Hearing: Physiology and Psychoacousticsotoole/PSY_4362/Ch09_Lecture-f.ppt.pdf1 9 Hearing: Physiology and Psychoacoustics 9 The Function of Hearing •the basics: –nature of sound

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9Hearing: Physiology and

Psychoacoustics

9 The Function of Hearing

• the basics:

– nature of sound

– anatomy and physiology of the auditory system

– loudness and pitch perception

– hearing impairments

9 What Is Sound?

• sounds are created when objects vibrate

– object vibrations cause molecules in object’ssurrounding medium to vibrate as well, whichcauses pressure changes in medium

9 Sound Wave and Air Pressure

9 What Is Sound? (cont’d)

• speed of sound wave propagation

– dependent on medium

– example:

• speed of sound through air is 340meters/second

• speed of sound through water is 1500meters/second


• Physical qualities of sound waves

– amplitude:

• displacement magnitude of sound pressurewave

– frequency:

• number of times per sec. that pressure changepattern repeats

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• Physical qualities of sound waves

– loudness:

• psychological aspect of sound related to perceivedintensity or magnitude


• frequency

– associated with pitch (but…)

• low-frequency sounds -> low pitch

• high-frequency sounds -> high pitch

9 Frequency and Amplitude 9 What Is Sound? (cont’d)

• human hearing uses a limited range ofelectromagnetic energy: From about 20 to 20,000 Hz


• Humans can hear wide range of sound intensities

– ratio between faintest and loudest sounds is morethan one to one million

– differences in amplitude, measured on alogarithmic scale, in units called decibels (dB)

– relatively small decibel changes can correspond tolarge physical changes

• e.g., increase of 6 dB corresponds to adoubling of the amount of pressure

9 Intensity of Environmental Sounds

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• 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

9 A Sine Wave


• Most sounds are “complex”

– e.g., human voices, birds, cars, etc.

• all sound waves can be described as somecombination of sine waves

9 Complex Sound Waves


• complex sounds are described by Fourier analysis

– mathematical theorem by which any sound can bedivided into a set of sine waves. Combining thesesine waves will reproduce the original sound

– results can be summarized by a spectrum

9 Wave Form and Spectrum (Part 1)

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9 Wave Form and Spectrum (Part 1) 9 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

9 Harmonic Sounds with the Same Fundamental 9 Basic Structure of the Mammalian Auditory System

• How are sounds detected and recognized by theauditory system?

– sense of hearing evolved over millions of years

9 Basic Structure of the Mammalian Auditory System (cont’d)

• 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 soundfrequencies

• purpose - insulate structure at its end:

– tympanic membrane

• ear drum

9 Mammalian Pinnae

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• tympanic membrane: (eardrum);

– thin sheet of skin at end of outer ear canal;

– it vibrates in response to sound


• Middle ear:

– pinnae and ear canal make up outer ear

– tympanic membrane is

• border between outer ear and middle ear

– middle ear ossicles

• 3 tiny bones that amplify sounds


• ossicles:

– malleus, incus, stapes; smallest bones in body

– stapes transmits vibrations of sound waves to ovalwindow, another membrane which representsborder between middle ear and inner ear

9 Structure of the Human Ear (Part 1)


• amplification by ossicles essential for hearing faintsounds

– impedance mismatch

• sound travel in air and fluid

• inner ear fluid


• middle ear reflex:

– two muscles

• tensor tympani

• stapedius

– 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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• inner ear:

– fine changes in sound pressure translated intoneural signals

– function

• roughly analogous to that of retina


• cochlear canals and membranes

– cochlea:

• spiral structure of the inner ear containing theorgan of Corti

• filled with watery fluids in three parallel canals

9 The Cochlea (Part 1) 9 The Cochlea (Part 2)

9 The Cochlea (Part 3) 9 The Cochlea (Part 4)

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• 3 canals of the cochlea

– tympanic canal

– vestibular canal

– middle canal

• 2 membranes

– Reissner’s membrane

– basilar membrane


• Cochlear mechanics

– vibrations transmitted over tympanic membrane

– middle-ear bones cause stapes to push and pullflexible oval window in and out of vestibular canalat base of cochlea

– extremely intense sounds,

• any remaining pressure is transmitted throughhelicotrema and back to cochlear base throughtympanic canal, where it is absorbed byanother membrane: round window


• Organ of Corti

– extends along top of basilar membrane

– movements of cochlear partition translated intoneural 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


• Hair cells in each human ear: Arranged in four rowsthat run down length of basilar membrane


• tectorial membrane:

– extends atop organ of Corti;

– gelatinous structure - like a flap

9 Vibration and the Tectorial Membrane

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• firing of auditory nerve fibers

– completes process of translating sound waves intopatterns of neural activity

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• End of notes for Test 2


• coding of amplitude and frequency in the cochlea

– place code:

• different parts of cochlea to “tuned to” differentfrequencies

• Therefore, information about the frequency ofincoming sound wave

– coded by place along cochlear partition withgreatest mechanical displacement


• inner and outer hair cells

– inner hair cells:

• convey almost all information about soundwaves to brain

– outer hair cells:

• convey information from brain (use ofefferents)

– involved in elaborate feedback system

9 The Cochlea is Tuned to Different Frequencies 9 Basic Structure of the Mammalian Auditory System (cont’d)

• auditory nerve (VIII cranial nerve)

– AN fibers response relate to place along thecochlear partition

– AN are frequency selectivity:

• best selectivity to faint sounds

– threshold tuning curve:

• thresholds of a neuron or fiber in response tosine waves with varying frequencies

– threshold - lowest intensity to give response

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9 Threshold Tuning Curves 9 Basic Structure of the Mammalian Auditory System (cont’d)

• two-tone suppression:

– decrease in firing rate of an auditory nerve fiberdue to one tone, when a second tone is presentedat the same time

9 Two-Tone Suppression 9 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 frequenciesat levels well above threshold as they are for the barelyaudible sounds?

• isointensity curves:

– AN fiber’s firing rate to wide range of frequencies, allpresented at same intensity level

9 Isointensity Functions 9 Basic Structure of the Mammalian Auditory System (cont’d)

• rate intensity function:

– firing rate of an auditory nerve fiber in response toa sound of constant frequency at increasingintensities

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9 Firing Rate vs. Sound Intensity 9 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 inthe period (cycle) of a sound wave at a givenfrequency

– evidence for phase locking:

• firing pattern of AN fiber carries a temporalcode

9 Neural Spikes 9 Basic Structure of the Mammalian Auditory System (cont’d)

• temporal code:

– tuning of different parts of the cochlea to differentfrequencies, in which information about theparticular frequency of an incoming sound wave iscoded by the timing of the neural firing as it relatesto the period of the sound


• Volley principle:

– multiple neurons provide a temporal code forfrequency if each neuron fires at a distinct point inthe period of a sound wave but does not fire onevery period


• Auditory brain structures

– AN (cranial nerve VIII) carries signals fromcochlea to brain stem

– There, all AN fibers initially synapse in cochlearnucleus

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9 Auditory System Pathways 9 Basic Structure of the Mammalian Auditory System (cont’d)

• sub-cortical auditory processing structures

– superior olive,

– inferior colliculus

– medial geniculate nucleus (MGN)


• tonotopic organization:

– arrangement in which neurons that respond todifferent frequencies are organized anatomically inorder of frequency

– maintained in primary auditory cortex (A1)

– neurons from A1

• project to belt area

• then to parabelt area

9 The First Stages of Auditory Processing


• structure of auditory versus visual systems

– auditory system:

• much processing is done before A1

– visual system:

• much processing occurs beyond V1

– differences may be due to evolutionary reasons

9 Basic Operating Characteristics of the Auditory System

• Psychoacoustics:

– Study of the psychological correlates of thephysical dimensions of acoustics;

• a branch of psychophysics

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9 Intensity and Loudness

• Audibility threshold: A map of just barely audibletones of varying frequencies

9 Intensity and Loudness (cont’d)

• Temporal integration:

– process by which a sound at a constant level isperceived as being louder when it is of greaterduration


• Tonotopic organization of auditory system

– suggests that frequency composition isdeterminant of how we hear sounds


• Psychoacousticians:

– Study how listeners perceive pitch

• research done using pure tones suggests thathumans are good at detecting small differencesin frequency


• Masking:

– using a second sound, “frequency noise”, to make thedetection of another sound more difficult;

– used to investigate frequency selectivity

• White noise:

– Consists of all audible frequencies in equal amounts; used inmasking

– band pass (pink) noise

• Critical bandwidth:

– range of frequencies that are conveyed within channel inauditory system

9 Critical Bandwidth and Masking

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9 Hearing Loss

• Hearing can be impaired by damage to any ofstructures along chain of auditory processing

– Peripheral

• obstructing the ear canal results in temporaryhearing loss (e.g., earplugs)

• excessive buildup of ear wax (cerumen) in earcanal

9 Hearing Loss

• Conductive hearing loss:

– problems with the bones of the middle ear, (e.g.,during ear infections, otitis media)

– Otosclerosis:

• more serious type of conductive loss

• abnormal growth of middle ear bones

– can be remedied by surgery

9 Hearing Loss (cont’d)

– sensorineural hearing loss:

• Most common & serious auditory impairment.

• defects in cochlea or auditory nerve;

– when hair cells are injured, (e.g., as result ofantibiotics or cancer drugs, ototoxic)

– common hearing loss:

• damage to hair cells due to excessive exposureto noise

9 Hearing Loss (cont’d)

• Hearing loss: Natural consequence of aging

– Young people:

• range of 20–20,000 Hz

– college age: 20–15,000 Hz

– Hearing aids:

• electronic aids

9 Hearing Loss in Easter Islanders 9 Hearing Loss with Age

Documents

Hearing: Physiology and Psychoacousticsotoole/PSY_4362/Ch09_Lecture-f.ppt.pdf1 9 Hearing: Physiology and Psychoacoustics 9 The Function of Hearing •the basics: –nature of sound