INTRODUCTION TO HEARING

WHAT IS SOUND?

amplitude

Intensity measured in decibels.

SAME PITCH

DIFFERENTTIMBRE

10 100 1000 10,000Frequency (Hz)

(high)(low)

020406080

100(loudest)

(quietest)

sound levelin decibels(dB)

can hear

can't hear

this line shows thequietest sounds youcan hear. At soundpressure levels belowthis line you can't hearthe sound.

10 100 1000 10,000

Frequency (Hz)

(high)(low)

020406080

100

(loudest)

(quietest)

sound levelin decibels(dB)

*

*

*

All points on this curve have the same perceived loudness as the standard (*)

All points on this curve have the same perceived loudness as the standard (*)

All points on this curve have the same perceived loudness as the standard (*)

EQUAL LOUDNESS CURVES

SUMMARY

Sound is variation in pressure

Frequency, pitchIntensity (SPL, “sound pressure level”), loudness, decibels

Timbre, harmonicsEqual loudnessHearing thresholds

THE EAR

pinna(external ear)

malleusincusstapes

little bones

oval windowtympanic membrane (ear drum)

leverage= 1.3 x

this section coiled upin the actual ear (cochlea)

(ossicles)}

areadecrease17 x

}

OUTER EAR MIDDLEEAR

INNER EAR

helicotrema

THE EAR

Amplification:

Bones 1.3xArea 17 x

Total = 22.1x

Density:Air: 1.2 kg/m3

Water: 1000 kg/m3

Ratio:

basilar membrane

top tube

middle tube

bottom tube

tectorial membrane

hair cells

vibrations pass down the top tube to theend (the HELICOTREMA) and thenback down the bottom tube.

helicotremaoval window

vibrates most tohigh frequencies(around 10 kHz)

vibrates most tomiddlefrequencies(around 1 kHz)

vibrates most tolow frequencies(down to around27 Hz)

LESS STIFFSTIFF

HIGH FREQUENCIES

LOW FREQUENCIES

hairs

hair cell

nucleus

fibre of 8th nerve (auditory nerve)

ACTION POTENTIALS TO BRAIN

Auditory system 13 - 5

SUMMARY

Outer, middle, inner ear

Oval window, round window, Basilar membrane, tectorial membrane, hair cells

THEORIES OF HEARING

PLACE THEORY (which fibres, labelled lines)Von Békésy (Nobel prize 1961)

1 - Travelling wave; stiffness varies2 - one place most active for a given frequency3 - Tonotopic code; coded as place

PERIODICITY THEORY (how they are firing, temporal code)1 - sound coded as pattern

vibrates most tohigh frequencies(around 10 kHz)

vibrates most tomiddlefrequencies(around 1 kHz)

vibrates most tolow frequencies(down to around27 Hz)

MODEL OF THE BASILAR MEMBRANE

Varies in stiffness…

RESONANCE

Traveling wave:

WHERE THE WAVE HAS ITS HIGHEST AMPLITUDE DEPENDS ON ITS FREQUENCY

Evidence against place-- Missing fundamental-- which can be masked -- some animals have no basilar membrane

Evidence against periodicity-- cells can’t fire fast enough-- diplacusis

Evidence for place-- physiology(basilar membrane)(cells tuned for frequencies)-- masking

Evidence for periodicity-- multiple cells could do it-- phase locking of cells

MASKING

Evidence against place-- Missing fundamental-- which can be masked -- some animals have no basilar membrane

Evidence against periodicity-- cells can’t fire fast enough-- diplacusis

Evidence for place-- physiology(basilar membrane)(cells tuned for frequencies)-- masking

Evidence for periodicity-- multiple cells could do it-- phase locking of cells

PHASE LOCKING

Temporal coding up to about 4,000 hz… but each spike takes about 2ms… therefore only up to around 500 hz…..

Temporal coding up to about 4,000 hz… but each spike takes about 2ms… therefore only up to around 500 hz…..

But could share it out over several cells One cell might only be able to follow every 4th cycle…..

… but others can share the task

Evidence against place-- Missing fundamental-- which can be masked -- some animals have no basilar membrane

Evidence against periodicity-- cells can’t fire fast enough-- diplacusis

Evidence for place-- physiology(basilar membrane)(cells tuned for frequencies)-- masking

Evidence for periodicity-- multiple cells could do it-- phase locking of cells

THE CASE OF THE MISSING FUNDAMENTAL

Pitch determined by fundamental….

So what happens if we remove the fundamental? What does it sound like?

Play it

Training a goldfish... CAN AN ANIMAL

WITH NO BASILAR MEMBRANE DISTINGUISH FREQUENCIES?

Evidence against place-- Missing fundamental-- which can be masked -- some animals have no basilar membrane

Evidence against periodicity-- cells can’t fire fast enough-- diplacusis

Evidence for place-- physiology(basilar membrane)(cells tuned for frequencies)-- masking

Evidence for periodicity-- multiple cells could do it-- phase locking of cells

Place theorysound coded as place

Periodicity theorysound coded as pattern

Duplicitybelow 1-4 kHz, coded by periodicityabove 1-4 kHz, coded by place

AUDITORY SYSTEM

hairs

hair cell

nucleus

fibre of 8th nerve (auditory nerve)

ACTION POTENTIALS TO BRAIN

Auditory system 13 - 5

Auditory cortex

Auditory thalamus

Superior colliculus

Inferior colliculus

cochlea

Cochlear nucleus

Superior olive

The AuditorySystem

The AuditorySystem(cortical route)

Cochlear nucleus

Inferior colliculus

thalamus

cortex

The AuditorySystem(sub-cortical route)

Cochlear nucleus

Superior olive

Inferior colliculus

Superior colliculus

The AuditorySystem

Cells in primary auditory cortex: -- tonotopically arranged-- respond to more complex features (eg howler monkey calls)

primary auditory cortex

front

back

left right

The SuperiorColliculus

a a

a

b

b

c

c

d

d

f

g

AUDITORY LOCALIZATION

Auditory localization

1 inter-aural time of arrival differences-- circle of confusion

2 inter-aural intensity differences3 pinnae (up/down front/back etc..)4 head movements

1 inter-aural time of arrival differences

1 inter-aural time of arrival differences

2 inter-aural intensity differences

3 pinnae (up/down front/back etc..)

4 head movements

4 head movements

AUDITORY SCENE ANALYSIS