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Chapter 9-15

PSYB51H3 Chapter Notes - Chapter 9-15: Oval Window, Tas1R2, Abducens Nerve

Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier
Chapter
9-15

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CHAPTER 9: HEARING: PHYSIOLOGY AND PSYCHOACOUSTICS
What is Sound?
- Sounds are created when objects vibrate
- Vibrations of an object cause its surrounding medium to vibrate as well and this vibration in turn causes
pressure changes in the medium
- Pressure changes best described as waves
- Sound waves travel at a particular speed depending on medium, moving faster thru denser substances
o Sound travels faster in water than in air
Basic Qualities of Sound Waves: Frequency and Amplitude
- Amplitude or intensity: the magnitude of displacement of a sound pressure wave. Amplitude is perceived as
loudness.
- Frequency: for sound, the number of times per second that a pattern of pressure change repeats. Frequency is
perceived as pitch.
- Hertz (Hz): a unit of measure for frequency. One hertz equals one cycle per second.
- Loudness: the psychological aspect of sound related to perceived intensity (amplitude)
- Pitch: the psychological aspect of sound related mainly to perceived frequency
- Low-frequency sounds correspond to low pitches and high-frequency sounds correspond to high pitches
- Humans can detect sounds that vary from about 20 to 20,000 Hz
- Elephants hear vibrations at very low frequencies that help detect presence of large animals
- Sonar systems used by some bats use sound frequencies above 60,000 Hz
- Decibel (dB): a unit of measure for the physical intensity of sound. Decibels define the difference between two
sounds as the ratio between two sound pressures. Each 10:1 sound pressure ratio equals 20 dB, and a 100:1
ratio equals 40 dB
o dB = 20 log(p/pâ‚€)
o p is the pressure of sound being described
o pâ‚€ is a reference pressure and is typically defined in auditory research contexts to be 0.0002 dyne/cmÂ˛
- Levels are defined as dB SPL (sound pressure level)
- Relatively small decibel changes can correspond to large physical changes
Sine Waves and Complex Sounds
- Sine wave or pure tone: a waveform for which variation as a function of time is a sine function
- Spectrum: a representation of the relative energy present at each frequency
- Harmonic spectrum: the spectrum of a complex sound in which energy is at integer multiples of the
fundamental frequency
- Fundamental frequency: the lowest frequency component of a complex periodic sound
- Timbre: the psychological sensation by which a listener can judge that two sounds with the same loudness and
pitch are dissimilar. Timbre quality is conveyed by harmonics and other high frequencies.
Basic Structure of the Mammalian Auditory System
Outer Ear
- Sounds first collected from the environment by the pinna: the outer, funnel-like part of the ear

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- Ear canal: the canal that conducts sound vibrations from the pinna to the tympanic membrane and prevents
damage to the tympanic membrane
- Tympanic membrane: the eardrum; a thin sheet of skin at the end of the outer ear canal. The tympanic
membrane vibrates in response to sound
- While a ruptured eardrum can be excruciating, in most cases a damaged tympanic membrane will heal itself, but
it is possible to damage it beyond repair
Middle Ear
- Pinna and ear canal make up the outer ear: the external sound-gathering portion of the ear
- Tympanic membrane is the border between outer and middle ear: an air-filled chamber containing the middle
bones, or ossicles. The middle ear conveys and amplifies vibration from the tympanic membrane to the oval
window.
- Ossicle: any of three tiny bones of the middle ear
- Malleus: the first ossicle (connected to the tympanic membrane), the malleus receives vibration from the
tympanic membrane and is attached to the incus
- Incus: the middle of the three ossicles, connecting the malleus and the stapes
- Stapes: the last ossicle, connected to the incus on one end, the stapes presses against the oval window of the
cochlea on the other end
o Stapes transmits vibrations of sound waves to the oval window: the flexible opening to the cochlea thru
which the stapes transmits vibration to the fluid inside
- Inner ear: a hollow cavity in the temporal bone of the skull, and the structures within this cavity; the cochlea
and the semicircular canals of the vestibular system
- Ossicles are the smallest bones in human body
- Amplify sound vibrations in two ways:
o Joints are hinged in a way that makes them work like levers
o Ossicles increase the energy transmitted to the inner ear by concentrating energy from larger to smaller
surface area
- Tensor tympani: the muscle attached to the malleus; tensing the tensor tympani decreases vibration
- Stapedius: the muscle attached to the stapes; tensing the stapedius decreases vibration.
- Main purpose of muscles is to tense when sounds are very loud, restricting the movement of the ossicles and
thus muffling pressure change that might be large enough to cause damage
- Acoustic reflex: a reflex that protects the ear from intense sounds, via contraction of the stapedius and tensor
tympani muscles
o Follows the onset of loud sounds by 1/5 of a second
Inner Ear
- It is here that the fine changes in sound pressure are translated into neural signals that inform the listener about
the world
Cochlear Canals and Membranes
- Cochlea: a spiral structure of the inner ear containing the organ of Corti
- Tympanic canal: one of three fluid-filled passages in the cochlea. The tympanic canal extends from the round
window at the base of the cochlea to the helicotrema at the apex. Also called scala tympani.
- Vestibular canal: extends from the oval window at the base of the cochlea to the helicotrema at the apex. Also
called scala vestibule.

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- Middle canal: sandwiched between the tympanic and vestibular canals and contains the cochlear partition. Also
called scala media.
- Helicotrema: the opening that connects the tympanic and vestibular canals at the apex of the cochlea
- Three canals of cochlea separated by two membranes
o Reissnerâ€™s membrane: a thin sheath of tissue separating the vestibular and middle canals
o Basilar membrane: a plate of fibers that forms the base of the cochlear partition and separates the
middle and tympanic canals in the cochlea
ď‚§ Not really a membrane because itâ€™s not a tin, pliable sheet, itâ€™s a plate made up of fibers that
have some stiffness
- Cochlear partition: the combined basilar membrane, tectorial membrane, and organ of Corti, which are
together responsible for the transaction of sound waves into neural signals
- Vibrations transmitted thru tympanic membrane and middle-ear bones cause the stapes to push and pull the
flexible oval window in and out of the vestibular canal at the case of the cochlea
- This movement of the oval window causes â€śtraveling wavesâ€ť t flow thru the fluid in the vestibular canal
- A displacement forms in the vestibular canal and travels from the base of the cochlea down to the apex
- By the time the traveling wave reaches the apex, its displacement has mostly dissipated
- Round window: a soft area of tissue at the base of the tympanic canal that releases excess pressure remaining
from extremely intense sounds
The Organ of Corti
- Organ of Corti: a structure on the basilar membrane of the cochlea that is composed of hair cells and dendrites
of auditory nerve fibers
- Hair cells: any cell that as stereocilia for transducing mechanical movement in the inner ear into neural activity
sent to the brain; some hair cells also receive inputs from the brain
- Auditory nerve fibers: a collection of neurons that convey information from hair cells in the cochlea to (afferent)
and from (efferent) the brain stem
- Stereocilia: hairlike extensions on the tips of hair cells in the cochlea that, when flexed, initiate the release of
neurotransmitters
o Inner hair cells are arranged in straight rows with shorter stereocilia in front and taller ones in back
o Outer hair cells stand in rows that form the shape of a V or W
- Tectorial membrane: a gelatinous structure, attached on one end, that extends into the middle canal of the ear,
floating above inner hair cells and touching outer hair cells
o Floats atop the organ of Corti
Inner and Outer Hair Cells
- Hair cells are specialized neurons that transduce one kind of energy into another form
- Deflection of a hair cellâ€™s stereocilia causes a change in voltage potential that initiates the release of
neurotransmitters, which encourage firing by auditory nerve fibers that have dendritic synapses on hair cells
- Cochlea has only 14,000 hair cells
- Tip link: a tiny filament that stretches from the tip of a stereocilium to the side of its neighbour
- When a stereocilium deflects, the tip link pulls on the taller stereocilium in a way that opens an ion pore
somewhat like opening a gate for just a tiny fraction of a second
o Allows potassium ions to flow rapidly into the hair cell, causing rapid depolarization
o Depolarization leads to rapid influx of calcium ions and initiation of the release of neurotransmitters
form the base of the hair cell to stimulate dendrites of the auditory nerve