CHAPTER 5: THE AUDITORY SYSTEM – SOUND AND EAR CHAPTER 5: THE AUDITORY SYSTEM – SOUND AND EAR Audition:
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The Physics of Sound The Physics of Sound
Essenti!" Re#ui$e%ents to he!$ sound: Essenti!" Re#ui$e%ents to he!$ sound:
-- SoSomemeththining tg thahat ct crreaeatetes ts the he sosounundd
-- SoSounund mud must pst proropapagagate tte thrhrouough a mgh a medediuiumm
o
o ell !ar e"periment# the sound produced by a bell becomes les ell !ar e"periment# the sound produced by a bell becomes les audible if enclosed inaudible if enclosed in
a $ar in which air is slowly pumped out a $ar in which air is slowly pumped out
-- %ech%echanisanism to tram to translanslate soute sound ennd energy ergy into binto bioloiologicagical signl signal to gal to generenerate thate the e"pee e"perienriencece &nown as hearing
&nown as hearing The C$e!tion of Sound The C$e!tion of Sound
-- 'ib'ibratiorational (nal (ropropertieerties of s of )b$ec)b$ects# *nts# *nertia ertia + Ela+ Elasticsticity ity 'ib'ibratiorationsns
o
o *nertia – an ob$ect will initially resist deformation, but once started it will continue*nertia – an ob$ect will initially resist deformation, but once started it will continue
until an opposing force acts on it until an opposing force acts on it
o
o Elasticity – The opposing force that brings an ob$ect to its original stateElasticity – The opposing force that brings an ob$ect to its original state o
o i.e. using a tuning fork i.e. using a tuning fork , vibrations of the prongs will create a, vibrations of the prongs will create a simple harmonicsimple harmonic
motion
motion represented by a represented by a sinusoidal function.sinusoidal function.
-- *mpact of a *mpact of a SounSound Soud Source rce on thon the %ede %edium# omium# omprepressiossion and n and rarrarefactefaction oion of airf air
o
o ompression– *ncreased air prompression– *ncreased air pressure bc of essure bc of prongs moving outward prongs moving outward pushing againstpushing against
it it
o
o /arefaction – 0ecrease air pressure bc of prongs moving inwards leaving a space/arefaction – 0ecrease air pressure bc of prongs moving inwards leaving a space o
o Sound wave – created by the propagation of the momentary compression Sound wave – created by the propagation of the momentary compression
rarefaction of air rarefaction of air The P$o&e$ties of Sound The P$o&e$ties of Sound
-- AmAmplplititudude e anand d SoSounund d *n*ntetensnsitity#y#
o
o Amplitude – pressure change from the baseline to the highest pea& 1measure in (aAmplitude – pressure change from the baseline to the highest pea& 1measure in (a
or &(a2 or &(a2
*ncrease in loudness of sound *ncrease in amplitude*ncrease in loudness of sound *ncrease in amplitude
o
o *ntensity – a word used to describe the loudness of sound*ntensity – a word used to describe the loudness of sound
*ntensity of an ob$ection is considered in relation to a reference point*ntensity of an ob$ection is considered in relation to a reference point
Refe$ence &ointRefe$ence &oint the the lowest sound level that can be heard 1345(a2lowest sound level that can be heard 1345(a2
*ntensity is represented by*ntensity is represented by Deci'e"s (d)*+Deci'e"s (d)*+ A log scale where 64d 6belA log scale where 64d 6bel
and 6bel log1* and 6bel log1*ss**rr22
SP,SP, Sound pressure level. Sound pressure level.
d)d)SP,SP, the reference value use to calculate intensity is the lowest sound the reference value use to calculate intensity is the lowest sound
level that can be
level that can be heardheard
-- SounSound 7d 7re8re8uencuencyy# 9 of c# 9 of compleomplete cyte cycles cles seco second. %nd. %easueasured red in :erin :ert; 1:t; 1:;2;2
o
o :umans can only hear between 34-3444 :;:umans can only hear between 34-3444 :; o
o N!tu$!" (Reson!nt* -$e#uencyN!tu$!" (Reson!nt* -$e#uency – fre8uency of an ob$ect due to its mass and – fre8uency of an ob$ect due to its mass and
sti<ness. sti<ness.
6 6
o As sti<ness 1or tension2 increases fre8uency increases
o As %ass increases fre8uency decreases
o As =ength increases fre8uency decreases
- Speed of Sound# >>6.?ms @ 4 in A*/
o Speed is entirely dependent on the characteristics of the medium – inertia and
elasticity
o *nertia 1density2 of medium is related to speed inversely – increase density
decrease speed
o Temperature – *ncrease temp increases speed
*ncreasing temperature causes e"pansion of molecules decreases density increase speed
6 increase 4.Bms increase in speed of sound
o Elasticity of medium is related to speed directly – increase elasticity increase
speed
i.e. sound actually travesl 6B" faster in steel than air bc of elasticity
Co%&"e. Sounds
/ omple" (eriodic Sounds# Chen the pattern of pressure change repeats itself at regular intervals over time
o *t does not have a sinusoidal function DT its formed by adding multiple sinusoidal
function with di<erent fre8uencies
o :armonic Series – a highly periodic wave characteri;ed by the summation of precise
series of sine waves 1i.e. sound produced on a musical instrument2
/ omple" Aperiodic Sounds# Chen the pattern of pressure change does not repeat itself at a regular interval
o A&a F)*SE# Chite noise – noise with fre8uencies within the human hearing range
1i.e. traGc sounds2
/ 7ourier Analysis# 0ecomposing comple" sound into the simple sine functions that formed it
o 7ourier Spectrum – a way to represent the deconstructed comple" sound by using
vertical lines
H 7re8uency 1location of vertical line2 and I amplitude 1height of the vertical line2
/emoves the need to plot each component of the sine wave functions / )hms law of :earing# :uman hear sound by deconstructing them into simple tones
similar to 7ourier Analysis Sound T$!ns%ission
- The *nverse S8uare =aw# Sound intensity is in0e$se"y related to the s#u!$e of the dist!nce from the sound source 1i.e. *f distance r is doubled to 3r intensity decreases to
6J of original2
o )nly true if there were no ob$ects in the way
- *nteraction of Sound with )b$ect#
o /eKection – when sound bounces o< a medium with greater resistance
Echo 1a&a reverberations2 reKected sound
/everberant room a room with highly reKective walls
o Absorption – depends on the absorption coeGcient 1 proportion of sound absorbed
compared to the initial sound2 L Anechoic a room with highly absorptive walls
o 0i<raction – when sound waves bend around an ob$ect
:igh fre8uencies sound cannot be di<racted as easily bc of more
compressed undulations of pressure change.
- Architectural Acoustics# warmth, brilliance, te"ture, and blend are important factors
o /everberation time – a factor used to 8uantied sound reKection
o %ore reverberation – oncert halls will be perceived as having more Fullness o =ess reverberation – oncert halls will be perceived as having more Clarity
o *ntimacy – the impression of being physically close to the sound source 1 occurs if
the sound reaches the listener really 8uic&ly2
Audito$y P$ocessin1 of Sound – Physic!" Ch!$!cte$istics An!to%ic!" Co%&onents of the Hu%!n E!$
- The )uter Ear# (inna, e"ternal auditory canal and tympanic membrane 1ear drum2
o (inna – funnels sounds into the ear. omposed of cartilages and s&in
o Auditory anal – S-shaped canal lined with wa"-secreting glands to protect the
insides of the ear
o Tympanic %embrane – thin elastic membrane that transfer vibrations to the middle
ear. Also have protective functions against foreign bodies. - The %iddle Ear#
o Eustachian Tube – connects the middle ear to the throat to e8uali;e pressure across
the ear drum
o )ssicles 1malleus, incus, stapes2 – transfer the vibration from the ear drum to the
oval window.
)ssicles are suspended by ligaments to allow free vibrations
- The *nner Ear# =ocated in a cavity of the temporal bone &nown as Bony Labyrinth
o ochlea – Kuid lled bony structure that contains the sensory transduction
apparatus of hearing
o *nterior of the ochlea is divided into 3 Kuid lled channels
Sc!"! 2esti'u"i 1biggest2 – uppermost channel. asal end is connected to
the oval window
Sc!"! ty%&!ni – lowermost channel. The basal end contains the round
window
Sc!"! %edi! 1cochlear duct2 – middle channel. Smallest.
• The Reissne$3s %e%'$!ne separate this channel from the Scala
vestibule
• The )!si"!$ %e%'$!ne separates this channel from the Scala
tympani
• The Kuid here is di<erent from the other 3 and they dont mi" o 'estibuli and Tympani are $oined together at the ape" by a small opening
He"icot$e%!
- Sound Transmission through the Ear
o (inna auditory canal tympanic membrane )ssicles oval window Kuids
in the scala vestibule and /eissners membrane scala tympani + basilar
membrane /ound window
A%&"itude P$ese$0!tion
/ The =ever E<ect# Small amplication of sound as it passes malleus to the incus. Dp to an amplication of 3d
/ The ondensation E<ect# =arge amplication of sound due to transfer of vibration from large surface area of the tympanic membrane to the small area of the stapes bone. A gain of 3?d.
/ The /esonance e<ect# The (inna and ear canal has a resonant fre8uence of J444-?444:; and 3?44 :; respectively. Therefore sound coming in at these fre8uencies will be be
amplied by 64-6?d.
- The 0irectional e<ect# the ossicular chain of bones in the middle ear channels all the incoming sound energy only upon the oval window.
o Cithout it, both oval and round window will respond e8ually to incoming pressure
resulting in no net movt of the cochlear Kuid. -$e#uency Re&$esent!tion
/ The 7re8uency Theory# Feural ring rate sound fre8uencyL neural ring amplitude intensityL DT
o Feurons re all-or-nothing therefore no di<erences in amplitude, all are the same
o Theres actually a time constraint that limits how fast neurons can re
o Cidth of basilar membrane is not uniform entire thing cannot vibrate at the same
fre8uency
- The (lace Theory# Sounds of di<erent fre8uencies produce a vibrational pattern whose ma"imum amplitude occurs at di<erent places along the basilar membrane
o This is possible because the basilar membrane is narrow at the basal end and gets
wider at the ape"
o The tension also decreases from the basal end to the ape"
o As a sound travels across, there will be ma"imum displacement of the basilar
membrane at a site where it matches the incoming fre8uency.
o Farrow and high tension at the basal end receptive to high fre8uency
o Cide and low tension at the ape" end receptive to low fre8uency
- Tonotopic )rgani;ation
o The way the basilar membranes resonant fre8uency is organi;ed creates a
tonotopic map.
o this allows e<ective neural coding of fre8uency
- 7re8uency Analysis of comple" Sounds
o The basilar membrane acts as a lter, a fre8uency analy;er, by separating comple"
sounds into simple sinusoidal functions
o As the comple" sounds enters the cochlea it creates a virbational disturbance in the
Kuids. :owever bc the basilar membrane only responds to certain fre8uency at specic places, where the incoming sound spectrum matches the resonant
fre8uency on the membrane there will be greater displacement. Audito$y P$ocessin1 of Sound – )io"o1ic!" Mech!nis%s
Audito$y T$!nsduction
/ )rgan of orti – Structural 7eatures
o The conversion of vibrations to neural signals occurs within this structure which lies
on top of the basilar membrane . *t is entirely within the cochlear duct.
o The Arch of orti – a rigid inverted M'N structure that divides the organ into an inner
and outer portion
o :air cells – most important cells with regard to auditory signal processing
*nner hair cells – located on the inner side of the arch and e"tends the full length of cochlea
)uter hair cells – located on the outer side of the arch and arranged into > rows e"tending the length of the cochlea
o Sterocilia – ne laments that protrude from the upper surface of hair cells. o Tectorial membrane – gelatinous structure that lies above the sterocillia
/ )rgan of orti – %echanical /esponse to Sound Stimulation
o Chen the basilar membrane moves up and down, the tectorial membrane also
bends creating a shearing force on to the steriocillia in between them
o The sterocillia hairs will bending according to yhr movement of the basilar
memebrane
Stereocillia bending outward upward deKection of the membrane
Stereocillia bending inward downward deKection of the membrane
/ Transudctional %echanism in :air ells# (ASS*'E response to auditory stimulation
o %ost of the neural output from the cochlear actually comes from the inner hair cells o Tip =in& – a very thin bre that connects all the sterocillia.
o ending of sterocillia ending of the tip lin& which uncovers the ion gates 1i.e. O+
and a3+2 inKu" of ions depolari;ation.
This is important so that we can hear /A(*0=I
o 3nd depolari;ation + increase in a3+ release of neurotransmitter at the base of
the hair cells neurotransmitter 1glutamate2 depolari;es the cochlear nerve bres signal carried to higher centres
/ /oles of )uter :air ells# AT*'E response to auditory stimulation %echanical or
Electrical
o %echanical /esponse of outer hair cells – net e<ect is to help bend the tectorial
membrane
6st physical change within the contractile elements of the outer hair cells
3nd slower response occurs through nerve stimulation
o Electrical mechanism – enhances the passive electrical response functions of the
inner hair cells
ochlear microphonic – an electrical response across the entire organ of orti
by the movement of outer hair cells in response to basilar membrane vibration
The ionic properties of the Kuid create a steady current that is modulated by
the sterocilia of the outer hair cells. The electrical waveform is e"actly the same as the sound wave
Neu$!" C!&tu$e of Audito$y Si1n!"s
- 'estibulocochlear Ferve 1F '***2# %ain pathway for the transmission of auditory signals out of the cochlea and into the FS
- Anatomical )rgani;ation
o 7ibres of the cochlear nerve originate from the cochlear ganglion. The cochlear
ganglion lies $ust outside of the cochlear and follows the cochlear spiral Spiral
Ganglion
o The neurons are bipolar neurons with the shorter branch innervating the hair cells o P4Q of the cochlear nerve bres terminate upon the inner hair cells.
- A<erent vs. E<erent nerve 7ibres
A4e$ent -i'$es E4e$ent -i'$es
/eceives signals mainly from *nner hair cells Sends signals mainly to )uter hair cells 6 neuron innervate 6 hair cell, but each hair cell
may have more than 6 nerve innervating it
*ncrease activity of these neurons reduce ring rate of action potentials of a<erent %ultiple nerves allow the di<erent properties of
sound to be transmitted independently
7unction un&nown
- Feural oding of Sound *ntensity
o Rreater amplitude greater deKection of basilar membrane greater bending of
sterocilia greater depolari;ation of inner hair cells greater release of neurotransmitter on to a<erent nerve
o The 64 a<erent nerves that innervate each hair cell have di<erent intensity
threshold ranging from 4d to 644d and beyond. This encodes the entire audible range of intensity.
o :air cells can control how much neurotransmitter it releases to each a<erent nerve
di<erence in neurotransmitter release means that it can trigger di<erent levels of
neural activity among a parallel set of bres
/ Feural oding of Sound 7re8uency – (lace Theory %echanisms
o Since the basilar membrane shows ma"imum vibrational amplitudes at di<erent
places along its length in response to di<erent sound fre8uencies, therefore a
particular a<erent bre will only carry signals that was depolari;edby the particular sound fre8uency associated with its place in the membrane .
o Tuning urve – displays the minimum sound intensity re8uired to obtain a neural
response as a function of fre8uency
haracteristic 7re8uency – the lowest point on the curve
o 7re8uency /esponse urve – Shows the neural response to fre8uency and intensity
/ Feural oding of Sound 7re8uency – 7re8uency Theory %echanisms
o (hase =oc& /esponse – All the a<erent nerves produce an action potential at the
same fre8uency as the incoming sound. 1i.e. an action potential is produced during each cycle2. )nly wor&s up to J444:;
Su'co$tic!" Audito$y St$uctu$es - 3 7unctions
o Serve as a relay mechanisms that transmits information from one site to another o They enhance, modify, and further process the auditory signals
- Ascending (athways# A<erent nerves come together forming the ranial Ferve '*** enter
brain stem and synapse at ochlear nucleus Superior )live + *nferior olliculus on both
sides *nferior olliculus will pro$ect to the %edial geniculate Fucleus on both side +
inferior colliculus of the opposite side 7rom %RF it goes to the Auditory orte"
o ochlear Fucleus – only receive inputs from their corresponding side. There is no
cross over at this level 1monaural neurons2. 3 types of cells /elay cells +
enhancing + sharpening signals cells
o Superior )live – Auditory (rocessing
o *nferior olliculus – Auditory relay + oordination of acoustic reKe"es 1midbrain2 o %edial Reniculate Fucleus – largely a relay signals to the corte"
- haracteristics of Subcortical Structures
o /esponse %odication – by way of inhibitory + e"citatory signals in the cochlear
nucleus
sharpens the neural representation important for sound locali;ation +
detecting sounds that are present in a noisy bac&ground
o (resence of a Tonotopic %ap – prominent in the inferior colliculus in which each
layer codes for one fre8uency 1*sofre8uency sheets2 ensures that fre8uency
information is retained
o =aterality – the e"tent to which the subcortical neurons can be separately driven by
the 3 ears
inaural Feurons – e"cited by contralateral sound stimulation
• This is consistent with the fact that neurons on one side of the brain
represents the opposite side of the body
%onaural Feurons – can be e"cited or inhibited by ipsilateral sound input
- 0escending (athway# %odulates auditory response to sound in 3 di<erent way
o inhibition of outer hair cell by )livocochlear neurons to allow the auditory system to
encode a higher range of sound intensities by preventing early saturation of discharge rates.
o Activation of the 3 small muscles in the middle ear, Tensor Tympani and Stapedius
%uscular contractions lead to decreased sound transmission to protect the
middle and inner ear component from sudden burst of loud sound. The Audito$y Co$te.
- Anatomical )rgani;ation#
o Area A6 – The primary Auditory corte" in the temporal lobe which receives signals
from the %RF
o Area A3 – Surrounds A6 and processes higher order acoustic information
o Cernic&es Area – located in the left hemisphere )F=I and is important for speech
comprehension - 7unctional )rgani;ation#
o Tonotopic Arrangement of the Auditory orte"
=ow 7re8uencies – towards the anterior end of A6 :igher fre8uencies towards the posterior end of A6
o olumnar )rgani;ation of Feural response resulting from inaural inputs
Summation /esponse – inputs from both ears are e"citatory and therefore the
neurons can be driven by sound stimulation of either ear
Suppression /esponse – the neurons is e"cited by sound input from the
opposite side but is inhibited by stimulation on the same side
- (arallel Auditory (athways# *t appears that neural circuits involved in sound pattern analysis are separate from those involved in sound locali;ation analysis
Audito$y Dysfunction C!uses of He!$in1 ,oss
- onductive =oss# )ccurs when the outer or middle ear is a<ected reduced sound
transmission to the cochlea
o %iddle ear infection 1a&a otitis media2 – most common cause
o )tosclerosis – inherited diseases that produces abnormal development and function
of the ossicles
caused by build up of calcium which restricts the movement of the bones
reducing the transfer of sound energy by >4d
o )ther auses – bloc&age of the outer ear by ear wa" or perforation of the ear drum
- Sensorineural =oss# occurs when there is damage to the cochlea or to the nerves of the inner ear
o *ngestion of )toto"ic drugs – some antibiotics and aspirin bc they destroy hair cells
leads to Tinnitus – the perception of Mringing in the earN in absence of
e"ternal sound
o Traumatic in$ury – fracture of the temporal bone
o Tumours – especially in cochlear nerve acoustic neurinomas
o 0iseases – rubella in pregnant mother, %enieres 0iseases e"cessive cochlear
Kuid production
o *ntense Environmental Foise – causes in$ury to the hair cells and the transduction
mechanism
a&a Foise-induced hearing loss. U4d will cause damage. 634d its
painful
- 0eaf-mutism# the absence of language vocali;ation ability due to hearing loss in pre-lingual children
- :ereditary 7actors and Aging#
o Renetic auses
Dsher Syndrome – autosomal recessive gene that causes variable degrees of
deafness either at birth or later in life. )ften accompanied by visual problems.
Caardenburg Syndrome – causes hearing loss and changes in s&in and hair
pigmentation.
• 3 di<erent eye colour, hearing loss can vary from moderate to severeL
0ominant gene
o (resbycusis – hearing loss due to aging. :igh pitch sounds are rst to go.
Di!1nosis !nd T$e!t%ent
/ one onduction test# use to distinguish bw conductive and sensorineural loss by applying a vibrating tuning for& somewhere on the s&ull
o Formal – hear sound whether it is touching the s&ull or not
o onductive – can only hear sound when the tuning for& touches the s&ull o Sensorineural – cannot hear sound with tuning for& touching the s&ull or not.
/ :earing Aids# amplies incoming sound and delivers them to the ear. )nly wor&s if cochlear is wor&ing
o ontains a microphone, amplier and spea&er
/ ochlear implant# contains a small microphone and processor that converts the sound signal into an electric current that is delivered to the inner ear via ne wires
o Single hannel *nputs – one wire to the cochlear nerve. Fo good.
o %ulti-channel *nputs – 34+ wires that delivers di<erent fre8uency to the di<erent
nerve along the basilar membrane
