CHAPTER 9
Hearing and Language
Hearing
Language
Sensory Receptors
• A receptor
– is a cell, often a specialized neuron,
– suited by its structure and function to respond to a particular form
of energy, such as sound.
– Function: Convert that energy into a neural response.
• Adequate stimulus: energy form for which receptor is
specialized to detect
– Due to the imperfect specialization of receptors, other stimuli will
often produce responses as well.
– For example, if you apply pressure to the side of your eyeball
(through the lid) you will see a circular dark spot.
Stimulus and patterning
• Several implications:
– A sensory system will register a sensory experience even
if the stimulus is inappropriate.
– For example:
• When neurosurgeon (or blow to head) stimulates auditory
cortex :the patient hears buzzing sound or even voices or music
• When you fall on your head rollerblading you really do see
“stars” if you hit occipital lobe
• Patterning of stimulation:
– the information contained in the stimulus
– This is what makes sensory information meaningful.
Hearing and Language
• Sensation versus perception:
– Sensation is the acquisition of sensory information.
– Perception is the interpretation of sensory information.
• Actually, very, very difficult distinction
– When does acquisition of information or input
end?
– When does processing or interpretation begin?
Audition
• Hearing:
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adequate stimulus for audition is vibration in a conducting medium.
Normally: Conducting medium is air
can also hear under water
Also: hear sound conducted through our skull.
• Air vibrates due to vibration of the sound source
– a person’s vocal cords
– a bell that has been struck
– stereo speaker.
• As the sound source vibrates, it alternately compresses and
decompresses the air.
Sensing air
pressure changes
• Audition or hearing involves detecting changes
in pressure
• Humans notice variety of pressure changes
– weather
– altitude changes
– colds, flus, water in ears, etc.
• Sound = pressure changes detected by cochlea
Important measures
• Psychological vs physical
• Loudness = decibels or dB
– Absolute threshold is about 5-10 db
– 70-80 dB starts to do damage
– 100-120 dB is damaging!
• Pitch = frequency or hertz
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Hearing range for humans from 20-20,0000 cycles/sec or Hertz (Hz)
Lower than 20 Hz you feel rather than hear
Perceive higher pitch as louder (even at same dB)
Perceive lower pitch as softer (Even at same dB)
• Timbre= harmonics or overtones of an instrument
– harmonics or overtones of a pitch
– unique sound quality of each voice or instrument
– sound recognition software uses this
Anatomy of Ear
• Pinna: outermost flap; funnels sound
• Outer ear canal:
– Cylindrical hole in skull between pinna and ear drum
– You can stick your finger in this part (but don’t!)
• Ear drum: tympanic membrane
– piece of living (skin) tissue
– can regenerate under many conditions
– sensitivity to air pressure: bounces in and out
Middle ear
• Middle ear: hollow region behind tympanic membrane
– approximately 2 ml in volume
– contains the OSSICLE CHAIN: three important bones
• Ossicles: malleus, incus and stapes
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hammer, anvil and stirrups
bones directly behind tympanic membrane
move back and forth in response to pressure changes
make middle ear canal more sensitivity to pressure: help move the air
back to cochlear
– malleus connects to tympanic membrane and transmits vibrations via
the incus and stapes
– stapes connects to membrane behind oval window
• Oval window
– opening (in skull) behind ossicles
– helps direct pressure waves (sound waves) back to cochlear
Vestibular membrane and
semicircular canals
• Detects static position
– allows us to pick up sense of gravity and pressure changes
– Tells us where our head and body are in position to one
another and to earth
• Movement detection:
– semi-circular fluid bends another set of hair cells when
stimulated
– this allows detection of acceleration and deceleration
– When get a cold/flu or pressure changes, this sense is
thrown off
Cochlea
• Snail shaped indentation in skull
• Three separate sections
– scala vestibula or vestibular stairway
– scala tympani or tympanic stairway
– scala media or middle stairway
• The stirrup rests on the oval window, a thin, flexible
membrane on the face of the vestibular canal.
• Vestibular canal: point of entry of sound into the cochlea.
– connects with tympanic canal at far end of the cochlea though opening
called helicotrema.
– Helicotrema: Allows pressure waves to travel through the cochlear fluid
into the tympanic canal more easily.
Cochlea
• Organ of Corti: receptive organ of the ear
– The vibration passes across the organ of Corti,
– Contains basilar, tectoral membrane
• Basilar Membrane: “base” membrane
– Consists of 4 rows of specialized cells: hair cells
– Flexible membrane
• Tectoral membrane:
– Membrane above these hair cells
– Like the roof of the organ of corti
– Is very rigid
• Auditory receptor cells = hair cells
Hair Cells
• The hair cells = receptors for auditory stimulation.
– Anchored by rod-like cells called Deiter’s cells to the basilar
membrane
– Ends of some of them attach to rigid tectorial membrane (this
projects overhead like a shelf)
• The human cochlea has about
– 12,000 outer hair cells, in three rows
– Single row of 3,400 inner hair cells.
• Inner hair cells receive 90-95% of the auditory neurons
– provide the majority of information about auditory stimulation
– But most hearing loss due to loss of OUTER hair cells
– Reduces amplification for inner hair cells
Auditory Nerve
• Auditory nerves (left ear and right ear):
– Contain neurons from the two cochlea
– Make up part of the auditory nerves (VIII cranial nerve)
– One of each nerve enters the brain on each side of the
brain stem.
• The neurons pass through:
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brain stem nuclei to
the inferior colliculi,
to the medial geniculate nucleus of the thalamus,
To auditory cortex in each temporal lobe.
Main pathway for Auditory system
Auditory nerve  dorsal cochlear nucleus
(crosses midline)  Inferior colliculus
 medial geniculate nucleus of thalamus
 primary auditory cortex of temporal lobe
Cortical processing
• Auditory cortex
– superior (upper) gyrus of the temporal lobe of each hemisphere.
– Area is topographically organized
• neurons from adjacent receptor locations project to adjacent cells in the
cortex.
• projections form a sort of map of the unrolled basilar membrane.
• Beyond the primary auditory cortex are additional
processing areas.
– These secondary auditory areas are involved in processing
complex sounds and understanding their meaning.
– Integration with visual and tactile/kinesthetic areas of brain
How does sound
stimulate hair cells?
• How does air get into cochlea and how does
liquid stay in?
– membrane covers round window
– allows fluid to stay in
– air pressure pushes on window, pushes fluid
• Wave like action which then pushes hair cells
back and forth
How does sound
stimulate hair cells?
• Vibratory energy exerted on oval window causes basilar
membrane to bend and move
• Moves and bends in two ways:
– Where it bends depends on frequency (pitch) of the sound
source
• High frequency sounds cause end nearest oval window to bend
• Low frequency sounds = apex of basilar membrane (farthest from oval
window) to bend
– How fast it bends and moves depends on frequency of the sound
source
• High frequency sounds: moves faster
• Low frequency sounds: moves slower
Theories of hearing
• Frequency theory: assumes that
– Auditory mechanism transmits the actual sound frequencies to
the auditory cortex for analysis there
– Different hair cells fire at different rates.
– Low rate of vibration for low frequency sounds, high rate of
vibration for high frequency sounds
• Telephone theory of frequency theory: assumes that
– individual neurons in the auditory nerve fire at the
same frequency as the rate of vibration of the sound
source
– Rate of vibration maps rate of sound frequency
Theories of hearing
• Place theory:
– identify frequency of a sound by location of maximal vibration on the
basilar membrane
– Thus depends on WHERE neurons are firing most.
• Volley Principle:
– Groups of neurons follow sound frequency
– Combination of place and frequency
– That is, both theories are correct
• Forms a Tonotopic map:
– Remember: auditory cortex topographically organized,
– Basilar membrane and hair cells respond in a way that can map the
frequency of sounds by place OR frequency of vibration
So: Place and Frequency
theories both correct
• At certain frequences, each theory is correct
– Place theory explains processing of sounds of 200-20,000 Hz
– frequency theory explains processing of sounds of 20-4,000 Hz
• Notice overlap from 200-4000 Hz
– at some frequencies both place and frequency of membrane
are affected
– rate of membrane vibration changes, but only for particular spot
– Seems to VOLLEY back and forth between places on the
membrane
– Membrane changes in both frequency and location of vibration
– Double coding
– Why? Human speech in this range!
Copyright © 2005 Allyn & Bacon
Processing Timbre
• Shape of a waveform repeats itself regularly at the fundamental
frequency
– corresponds to perceived pitch of note (think middle C and octaves of C above
and below middle C)
• But: Also produces many different overtones
– multiples of the fundamental frequency
– Different instruments produce different series of overtones
– Each instrument has unique timbre
• Sound of an instrument is dynamic
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changes over time
beginning sound of a tone is different than end across instruments
different pattern of harmonics or overtone
thus, must be able to code the sequence of overtones as well as the different
overtones
• This is how voice recognition software works:
– Maps pattern of overtones and fundamental frequency of voice
– Compares speaker to that of record: if same = match!
Perception of
spatial location
• How do organisms tell WHERE the sound is coming from?
– Most animals quite good at this
– humans are very poor at this!~
– Humans can tell if left or right, in general, but not front/back
(WHY?)
• Has to do with phase differences
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Arrival of sound to each ear does not occur simultaneously
Slight difference = phase difference
Out of phase = cue for distance
Have specialized neurons to detect phase differences
• works best for low frequencies
• why surround sound = low sounds then?
So why can humans detect left/right sound differences but not front and back?
What would we need to change about our hearing system to do this?
Put all together =
pattern recognition
• Pattern recognition must include the ability to process:
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pitch
loudness
timbre
location
• Each “voice” or language sound is unique pattern
• The job of ears/brain is to make sense of this
• Auditory agnosia
– inability to process sounds/speech
– use it or lose it
– This becomes critical for deafness and the remediation of deafness!