Chapter 10Chapter 10

The Ear and Auditory SystemThe Ear and Auditory System

Sound wavesSound waves

If a tree falls in the woods and no one is If a tree falls in the woods and no one is around to hear it ….around to hear it ….

Distinguish between perceptual qualities Distinguish between perceptual qualities and physical qualitiesand physical qualities

The physical quality relevant for hearing is The physical quality relevant for hearing is mechanical disturbances in particular mechanical disturbances in particular media.media.

This is acoustic energy.This is acoustic energy.

Robert Boyle shows that typical sounds Robert Boyle shows that typical sounds are vibrations in the molecules of air.are vibrations in the molecules of air.

Speed of soundSpeed of sound

343 meters/second in air343 meters/second in air1,500 meters/second in water1,500 meters/second in water5,000 meters/second in steel5,000 meters/second in steel

Speed is constant in any given medium, Speed is constant in any given medium, although sounds fade with increasing although sounds fade with increasing distance. Signal strength declines with the distance. Signal strength declines with the square of the distance. (double distance = square of the distance. (double distance = 4x reduction in sound)4x reduction in sound)

EchoesEchoes

Sound, unlike light, can propagate around Sound, unlike light, can propagate around and through objects. This makes it more and through objects. This makes it more difficult to block out sounds than light.difficult to block out sounds than light.

Objects do reflect sound waves as Objects do reflect sound waves as echoes.echoes.

These echoes can be used in sonar These echoes can be used in sonar ((sosound und nanavigation vigation rranging)anging)

EchoesEchoes

Direction of reflections make a vast Direction of reflections make a vast difference in acoustic properties of rooms, difference in acoustic properties of rooms, e.g. concert halls.e.g. concert halls.

Plaster or tile absorbs about 3% of incident Plaster or tile absorbs about 3% of incident sound waves.sound waves.

Carpet absorbs about 25% of incident sound Carpet absorbs about 25% of incident sound waves.waves.

EchoesEchoes

Anechoic chambers use foam wedges to Anechoic chambers use foam wedges to eliminate all echoes.eliminate all echoes.

These rooms have a “dead” feel to them.These rooms have a “dead” feel to them.

Echoes do appear to enable humans to Echoes do appear to enable humans to navigate and can be used to identify navigate and can be used to identify materials.materials.

Nature of sound wavesNature of sound waves

Sound waves are variations in the density of Sound waves are variations in the density of molecules in the air.molecules in the air.

Sounds levelsSounds levels

Measured in decibels (dB).Measured in decibels (dB).

Decibel scale is logarithmic.Decibel scale is logarithmic.

dB = 20 log (p1/p0)dB = 20 log (p1/p0)

20 20 μμPa ≈ softest sound humans can hear.Pa ≈ softest sound humans can hear.

The Auditory System: The EarThe Auditory System: The Ear

Outer Ear TriviaOuter Ear Trivia

Pinna “colors” the sounds we hear.Pinna “colors” the sounds we hear.Ear canal is about 2.5 cm long x 7mm in Ear canal is about 2.5 cm long x 7mm in diameter.diameter.Ear drum (tympanic membrane) can Ear drum (tympanic membrane) can detects sounds using a displacement of detects sounds using a displacement of one millionth of a centimeter.one millionth of a centimeter.Ear drum has surface area of 68mmEar drum has surface area of 68mm22

Gerbil area = 15 mmGerbil area = 15 mm22

Elephant area = 450 mmElephant area = 450 mm22

Middle EarMiddle Ear

Note: Error in Figure 10.9, p. 362, Labeling of middle ear.

Each bone is about the size of a letteron a printed page.

Ossicles vary in size across species:Human: 28.5 mgGerbil: 1.15 mgElephant: 335 mg

Why have the ossicles?Why have the ossicles?

Sound waves in air do not transmit well to Sound waves in air do not transmit well to sound waves in water.sound waves in water.

The ossicles decrease the loss in signal The ossicles decrease the loss in signal strength.strength.

The Eustachian tube maintains air The Eustachian tube maintains air pressure on both side of the ear drum.pressure on both side of the ear drum.

The acoustic reflexThe acoustic reflex

The tensor tympani connects to the ear drum.The tensor tympani connects to the ear drum.

The stapedius connects to the stapes. The stapedius connects to the stapes.

These muscles flex during loud noises to reduce These muscles flex during loud noises to reduce the response of the ossicles.the response of the ossicles.

This reduces intensity of sound transmission by This reduces intensity of sound transmission by the equivalent of 30 dB.the equivalent of 30 dB.

The Acoustic ReflexThe Acoustic Reflex

Is more effective at damping low Is more effective at damping low frequencies than at damping high frequencies than at damping high frequencies.frequencies.

Takes roughly 1/20 of a second to take Takes roughly 1/20 of a second to take effect.effect.

Perhaps reduces ability to hears one’s Perhaps reduces ability to hears one’s own voice.own voice.

The Inner EarThe Inner Ear

CochleaCochlea

Three chambers:Three chambers: Vestibular canal (aka scala vestibuli)Vestibular canal (aka scala vestibuli) Cochlear duct (aka scala media)Cochlear duct (aka scala media) Tympanic Canal (aka scala timpani)Tympanic Canal (aka scala timpani)

http://www.vimm.it/cochlea/index.htm

Hair cell triviaHair cell trivia

~20,000 total hair cells~20,000 total hair cells~4,500 Inner hair cells~4,500 Inner hair cells~15,500 outer hair cells~15,500 outer hair cells

OHC arranged in “v”sOHC arranged in “v”sIHC are linearly arranged.IHC are linearly arranged.

OHC attached to tectorial membrane;OHC attached to tectorial membrane;IHC are not attached.IHC are not attached.

OHC amplify signals of IHC.OHC amplify signals of IHC.

Background to BekesyBackground to Bekesy

Temporal theory: Basilar membrane Temporal theory: Basilar membrane vibrates at frequency of incoming sound vibrates at frequency of incoming sound waves.waves.

Place theory: Basilar membrane vibrates Place theory: Basilar membrane vibrates at a place corresponding to the frequency at a place corresponding to the frequency of incoming sound.of incoming sound.

Temporal theoryTemporal theory

Temporal theory: Basilar membrane Temporal theory: Basilar membrane vibrates at frequency of incoming sound vibrates at frequency of incoming sound waves.waves.

Rutherford: A 500 Hz sound would cause Rutherford: A 500 Hz sound would cause basilar membrane to vibrate at 500 Hz; a basilar membrane to vibrate at 500 Hz; a 1,200 Hz sound would cause basilar 1,200 Hz sound would cause basilar membrane to vibrate at 1,200 Hz.membrane to vibrate at 1,200 Hz.

Problems for temporal theoryProblems for temporal theory

Basilar membrane varies in width and stiffness Basilar membrane varies in width and stiffness over its length, so cannot vibrate uniformly over over its length, so cannot vibrate uniformly over its length.its length.

Nerve cells cannot fire faster than about 1,000 Nerve cells cannot fire faster than about 1,000 Hz, but sounds are much high pitched than this.Hz, but sounds are much high pitched than this. Maybe two or more neurons act together to coarse Maybe two or more neurons act together to coarse

code frequency information. (Volley theory)code frequency information. (Volley theory)

Place TheoryPlace Theory

Frequency information is encoded by the place Frequency information is encoded by the place along the basilar membrane disturbed by the along the basilar membrane disturbed by the fluid vibration.fluid vibration.

Hermann von Helmholtz

Problems for Place TheoryProblems for Place Theory

Basilar membrane is not composed of Basilar membrane is not composed of fibers along its width. It is a continuous fibers along its width. It is a continuous strip.strip.

Basilar membrane is not under tension.Basilar membrane is not under tension.

Bekesy & traveling wavesBekesy & traveling waves

1920’s Georg von Bekesy could not image 1920’s Georg von Bekesy could not image the cochlea or basilar membrane in action.the cochlea or basilar membrane in action.

So, von Bekesy built a model of the So, von Bekesy built a model of the cochlea.cochlea.

Waves travel along the basilar membrane.Waves travel along the basilar membrane.

A wave reaches a peak, then quickly dissipates.A wave reaches a peak, then quickly dissipates.

This peak is the peak sensitivity.This peak is the peak sensitivity.

Lower tones travel farther.Lower tones travel farther.

This yields tonotopic representation (cf. This yields tonotopic representation (cf. retinotopic representation).retinotopic representation).

LoudnessLoudness

Louder noises correspond to sound waves Louder noises correspond to sound waves of higher amplitude.of higher amplitude.

In the ear, this leads to waves of greater In the ear, this leads to waves of greater amplitude in the basilar membrane.amplitude in the basilar membrane.

In the IHC, this leads to a larger neural In the IHC, this leads to a larger neural response.response.

Cochlear emissionsCochlear emissions

Sound in airSound in airMovement of ear drumMovement of ear drumMovement of ossiclesMovement of ossiclesMovement of oval windowMovement of oval windowFluid-borne pressure wavesFluid-borne pressure wavesDisplacement of basilar membraneDisplacement of basilar membraneStimulation of hair cellsStimulation of hair cells

(cf. p. 374).(cf. p. 374).

Cochlear emissionsCochlear emissions

Typically have a narrow band of Typically have a narrow band of frequencies.frequencies.

Roughly 66% of those tested display Roughly 66% of those tested display cochlear emissions.cochlear emissions.

Frequency of emission is idiosyncratic.Frequency of emission is idiosyncratic.

More prevalent and stronger in women.More prevalent and stronger in women.

Dogs, cats, and birds have cochlear Dogs, cats, and birds have cochlear emissions.emissions.

Cochlear emissionsCochlear emissions

Can be induced by clicks near the ear.Can be induced by clicks near the ear.

Can be diagnostic of early ear damage.Can be diagnostic of early ear damage.

TinnitusTinnitus

Ringing in the earsRinging in the ears

Occurs in about 35% of people at some point in Occurs in about 35% of people at some point in their lives.their lives.

Can be caused temporarily by large dose of Can be caused temporarily by large dose of aspirin.aspirin.

Appears to have a cortical basis.Appears to have a cortical basis.

Auditory system: Auditory system: The auditory pathwaysThe auditory pathways

Feedforward: auditory nerve, superior Feedforward: auditory nerve, superior olive, medial geniculate nucleus and olive, medial geniculate nucleus and inferior colliculus, auditory cortex.inferior colliculus, auditory cortex.

Feedback:Feedback: Medial portion of superior olivary complex to Medial portion of superior olivary complex to

OHCs.OHCs. Lateral portion of superior olivary complex to Lateral portion of superior olivary complex to

auditory nerve.auditory nerve.

Auditory system: Auditory system: The auditory pathwaysThe auditory pathways

Auditory system: Auditory system: The auditory pathwaysThe auditory pathways

The auditory nerveThe auditory nerve

The “what” pathwayThe “what” pathway

The “where” pathway The “where” pathway These last two are analogous to those found These last two are analogous to those found

in vision.in vision.

The auditory nerveThe auditory nerve

~20,000 total hair cells ~20,000 total hair cells

~4,500 Inner hair cells~4,500 Inner hair cells

~15,500 outer hair cells~15,500 outer hair cells

~30,000 nerve fibers from these cells ~30,000 nerve fibers from these cells constitute the auditory nerveconstitute the auditory nerve

Most (~95%) auditory nerve fibers connect Most (~95%) auditory nerve fibers connect to IHCs.to IHCs.

Frequency tuning curvesFrequency tuning curves

Different fibers have different Different fibers have different characteristic frequencies.characteristic frequencies.

All fibers are asymmetric (sharp high drop-All fibers are asymmetric (sharp high drop-off, flat low drop-off).off, flat low drop-off).

Narrow characteristic frequencies.Narrow characteristic frequencies.

The auditory nerve coarse codes The auditory nerve coarse codes information from the IHC along the basilar information from the IHC along the basilar membrane.membrane.

This is because a single fiber is This is because a single fiber is ambiguous as to what combination of ambiguous as to what combination of intensity and frequency of sound wave is intensity and frequency of sound wave is impinging on the ear.impinging on the ear.

Sound localization: Sound localization: The “where” pathwayThe “where” pathway

Works primarily on two types of localization Works primarily on two types of localization cues: interaural time differences and interaural cues: interaural time differences and interaural intensity differences.intensity differences.

Cells in the cochlear nucleus are monaural.Cells in the cochlear nucleus are monaural.

Cells beyond the cochlear nucleus are bimaural.Cells beyond the cochlear nucleus are bimaural.

Some binaural cells act in complementary Some binaural cells act in complementary fashion.fashion. These prefer low frequencies.These prefer low frequencies.

Other binaural cells act antagonistically.Other binaural cells act antagonistically. These prefer high frequencies.These prefer high frequencies.

Some binaural cells differ in characteristic Some binaural cells differ in characteristic frequency for left and right ears.frequency for left and right ears.

Some binaural cells are sensitive to the speed of Some binaural cells are sensitive to the speed of moving sounds.moving sounds.

Binaural timing differencesBinaural timing differences

Some binaural cells respond maximally to Some binaural cells respond maximally to simultaneous combination of inputs from simultaneous combination of inputs from both ears.both ears.

Axons can differ in length, so use this to Axons can differ in length, so use this to measure time between signals.measure time between signals.

This is Jeffress, (1948), delay line theory.This is Jeffress, (1948), delay line theory.

Binaural intensity differencesBinaural intensity differences

““When sound energy passes through a dense barrier – When sound energy passes through a dense barrier – such as your head – some sound energy is lost” (Blake such as your head – some sound energy is lost” (Blake & Sekuler, 2005, p. 382).& Sekuler, 2005, p. 382).

Some neurons respond maximally to slightly different Some neurons respond maximally to slightly different intensities of sound.intensities of sound.

An ear plug in one ear tends to distort sound localization An ear plug in one ear tends to distort sound localization abilities.abilities.

Organisms with large olivary structures are also better at Organisms with large olivary structures are also better at sound localization.sound localization.

Primary auditory cortex:Primary auditory cortex:

Laid out in concentric rings, beginning with A1 (aka Laid out in concentric rings, beginning with A1 (aka Brodmann’s 41 & 42)Brodmann’s 41 & 42)

A1 is comparable to V1 and S1.A1 is comparable to V1 and S1.

In the “core,” which includes A1, single-cell recordings In the “core,” which includes A1, single-cell recordings show that sounds are represented tonotopically (3 show that sounds are represented tonotopically (3 times).times).

There is cortical magnification in the auditory “core,” There is cortical magnification in the auditory “core,” especially human speech and animal vocalizations.especially human speech and animal vocalizations.

The spatial arrangement of the whiskers on a rat’s face is illustratedabove (A), as well as the corresponding matrix of cell rings in the somatosensorycortex (B).  Actual barrels from layer IV are shown as well (C).  (From Blakemore, 1977)

Secondary auditory cortex Secondary auditory cortex (a.k.a. “belt” areas)(a.k.a. “belt” areas)

Single-cell recordings show that these cells Single-cell recordings show that these cells respond relatively weakly to pure tones.respond relatively weakly to pure tones.

They respond to more complicated features of They respond to more complicated features of sounds, e.g. frequency modulations, intensity sounds, e.g. frequency modulations, intensity modulations.modulations.

Natural sounds are non-harmonic; animal Natural sounds are non-harmonic; animal sounds are harmonic. (To be explained in sounds are harmonic. (To be explained in Chapter 11)Chapter 11)

Human auditory cortexHuman auditory cortex

Has tonotopic maps revealed by fMRI.Has tonotopic maps revealed by fMRI.

There are “phantom” tones associated There are “phantom” tones associated with particular regions of the brain.with particular regions of the brain.

Sometimes cortex deprived of input of a Sometimes cortex deprived of input of a given tone will adjust its sensitivity to given tone will adjust its sensitivity to include other tones.include other tones.

Music and speech have idiosyncratic Music and speech have idiosyncratic auditory features, to be discussed later.auditory features, to be discussed later.