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Tomer Noff M.D.
Maalot Education Network
4/25/12
Anatomy and Physiology of the Ear
Topics to be covered
Hearing- Basic Concepts
Anatomy
Perception of Sound
Hearing - Pathology
Psychoacoustics
Hearing aids and cochlear implants
The incredible sense of hearing
“Behind these unprepossessing flaps lie structures of such delicacy that they shame the most skillful craftsman"
-Stevens, S.S. [Professor of Psychophysics, Harvard University]
Why study hearing?
• Best example of speech recognition– Mimic human speech processing
• Hearing aids/ Cochlear implants
• The stapes or stirrup is the smallest bone in our body. – It is roughly the size of a grain of rice ~2.5mm
• Eardrum moves less than the diameter of a hydrogen atom– For minimum audible sounds
• Inner ear reaches its full adult size when the fetus is 20-22 weeks old.
• The ears are responsible for keeping the body in balance
• Hearing loss is the number one disability in the world. – 76.3% of people loose their hearing at age 19 and over
Interesting facts
Specifications
Frequency range: 20Hz-20kHz
Size of cochlea: smaller than a dime
Anatomy
Understand:
Inner vs. Middle vs. Outer Ear
hearing.ppt8
Adapted from: (http://www.teleport.com/~veda/gallery.html)
The Ear
Hearing Anatomy & Physiology
PinnaAuditory
CanalTympanicMembrane
Ossicles
OvalWindow
CochleaAuditory
NerveAuditoryCortex
Malleus (hammer)Incus (anvil)Stapes (stirrup)LigamentsMuscles
Amplitude reductionPressure amplificationAttenuation reflex
(protection, lowfrequency masking)
A
N
A
T
O
M
Y
Outer ear
Importance of the PinnaFocuses sound waves (variations in pressure) into the ear canal
Pinna size:• Inverse Square Law• Larger pinna captures more of the wave • Elephants: hear low frequency sound from up to 5 miles away
Human Pinna structure: • Pointed forward & has a number of curves• Helps in sound localization• More sensitive to sounds in front
Dogs/ Cats- Movable Pinna => focus on sounds from a particular direction
Pinna /Auricle
Auditory Canal
Interaural Time Difference (ITD)
Interaural Intensity Difference (IID)
Horizontal localization
Vertical localization
Sound Localization
Outer earPinna /Auricle
Auditory Canal
Is sound on your right or left side?
Interaural differences
- The signal needs to travel further to more distant ear- More distant ear partially occluded by the head
Two types of interaural difference will emerge
- Interaural time difference (ITD)- Interaural intensity difference (IID)
Illustration of interaural differences
Leftear
Rightear
soundonset
left right
time
Leftear
Rightear
soundonset
time
arrival timedifference
Illustration of interaural differences
Leftear
Rightear
soundonset
time
ongoing timedifference
Illustration of interaural differences
Leftear
Rightear
soundonset
time
inte
nsity
di
ffere
nce
Illustration of interaural differences
Pinna Directional Filtering
Horizontal localization
Vertical localizationSound Localization
•Pinna amplifies sound above and below differently
•Curves in structure selective amplifies certain parts of the sound spectrum
Outer earPinna /Auricle
Auditory Canal
Is sound above or below?
Think
• In a Barn Owl, the left ear left opening is higher than the right
• A sound from below the owl will reach which ear
• a sound coming from below the Owl's line of site will reach the right ear first.
• This allows detection of sound origin
Caveman science
Pinna /Auricle
Auditory Canal
•Closed tube resonance: ¼ wave resonator
•Auditory canal length 2.7cm
•Resonance frequency ~3Khz
•Boosts energy between 2-5Khz upto 15dB
Outer ear
A
N
A
T
O
M
Y
Middle Ear
Impedance matching– Acoustic impedance of the fluid is 4000 x that of air– All but 0.1% would be reflected back
Amplification– By lever action < 3x– Area amplification [55mm2 3.2mm2] 15x
Stapedius reflex – Protection against low frequency loud sounds– Tenses muscles stiffens vibration of Ossicles– Reduces sound transmitted (20dB)
Eardrum
Ossicles
Oval window
Pressure variations are converted to mechanical motionEardrum OssiclesOval Window
Ossicles: Malleus, Incus, Stapes
A
N
A
T
O
M
Y
Inner EarSemicircular Canals
Cochlea
Body's balance organsAccelerometers in 3 perpendicular planes
•Hair cells detect fluid movements•Connected to the auditory nerve
Cochlea is a snail-shell like structure 2.5 turns
3 fluid-filled parts:
•Scala tympani
•Scala Vestibuli
•Cochlear duct (Organ of Corti)
(1) Organ of Corti
(2) Scala tympani
(3) Scala vestibulli
(4) Spiral ganglion
(5) auditory nerve fibres
Semicircular Canals
Cochlea
Inner Ear
Semicircular Canals
Cochlea
Organ of CortiBasilar membraneInner hair cells and outer hair cells (16,000 -20,000)IHC:100 tiny stereocilia
The body's microphone:•Vibrations of the oval window causes the cochlear fluid to vibrate•Basilar membrane vibration produces a traveling wave•Bending of the IHC cilia produces action potentials•The outer hair cells amplify vibrations of the basilar membrane
Inner Ear
The cochlea works as a frequency analyzer It operates on the incoming sound’s frequencies
Place Theory
• Each position along the BM has a characteristic frequency for maximum vibration
• Frequency of vibration depends on the place along the BM
• At the base, the BM is stiff and thin (more responsive to high Hz)
• At the apex, the BM is wide and floppy (more responsive to low Hz)
32-35 mm long
4mm21mm2
Tuning curves of auditory nerve fibers
Response curve is a BPF with almost constant Q(=f0/BW)
To determine the tonotopic map on Cochlea
•Apply 50ms tone bursts every 100ms
•Increase sound level until discharge rate increases by 1 spike
•Repeat for all frequencies
Auditory Neuron
Carries impulses from both the cochlea and the semicircular canalsConnections with both auditory areas of the brainNeurons encode– Steady state sounds– Onsets or rapidly changing frequencies
Auditory Area of Brain
Auditory Neurons Adaptation
•At onset, auditory neuron fiber firing increases rapidly
•If the stimulus remains (a steady tone for eg.) the rate decreases exponentially
•Spontaneous rate: Neuron firings in the absence of stimulus
Neuron is more responsive to changes than to steady inputs
Threshold variation with age
•Presbycusis
•Hearing sensitivity decreases with age especially at High frequencies
•Threshold of pain remains the same
•Reduced dynamic range
32-35 mm long
4mm21mm2
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