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George Berkeley (1710) “A Treatise Concerning the Principles of Human Knowledge”
• “If a tree falls in the woods, and there’s no one around to hear it… does it make a sound?”
George Berkeley (1710) “A Treatise Concerning the Principles of Human Knowledge”
• “If a tree falls in the woods, and there’s no one around to hear it… does it make a sound?”
• Is there “objective” reality or only “subjective” perspective?– Objective reality defined: pressure
changes in the air or other medium (water).
– Subjective reality defined: the experience of hearing (perceiving).
objectvibrates
air moleculesvibrate
eardrumvibrates
You hear vibrations
sound waves
Periodicity of waves
The simplest sound waves – Pure Tones – a sound that is portrayed by a single sine wave
A
time
Frequency (Hz) = 1 cycle/t (unit of time or distance)
Amplitude (dB) = difference between atmospheric pressure & maximum pressure of a sound wave
Compression (density increase) of air molecules
Depression (density decrease) of air molecules
Pure tones can vary in frequency (Hertz (Hz = cycles/second))
Pure tones can vary in amplitude (Decibels (dB) – Sound Pressure Level (SPL))
the softest sounds we hear are about 1/10,000,000th the amplitude of theloudest sounds
Sound SPL (dB)Barely audible sound (whisper) 0+-ishLeaves rustling 20Quiet residential community 40Average speaking voice 60Loud music from radio/heavy traffic 80Express subway train 100Propeller plane at takeoff 120Jet engine at takeoff (pain threshold) 140Spacecraft launch at close range 160
Elements of SoundMagnitude of displacement
Intensity Loudness: amount of sound energy falling on a unit area (cm2)
Because air pressure changes are periodic (i.e. they repeat a regular pattern), one can do a Fourier analysis
0
Frequency (Hz)440 880 1320
Re
lativ
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ower
The auditory system does break down tones intosimpler components = Ohm's acoustic law
Fourier Analysis reduces sound wave to its fundamental frequency and harmonics frequencies
0
Frequency (Hz)
Fourier frequency spectrum
440 880 1760
Re
lativ
e P
ower
“Fundamental Frequency” is the lowest frequency of a vibrating object
Additive synthesis & Fourier analysis
Fundamental Frequency (“1st harmonic”)
2nd Harmonic
3rd Harmonic
Perception of sound
Pitch:
Subjective quality of “high” or “low” tone (tone height)
Constructed from fundamental & harmonics
Octave: multiples of pitch (200 Hz, (x2) 400 Hz, (x4) 800 Hz, (x8) 1600 Hz, etc.)
Perception of sound
Timbre (or “tambre”):
Subjective quality of sounding different at same pitch and loudness (flute v. bassoon)
Constructed from differential energies (loudness) at different fundamental & harmonic frequencies, and differences in the number of harmonics
Timbre: differences in number & relative strength of harmonics
guitar- many harmonics
400 800 1200 1600 2000 2400
Frequency (Hz)
flute - few harmonics (only one actually)
400 800 1200 1600 2000 2400
Ear—overall view
Redrawn by permission from Human Information Processing, by P. H. Lindsay and D. A. Norman, 2nd ed. 1977, pg 229. Copyright ©1977 by Academic Press Inc.
Outer Ear
• pinna(e)
• auditory canal - amplifies sounds in 2-5 kHz range
• tympanic membrane (eardrum)
Middle Ear• Ossicles: malleus (hammer); incus (anvil); stapes (stirrup) • Oval Window • Middle ear muscles - at high intensities they dampen the vibration of the ossicles
Inner Ear
• Cochlea
Outer ear Airborne vibratory processing – helps capture, direct & modulate sound
Auditory canal amplifies sounds through Resonance (combining reflected sound in canal with newly entering sound)
Middle ear
Tympanic membrane vibrates
Mechanical processing – vibration from airborne processes translate into movement of bones & muscles -- causing a second level of vibrations on the oval window
Inner ear Liquid medium – Neural processing – translation of vibrations into neural signals
Figure 10.14, page 345
Middle ear
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tympanicmembrane
ossiclessurface area of stapes
The ossicles concentrate thevibration on a smaller surface areawhich increase the pressure per unit area (factor of 17)
The ossicles act as a lever, thereby increasing the vibration by a factor of 1.3
Ear—overall view
Redrawn by permission from Human Information Processing, by P. H. Lindsay and D. A. Norman, 2nd ed. 1977, pg 229. Copyright ©1977 by Academic Press Inc.
The inner ear is responsible for both:
• Balance (vestibular system)• Transduction for hearing (Cochlea)
The outer and middle ear are purely mechanical.
Neural transduction and
analysis begin at the inner ear. • The cochlea is the site at which
vibrations of the stapes and inner ear fluid are transduced to neural responses in fibers of the auditory nerve.
The cochlea is a coiled tube that resembles a snail shell
• In humans, the cochlea coils about 2.5 times
• 2 mm diameter• 35 mm long• A cross-section
through the coiled cochlear tube reveals that the inside is divided into three compartments
The receptor cells are located in the Scala Media,
part of the Organ of Corti.
Organ of corti
The Organ of Corti – located on the basilar membrane in the Scala Media
• Three rows of outer hair cells
• One row of inner hair cells – main receptor cells involved in hearing
• Tectorial membrane covers the tops of the hair cells
• Basilar membrane sits underneath the Organ of Corti (underneath the hair cells)
Hair Cells3,500 inner hair cells12,000 outer hair cells
inner hair cells diverge - each connects to 8-30Ganglion Cells (auditory nerve fibres)
-connected to fibres which leave the cochlea as the auditory nerve
outer hair cells converge - each auditory nerve fibreis connected to many outer hair cells
hair cells have cilia - when they bend, the transduction process starts
Innervation of hair cells
• Each inner hair cell innervated many spiral ganglion cells
• A single spiral ganglion cell is innervated by many outer hair cells
Transduction in auditory receptor cells
• The tops of the cilia extend up to the tectorial membrane
• The base of each hair cell is contacted by one or more spiral ganglion cells
Stapes vibrates - oval window vibrates- this causes pressure changes in the fluid
- the basillar membrane vibrates at the same frequency as the stapes
- the cilia of the hair cells are embedded in the tectorial membrane
-when the basillar membrane vibrates, the haircells bend
tectorialmembrane
basillar membranedepolarization hyperpolarization
Release of neurotransmitter Termination of neurotransmitter
The traveling wave of sound transduction across the Basilar Membrane
Movement of the cilia results in transduction
• When pressure at the oval window increases, the basilar membrane moves downward. When pressure decreases, it moves upward. These movements drag the stereocilia in opposite directions.
The auditory nerve and central auditory system
• The auditory nerve projects in an organized way so that the map of frequency is maintained at every level of the central auditory system.
Tonotopic processing
Cochlear Base
Cochlear Apex
The location of the peak of the envelope is frequency dependent
high frequency close to base
low frequency close to apex
Hi Hz Low Hz
1. Place code - von Bekesybasilar membrane is narrower at the base than at the apex
stiffer at base
-present vibration to basilar membrane traveling wave results
baseapex
Peak envelope
Tonotopic Map of the Cochlea
Base
Apex
#’s indicate fundamental frequency (& pitch)
Hair cells along the basilar membrane are sharplytuned to a best or characteristic frequency
single-unit recording
30
40
50
60
70
1052 20
thre
shol
d (d
B S
PL)
Frequency (kHz)
Tuning frequency curve:
Specific hair cell locations respond to specific frequencies across the Basilar Membrane
How do we code for frequency (pitch)?
2,000 Hz 1,000 Hz 500 Hz
oval window
basilarmembrane
neurons on basilar membrane code different frequencies 1. Place coding
2. Rate coding
-the frequency (& amplitude) of the sound is reflectedin the firing rates of neuron(s)
The traveling wave of sound transduction across the Basilar Membrane
Sound Complexity: (i) number of peaks in the traveling wave, and (ii) how much “side to side” activation occurs.
Basilar Membrane’s response to complex tones
Fourier Analysis reduces sound wave to its fundamental frequency and harmonics frequencies
0
Frequency (Hz)
Fourier frequency spectrum
440 880 1320 1760
Re
lativ
e P
ower
The auditory system does break down tones intosimpler components = Ohm's acoustic law
COMPLEX TONES
The Central Auditory System
Where do we go after the cochlea?
Nuclei of Lateral
Lemniscus
“Where?” information
“What?” information
The Central Auditory System
Where do we go after the cochlea?
The cochlear nucleus is the first stage in the central auditory pathway
• Each auditory nerve fiber diverges to three divisions of the
cochlear nucleus.
This means that thereAre three tonotopic maps In the cochlear nucleus.
Each map provides a separate set of pathways & neural code patterns.
Each division of the cochlear nucleus contains specialized cell types
• Different cell types receive different types of synaptic endings, have different intrinsic properties, and respond differently to the same input.
Changes in response pattern
• The stereotyped auditory nerve discharge is converted to many different temporal response patterns.
• Each pattern emphasizes different information.
Neurons in the cochlear nucleus (and at higher levels)
transform the incoming signal in many different ways.
• The auditory nerve input is strictly excitatory (glutamate). Some neurons in the cochlear nucleus are inhibitory (GABA or glycine).
• Auditory nerve discharge patterns are converted to many other types of temporal pattern.
• There is a progressive increase in the range of response latencies.
Increase in range of latencies
• Each synapse adds approximately 1 ms latency.
• Any integrative process that requires time adds latency.
The Central Auditory System
• There are many parallel pathways in the auditory brainstem.
• The binaural system receives input from both ears.
• The monaural system receives input from one ear only.
• 1st place that binaural information can merge: Superior Olivary Nuclei
Nuclei of Lateral
Lemniscus
“Where?” information
“What?” information
Monaural pathways (“what?”)
Information from each ear is distributed to parallel pathwaysIn the cochlear nucleus and from there, the contralateral laterallemniscus
Binaural Pathways: (“Where?”)
The superior olivary complex receives input from both cochlear nuclei andcompares the input to the right and left ears.
Auditory information from receptors to Auditory Association Areas:
Each set of auditory pathways has a specialized function
More Analysis & Transduction
Route of auditory impulses from the receptors in the ear to the auditory cortex: “SONIC MG”
SON (Superior Olivary Nucleus) IC (Inferior Colliculus)
MG (Medial Geniculate Nucleus)
“A1”
Auditory areas in monkey cortex: lobe between (and under) the Lateral and Superior Temporal Sulcus
Lateral Sulcus (LS)
Superior Temporal Sulcus (STS)
Information flow within the auditory cortex:
Received in Core Area (A1)
Secondary area
Association area
Outside the primary auditory area are other specialized areas for speech, language, and probably other purposes
Unlike vision, much less overlap of left & right hemispheres: called “hemispheric lateralization”
Cortical Representation of Frequency: tonotopic representation
columnar organization in cortex: neurons in same column prefer the same
characteristic frequency
A1
Cochlea
Combination & integration of sound happens “above” the cochlea (at the level of the cortex?): “periodicity
pitch”
• Pitch perception significantly influenced by the fundamental frequency
• Pitch can be processed at the level of the cochlea (by analyzing different “peak waves” on the basilar membrane)
• However, what happens if you:– take out the fundamental frequency (no 400 Hz)
– only play some harmonics in one ear (800, 1600)
– and other harmonics in the other ear (1200, 2000)
• Effect of the “missing fundamental:” we hear the pitch (400 Hz) from only hearing the harmonics
• Called “periodicity pitch”
Periodicity Pitch
The effect of the missing fundamental
Merging left-right information in the Central
Auditory System
• The monaural system receives input from one ear only
• The binaural system receives input from both ears
• 1st place that binaural information can merge: Superior Olivary Nuclei
Nuclei of Lateral
Lemniscus
“Where?” information
“What?” information
Combination & integration of sound happens “above” the cochlea (at the level of the cortex?): “periodicity
pitch”
• Can’t be explained by pitch processing at cochlea (neither cochlea has all harmonic information)
• Could be happening at the level of Superior Olivary nucleus or Nuclei of Lateral Lemniscus (both have first combination of information from both ears)
Combination & integration of sound happens “above” the cochlea (at the level of the cortex?): “periodicity
pitch”
• However, patients with damage to a specific area auditory cortex of the right hemisphere no longer can process periodicity pitch
– Hence, use of “merged” information may be processed at the level of the cortex
– “Hemispheric lateralization”: differences in what left-right hemispheres process (i.e., left language; right non-lang sound)
Cortical response to sound: Rhesus monkey calls
• Cells in the non-associative area (i.e., beyond A1 in right temporal lobe) of the monkey cortex respond poorly to pure tones, and respond best to “noisy” sounds (lots of fundamental and harmonic frequencies combined)
• Some cells respond “best” to monkey calls (same left temporal lobe area responsive to language in humans)