Download Lecture 9

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The Function of Hearing
• the basics:
– nature of sound
– anatomy and physiology of the auditory system
Hearing: Physiology and
Psychoacoustics
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What Is Sound?
– loudness and pitch perception
– hearing impairments
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Sound Wave and Air Pressure
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What Is Sound? (cont’d)
• sounds are created when objects vibrate
– object vibrations cause molecules in object’s
surrounding medium to vibrate as well, which
causes pressure changes in medium
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What Is Sound? (cont’d)
• speed of sound wave propagation
– dependent on medium
– example:
• speed of sound through air is 340
meters/second
• speed of sound through water is 1500
meters/second
• Physical qualities of sound waves
– amplitude:
• displacement magnitude of sound pressure
wave
– frequency:
• number of times per sec. that pressure change
pattern repeats
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What Is Sound? (cont’d)
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What Is Sound? (cont’d)
• frequency
• Physical qualities of sound waves
– loudness:
– associated with pitch (but…)
• psychological aspect of sound related to perceived
intensity or magnitude
• low-frequency sounds -> low pitch
• high-frequency sounds -> high pitch
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Frequency and Amplitude
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What Is Sound? (cont’d)
• human hearing uses a limited range of
electromagnetic energy: From about 20 to 20,000 Hz
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What Is Sound? (cont’d)
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Intensity of Environmental Sounds
• Humans can hear wide range of sound intensities
– ratio between faintest and loudest sounds is more
than one to one million
– differences in amplitude, measured on a
logarithmic scale, in units called decibels (dB)
– relatively small decibel changes can correspond to
large physical changes
• e.g., increase of 6 dB corresponds to a
doubling of the amount of pressure
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What Is Sound? (cont’d)
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A Sine Wave
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Complex Sound Waves
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Wave Form and Spectrum (Part 1)
• Sine wave (pure tone)
– simplest of sounds
• sine wave:
– SPL varies sinusoidally as f(time)
– parameters
• period
– time for complete cycle of sine wave
• phase
– 360 degrees of phase across one period
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What Is Sound? (cont’d)
• Most sounds are “complex”
– e.g., human voices, birds, cars, etc.
• all sound waves can be described as some
combination of sine waves
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What Is Sound? (cont’d)
• complex sounds are described by Fourier analysis
– mathematical theorem by which any sound can be
divided into a set of sine waves. Combining these
sine waves will reproduce the original sound
– results can be summarized by a spectrum
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Wave Form and Spectrum (Part 1)
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What Is Sound? (cont’d)
• harmonics:
– integer multiples of the fundamental frequency
– first harmonic
• fundamental frequency
– lowest frequency component of the sound
– timbre:
• psychological sensation of sound quality
– “remainder” after equate loudness and pitch
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Harmonic Sounds with the Same Fundamental
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Basic Structure of the Mammalian Auditory System
• How are sounds detected and recognized by the
auditory system?
– sense of hearing evolved over millions of years
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Basic Structure of the Mammalian Auditory System (cont’d)
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Mammalian Pinnae
• Outer ear:
– pinnae
• collects sounds from environment
– ear canal (external auditory meatus)
• sound waves funneled by the pinnae into canal
• length and shape of ear canal boosts sound
frequencies
• purpose - insulate structure at its end:
– tympanic membrane
• ear drum
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Basic Structure of the Mammalian Auditory System (cont’d)
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• tympanic membrane: (eardrum);
Basic Structure of the Mammalian Auditory System (cont’d)
• Middle ear:
– thin sheet of skin at end of outer ear canal;
– pinnae and ear canal make up outer ear
– it vibrates in response to sound
– tympanic membrane is
• border between outer ear and middle ear
– middle ear ossicles
• 3 tiny bones that amplify sounds
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Basic Structure of the Mammalian Auditory System (cont’d)
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Structure of the Human Ear (Part 1)
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Basic Structure of the Mammalian Auditory System (cont’d)
• ossicles:
– malleus, incus, stapes; smallest bones in body
– stapes transmits vibrations of sound waves to oval
window, another membrane which represents
border between middle ear and inner ear
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Basic Structure of the Mammalian Auditory System (cont’d)
• amplification by ossicles essential for hearing faint
sounds
– impedance mismatch
• sound travel in air and fluid
• middle ear reflex:
– two muscles
• tensor tympani
• stapedius
• inner ear fluid
– purpose: contract in response to loud sounds
– However,
• acoustic reflex follows onset of loud sounds by 200ms,
so cannot protect against abrupt sounds, (e.g., gun shot)
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Basic Structure of the Mammalian Auditory System (cont’d)
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• inner ear:
Basic Structure of the Mammalian Auditory System (cont’d)
• cochlear canals and membranes
– fine changes in sound pressure translated into
neural signals
– cochlea:
• spiral structure of the inner ear containing the
organ of Corti
– function
• roughly analogous to that of retina
• filled with watery fluids in three parallel canals
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The Cochlea (Part 1)
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The Cochlea (Part 2)
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The Cochlea (Part 3)
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The Cochlea (Part 4)
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Basic Structure of the Mammalian Auditory System (cont’d)
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• 3 canals of the cochlea
• Cochlear mechanics
– tympanic canal
– vibrations transmitted over tympanic membrane
– vestibular canal
– middle-ear bones cause stapes to push and pull
flexible oval window in and out of vestibular canal
at base of cochlea
– middle canal
• 2 membranes
– extremely intense sounds,
– Reissner’s membrane
• any remaining pressure is transmitted through
helicotrema and back to cochlear base through
tympanic canal, where it is absorbed by
another membrane: round window
– basilar membrane
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Basic Structure of the Mammalian Auditory System (cont’d)
Basic Structure of the Mammalian Auditory System (cont’d)
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• Organ of Corti
Basic Structure of the Mammalian Auditory System (cont’d)
• Hair cells in each human ear: Arranged in four rows
that run down length of basilar membrane
– extends along top of basilar membrane
– movements of cochlear partition translated into
neural signals by structures in the organ of Corti;
– made up of
• specialized neurons called hair cells
• dendrites of auditory nerve fibers
– terminate at base of hair cells
• scaffold of supporting cells
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Basic Structure of the Mammalian Auditory System (cont’d)
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Vibration and the Tectorial Membrane
• tectorial membrane:
– extends atop organ of Corti;
– gelatinous structure - like a flap
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Basic Structure of the Mammalian Auditory System (cont’d)
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• firing of auditory nerve fibers
• End of notes for Test 2
– completes process of translating sound waves into
patterns of neural activity
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Basic Structure of the Mammalian Auditory System (cont’d)
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• coding of amplitude and frequency in the cochlea
• inner and outer hair cells
– place code:
– inner hair cells:
• different parts of cochlea to “tuned to” different
frequencies
• convey almost all information about sound
waves to brain
• Therefore, information about the frequency of
incoming sound wave
– outer hair cells:
• convey information from brain (use of
efferents)
– coded by place along cochlear partition with
greatest mechanical displacement
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The Cochlea is Tuned to Different Frequencies
Basic Structure of the Mammalian Auditory System (cont’d)
– involved in elaborate feedback system
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Basic Structure of the Mammalian Auditory System (cont’d)
• auditory nerve (VIII cranial nerve)
– AN fibers response relate to place along the
cochlear partition
– AN are frequency selectivity:
• best selectivity to faint sounds
– threshold tuning curve:
• thresholds of a neuron or fiber in response to
sine waves with varying frequencies
– threshold - lowest intensity to give response
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Threshold Tuning Curves
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Basic Structure of the Mammalian Auditory System (cont’d)
• two-tone suppression:
– decrease in firing rate of an auditory nerve fiber
due to one tone, when a second tone is presented
at the same time
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Two-Tone Suppression
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Basic Structure of the Mammalian Auditory System (cont’d)
• rate saturation
– Saturation point of a nerve fiber
• it is firing as rapidly as possible
• further stimulation incapable of increasing the firing rate
• Answer to:
– Are AN fibers as selective for their characteristic frequencies
at levels well above threshold as they are for the barely
audible sounds?
• isointensity curves:
– AN fiber’s firing rate to wide range of frequencies, all
presented at same intensity level
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Isointensity Functions
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Basic Structure of the Mammalian Auditory System (cont’d)
• rate intensity function:
– firing rate of an auditory nerve fiber in response to
a sound of constant frequency at increasing
intensities
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Firing Rate vs. Sound Intensity
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Basic Structure of the Mammalian Auditory System (cont’d)
• temporal code for sound frequency
– (complementary) alternative to place code
– Phase locking:
• firing of a single neuron at one distinct point in
the period (cycle) of a sound wave at a given
frequency
– evidence for phase locking:
• firing pattern of AN fiber carries a temporal
code
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Neural Spikes
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Basic Structure of the Mammalian Auditory System (cont’d)
• temporal code:
– tuning of different parts of the cochlea to different
frequencies, in which information about the
particular frequency of an incoming sound wave is
coded by the timing of the neural firing as it relates
to the period of the sound
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Basic Structure of the Mammalian Auditory System (cont’d)
• Volley principle:
– multiple neurons provide a temporal code for
frequency if each neuron fires at a distinct point in
the period of a sound wave but does not fire on
every period
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Basic Structure of the Mammalian Auditory System (cont’d)
• Auditory brain structures
– AN (cranial nerve VIII) carries signals from
cochlea to brain stem
– There, all AN fibers initially synapse in cochlear
nucleus
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Auditory System Pathways
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Basic Structure of the Mammalian Auditory System (cont’d)
• sub-cortical auditory processing structures
– superior olive,
– inferior colliculus
– medial geniculate nucleus (MGN)
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Basic Structure of the Mammalian Auditory System (cont’d)
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The First Stages of Auditory Processing
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Basic Operating Characteristics of the Auditory System
• tonotopic organization:
– arrangement in which neurons that respond to
different frequencies are organized anatomically in
order of frequency
– maintained in primary auditory cortex (A1)
– neurons from A1
• project to belt area
• then to parabelt area
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Basic Structure of the Mammalian Auditory System (cont’d)
• structure of auditory versus visual systems
– auditory system:
• much processing is done before A1
– visual system:
• Psychoacoustics:
– Study of the psychological correlates of the
physical dimensions of acoustics;
• a branch of psychophysics
• much processing occurs beyond V1
– differences may be due to evolutionary reasons
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Intensity and Loudness
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• Audibility threshold: A map of just barely audible
tones of varying frequencies
Intensity and Loudness (cont’d)
• Temporal integration:
– process by which a sound at a constant level is
perceived as being louder when it is of greater
duration
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Intensity and Loudness (cont’d)
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• Tonotopic organization of auditory system
• Psychoacousticians:
– suggests that frequency composition is
determinant of how we hear sounds
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Intensity and Loudness (cont’d)
Intensity and Loudness (cont’d)
– Study how listeners perceive pitch
• research done using pure tones suggests that
humans are good at detecting small differences
in frequency
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Critical Bandwidth and Masking
• Masking:
– using a second sound, “frequency noise”, to make the
detection of another sound more difficult;
– used to investigate frequency selectivity
• White noise:
– Consists of all audible frequencies in equal amounts; used in
masking
– band pass (pink) noise
• Critical bandwidth:
– range of frequencies that are conveyed within channel in
auditory system
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Hearing Loss
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• Conductive hearing loss:
• Hearing can be impaired by damage to any of
structures along chain of auditory processing
– problems with the bones of the middle ear, (e.g.,
during ear infections, otitis media)
– Peripheral
– Otosclerosis:
• obstructing the ear canal results in temporary
hearing loss (e.g., earplugs)
• more serious type of conductive loss
• abnormal growth of middle ear bones
• excessive buildup of ear wax (cerumen) in ear
canal
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Hearing Loss (cont’d)
Hearing Loss
– can be remedied by surgery
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– sensorineural hearing loss:
Hearing Loss (cont’d)
• Hearing loss: Natural consequence of aging
• Most common & serious auditory impairment.
• defects in cochlea or auditory nerve;
– Young people:
– when hair cells are injured, (e.g., as result of
antibiotics or cancer drugs, ototoxic)
• range of 20–20,000 Hz
– college age: 20–15,000 Hz
– common hearing loss:
• damage to hair cells due to excessive exposure
to noise
– Hearing aids:
• electronic aids
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Hearing Loss in Easter Islanders
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Hearing Loss with Age
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