is defined physically in terms
of changes in air pressure. These changes in air
pressure could be recorded in the absence of an
observer.
If, however, the question is “Is there any noise?,”
then the answer is quite different…
No, because noise is a psychological correlate of
sound generated by brain activity. Without an
observer, there is no noise.
Hearing: The Auditory System
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Stimulus = sound waves
(vibrations of molecules
traveling in air)
• Amplitude (loudness)
• Wavelength (pitch)
Wavelength/Frequency
The number of complete wavelengths that pass through
a point at a given time determines the pitch (range of
high and low sounds) of a sound.
• Amplitude is a measure of the physical strength
of the sound wave (shown in its peak-to-valley
height).
*It is a description of sound pressure and it is
measured in decibels (db).
*The amplitude determines how loud the sound is.
The higher the crest of the wave is, the louder the
sound is perceived.
*When you turn the volume down on your stereo,
you are decreasing the amplitude of the sound
waves
The Structure of the Ear
The External Ear, Middle Ear,
and Inner Ear
The Outer/External Ear
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The outer ear is the part of the ear that people
can see. It's what people pierce to wear earrings.
• The main job of the outer ear is to
________________.
The Middle Ear: Good Vibrations
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After sound waves
enter the outer ear,
they travel through the
ear canal and make
their way to the middle
ear.
The middle ear's main
job is to take those
sound waves and turn
them into vibrations
that are delivered to
the inner ear.
To do this, it needs the
eardrum, called the
tympanic
membrane, which is a
thin piece of skin
stretched tight like a
drum.
The Middle Ear: Good Vibrations (cont.)
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The eardrum separates
the outer ear from the
middle ear and the
ossicles.
Ossicles: the three
tiniest, most delicate
bones in your body.
They include:
• The malleus, which is
attached to the eardrum
and means "hammer" in
Latin
• The incus, which is
attached to the malleus
and means "anvil" in
Latin
• The stapes, the smallest
bone in the body, which
is attached to the incus
and means "stirrup" in
Latin
The Middle Ear: Good Vibrations (cont.)
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When sound waves reach the
eardrum, they cause the
eardrum to vibrate.
When the eardrum vibrates,
it moves the tiny ossicles —
from the hammer to the anvil
and then to the stirrup.(help
sound move along on its
journey into the inner ear)
The Inner Ear: Nerve Signals Start Here
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When the stirrup vibrates, it hits
against the oval window, the
membrane surrounding a snailshaped structure called the
cochlea
The cochlea is a small, curled
tube in the inner ear that is
filled with liquid, which is set
into motion, like a wave, when
the ossicles vibrate.
The formerly airborne sound
wave becomes “seaborne” as
the vibrations set the fluid into
wave motion.
This fluid wave spreads through
the cochlea, causing a
sympathetic vibration in the
basilar membrane, a thin strip
of tissue running through the
cochlea.
This is where transduction, the
conversion of vibrations into
neural messages, will happen…
Transduction in the Cochlea
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Much like vision, the
psychological sensation of
sound requires that waves be
transduced into neural
impulses and sent to the
brain.
The swaying of tiny hair cells
(the auditory receptors) on
the vibrating basilar
membrane (much like the
swaying of buildings during
an earthquake) stimulates
sensory nerve endings
connected to the hair cells.
The axons of these nerve cells
converge to form the
auditory nerve, which sends
neural messages (via the
thalamus) to the temporal
lobe’s auditory cortex in the
temporal lobes.
Ear Sound Waves 1
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Instructor’s Notes
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Ear Sound Waves 2
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Eustachian Tube
See
http://www.sumanasinc.co
m/webcontent/animations/
content/soundtransduction.
html for an animated demo
of the process of hearing.
How do we perceive
differences in pitch?
There are two theories:
Place Theory and
Frequency Theory
Helmholtz’s Place Theory
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Hermann von Helmholtz
(1863) proposed that
perception of pitch
corresponds to the
vibration of different
portions, or places, along
the basilar membrane.
Thus, different places have
different pitches, like keys
on a piano.
• So some hairs vibrate when
they hear high pitches and
others vibrate when they hear
low pitches.
Helmholtz’s Place Theory
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Problem with the theory?
It can’t explain how we hear
low-pitched sounds,
because the neural signals
for low-pitched sounds are
not so neatly localized on
the basilar membrane.
Frequency Theory
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Frequency theory: which holds
that perception of pitch
corresponds to the rate at which
the entire basilar membrane
vibrates
• Causing the auditory nerve to fire at
different rates for different
frequencies.
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Thus, according to this theory, the
brain detects the frequency of a
tone by the rate at which the
auditory nerve fires.
All the hairs
vibrate but at
different
speeds.
Frequency Theory
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We sense pitch by the basilar
membrane vibrating at the same
rate as the sound (if a sound wave
has a frequency of 100 waves per
second, then 100 impulses per
second travel up the auditory
nerve).
Problem with the theory?
• An individual neuron cannot fire faster
than 1000 times per second. How,
then, can we sense sounds with
frequencies above 1000 waves per
second (roughly the upper third of a
piano keyboard)?
All the hairs
vibrate but at
different
speeds.
Frequency Theory
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This problem can be explained
using the volley principle.
• Volley principle – like soldiers who
alternate firing so that some can shoot
while others reload, neural cells can
alternate firing.
• By firing in rapid succession, they can
achieve a combined frequency above
1000 waves per second.
All the hairs
vibrate but at
different
speeds.
So which theory of pitch is right?
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Like with research in theories of color vision,
researchers argued about these two competing
theories for almost a century.
It turns out that both are valid - in part.
The two were reconciled by Georg von Bekesy,
1947, with his traveling wave theory. Basically,
von Bekesy said that the whole basilar membrane
does move, but the waves peak at particular
places, depending on frequency.
• Place theory best explains how we sense high pitches,
frequency theory best explains how we sense low
pitches, and some combination of place and frequency
(Bekesy’s theory) seems to handle the pitches in the
intermediate range.
Hearing Loss
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Conduction Hearing Loss: caused by damage to
mechanical system of ear.
• Ex: punctured eardrum, inability of the tiny bones of
the middle ear to vibrate, inability of the ear to
conduct vibrations, etc.
Sensorinueral hearing loss (Nerve deafness):
caused by damage to cochlea’s hair cell
receptors or their associated auditory nerves.
• More common
• Usual causes are heredity, aging, and prolonged
exposure to ear-splitting noise or music.
Deafness
Nerve (sensorineural)
Deafness
Conduction Deafness
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Something goes wrong
with the sound and the
vibration on the way to
the cochlea.
You can replace the
bones or get a hearing
aid to help.
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The hair cells in the cochlea
get damaged.
Loud noises can cause this
type of deafness.
NO WAY to replace the hairs.
Cochlear implant is possible.
Cultural Differences in
Hearing Loss?
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While the majority of 70 year olds living
near the Sudanese-Ethiopian border could
hear a whisper from 100 yards away,
about 1 in 4 Americans over 65 needs a
hearing aid to detect whispers across the
room.
The data from Africa suggest that hearing
loss may not be a physiological
consequence of aging but could be the
cumulative effect of a lifetime’s exposure
to environmental noise.
Why does our own voice sound
unfamiliar when we hear it on tape?
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The answer? Bone conduction!
When we listen to ourselves speak, we hear both the sound
conducted by air waves to the outer ear and that carried
directly to the auditory nerve by bone conduction. The latter is
easily demonstrated by clicking the teeth or munching
popcorn, or by striking the prongs of a fork on a table and
quickly applying its handle to the bone behind the ear. An even
more resounding effect will be produced if the handle is
clenched between the teeth. The strictly airconducted sound
that others normally hear (like a sound we hear when our voice
is on tape) is thinner. Students can hear the sound waves
conducted by bone if they plug their ears and talk in a normal
voice.
You can also demonstrate boneconducted sound with a metal
coat hanger tied to the center of a thin string about four feet
long.
You should first press one end of the string into each ear with
the tips of the index fingers while plugging your ears. Then ask
someone to tap the coat hanger with a knife or fork. John
Fisher reports that the effect will sound like “Big Ben.”
Are all deaf people really deaf?
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People who are deaf due to a defect in either the
inner or middle ear may still be able to hear by
bone conduction.
When Beethoven became deaf, he could still hear
a piano being played by placing one end of his
walking stick against it and gripping the other
end between his teeth.
To determine the nature and degree of their
hearing loss, deaf violinists reportedly applied
their teeth to some part of their vibrating
instruments. If they could not hear sound, they
concluded that the auditory nerves were the
problem and the deafness was past cure.