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Coding of Sound Qualities
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• Firing of auditory nerve fibers into patterns of neural
activity finally completes process of translating sound
waves into patterns of neural activity
• How do auditory nerve fibers convey frequency info?
• Place Code vs. Temporal Code
– Place code: Tuning of different parts of cochlea to
different frequencies
• Transduction - presence of sound in environment
• Need more info (e.g., concert)
– Frequency
– Intensity
– Waveform
– Location
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Basilar Membrane Characteristics
Frequency Coding
• Information about the particular frequency of
incoming sound wave is coded by place along
basilar membrane
• Location with greatest mechanical displacement
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The Cochlea is Tuned to Different Frequencies
• Basilar membrane
– Narrow and stiffer near oval window (base)
– Wider and more flexible near helicotrema (apex)
• Different resonant frequencies
– Higher frequency – near base
– Lower frequency – near apex
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Inner and Outer Hair Cells
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• Inner hair cells: Convey almost all information about
sound waves to brain
• The auditory nerve
– Responses of individual AN fibers to different
frequencies are related to their place along the
cochlear partition
– 10-30 AN fibers synapse on each IHC
– Most of the sensory information comes from IHC
– Frequency selectivity: Clearest when sounds are
very faint
• Outer hair cells:
– Feedback -- controls stereocilia
– Threshold tuning curve: Map plotting thresholds of
a neuron or fiber in response to sine waves with
varying frequencies at lowest intensity that will
give rise to a response
– Modulate movement of basilar membrane
– “Sharpen ” wave envelopes
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Basic Structure of the Mammalian Auditory System (cont’d)
Threshold Tuning Curves
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Rate Saturation
• Rate saturation
– Are AN fibers as selective for their characteristic
frequencies at levels well above threshold as they
are for the barely audible sounds?
– Isointensity curves: Chart AN fiber’s firing rate to
wide range of frequencies, all presented at same
intensity level
– Rate saturation: Point at which a nerve fiber is
firing as rapidly as possible and further stimulation
is incapable of increasing the firing rate
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Isointensity Functions
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Rate Saturation
• Potential problem: Lose information about frequency?
• Rate intensity function: A map plotting firing rate of an
auditory nerve fiber in response to a sound of
constant frequency at increasing intensities
• Multiple AN fibers synapse with each inner hair cell
• The AN fibers have different dynamic ranges
– High spontaneous fibers
• More sensitive – saturate at lower intensities
– Low spontaneous fibers
• Less sensitive – saturate at higher intensities
– Range of neurons in between between
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Firing Rate vs. Sound Intensity
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Intensity and Firing Rate
• At higher intensities
– Low spontaneous rate neurons won’t saturate
– Will still provide precise information about
frequency
• Firing rate can be used to convey information about
intensity
– Some AN fibers respond 0 -25 dB
– Other AN fibers 15-40 dB
– Other AN fibers 30-55 dB
– Etc.
– Gives more precise info about intensity
• Will be important for sound localization
3
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Temporal Code for Frequency
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Neural Spikes
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Volley Principle
• Temporal code for sound frequency
– Auditory system has another way to encode
frequency aside from the cochlear 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
– Existence of phase locking: Firing pattern of an
AN fiber carries a temporal code
• 100 Hz sound – AN fiber fires at 100 Hz
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Temporal Code
• Problem with temporal code
• AN fibers – absolute refractory period
– Can only fire so fast (~500 Hz)
– Can’t follow higher frequencies
• The volley principle: neurons fire at a distinct point in
the period of a sound wave but does not fire on every
period
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Place theory problem
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• Place Coding Problem
– Low frequencies -- peak off
of Basilar Membrane
– “run out of room”
– Ambiguous low frequency
information
• Temporal Code Problem
– Can’t follow high frequencies
– Even with volley principle
Which is being used?
•
•
•
•
Both are operating
Frequency theory -- low frequencies
Place theory -- high frequencies
Both -- intermediate frequencies
Frequency Theory
Place Theory
20 50
125 250 500
1000
2000 4000
8000 16,000 20,000
Stimulus Frequency (Hz)
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Auditory System Pathways
• Auditory brain structures
– AN fibers carries
signals from cochlea
to brain stem
– All AN fibers initially
synapse in cochlear
nucleus
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Organization
• Tonotopic organization: An 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
– Superior olive, inferior
colliculus, and medial
geniculate nucleus all
play roles in auditory
process
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The First Stages of Auditory Processing
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Basic Structure of the Mammalian Auditory System (cont’d)
• Comparing overall structure of auditory and visual
systems
– Auditory system: Large proportion of processing is
done before A1
– Visual system: Large proportion of processing
occurs beyond V1
– Differences may be due to evolutionary reasons
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Basic Operating Characteristics of the Auditory System
• Psychoacoustics: The study of the psychological
correlates of the physical dimensions of acoustics; a
branch of psychophysics
• Distinction between physical and psychological
– Frequency and pitch
– Intensity and loudness
• Example: Relationship between intensity and
loudness (need to take frequency into consideration)
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Equal Loudness Contours
• Procedure:
– Standard tone (e.g., 1000 Hz at 40 dB)
– Comparison tone (e.g., 100 Hz)
• Adjust intensity of comparison
• Loudness match
– Repeat for other frequencies (e.g., 100, 500, . . . ,
3000, 5000)
– Record “matching” intensity for each
– Set of sounds -- Matched in loudness
• Equal loudness contour
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Equal Loudness Demo
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• Audibility threshold: A map of just barely audible
tones of varying frequencies
• Web Demo
125 Hz
1000 Hz
Intensity and Loudness
3000 Hz
“46”
“44”
“42”
“40”
“38”
“36”
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Hearing Loss
• Hearing can be impaired by damage to any of
structures along chain of auditory processing
– Obstructing the ear canal results in temporary
hearing loss (e.g., earplugs)
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Hearing Impairments
• Review Transduction
– Air pressure fluctuations
– Action potentials in auditory nerve fibers
– Excessive buildup of ear wax (cerumen) in ear
canal
– Conductive hearing loss: Caused by problems
with the bones of the middle ear, (e.g., during ear
infections, otitis media)
– Otosclerosis: More serious type of conductive
loss. Caused by abnormal growth of middle ear
bones; can be remedied by surgery
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Hearing impairments
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Conductive & Sensorineural Hearing Loss
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Sensorineural Hearing Loss
• Problems in any stage
– Hearing impairment
• Two categories
– Conductive hearing loss
– Sensorineural hearing loss
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Conductive Hearing Loss
• Problems transmitting pressure waves to cochlear fluid
• Variety of causes
– Swimmer’s ear -- swelling of ear canal
– Ruptured tympanic membrane
– Otitis media
• Middle ear infection
• Fluid fills middle ear
– Otosclerosis
• Abnormal bone growth -- ossicles fused in place
• Loss: All frequencies
• Problems in transduction or transmission
• Caused by
– Hair cell damage
• Abrupt noise
• Prolonged exposure
• Some drugs -- ototoxic
• Presbycusis -- old hearing
– Tumors along auditory pathway
• Auditory nerve
• Nuclei of brainstem
• Loss: Frequency specific
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Hearing Evaluations
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Audiogram
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Conductive Hearing Loss
• Pure-tone Audiometry
– Use pure tones -- sine waves (single frequency)
– Measures absolute threshold across range of
frequencies
– Variant of Method of Limits
– Create Audiogram
• Threshold relative to normal
• Test range of frequencies
• Headphones (air conduction)
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Hearing Evaluations
• Bone conduction
– Cochlea -- cavity in skull
– Sound - vibrations in cochlear fluid
– Vibrate skull -- vibrate fluid
• Beethoven
• Air conduction XO
– Increased
threshold
– All frequencies
• Bone conduction [ ]
– normal thresholds
– Vibrate skull at different frequencies
– Create Bone -conduction Audiogram
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Sensorineural Hearing Loss
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Presbycusis
•“Old hearing”
•Sensorineural hearing loss
•Increase in threshold
–More pronounced for high frequencies
–Damage to hair cells (years of wear and tear)
• Air Conduction XO
– Increased
thresholds
– Frequency specific
• Bone conduction
– Same pattern
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Hearing Loss in Easter Islanders
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Cochlear Implant
• Environmental
Contributions to
Presbycusis
– Sex differences
– Location differences
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