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

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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

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Frequency (Hz)

Fourier frequency spectrum

440 880 1760

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“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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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

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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

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lativ

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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)