Hearing and Deafness 1. Anatomy & physiology

Chris DarwinWeb site for lectures, lecture notes and filtering lab:http://www.lfesci.sussex.ac.uk/home/Chris_Darwin/

look under: "Teaching material for students" "Perception & Attention"

Protection

Impedance match

Capture; Amplify mid-freqs

Vertical direction coding

Frequency analysis

Transduction

Outer, middle & inner ear

Middle ear structure

Conductive hearing loss

• Sounds don’t get into cochlea

• Middle ear problems

• Helped by surgery and by amplification

Protection

Impedance match

Capture; Amplify mid-freqs

Vertical direction coding

Frequency analysis

Transduction

Outer, middle & inner ear

Cochlea

Cochlea cross-section

Travelling wave on basilarmembrane sorts sounds by frequency

distance along basilar membrane

amp

litu

de

of

vib

rati

on

base apex

high frequencies low frequencies

0

Reponse of basilar membraneto sine waves

Each point on the membrane responds best to a different frequency:high freq at base, low at apex.

amadeus praat

Organ of Corti

Innerhair cell

Hair CellStereocilia

Auditory nerve innervation

OHC (2)spiral afferent (green)medial efferent (red)

IHC (1)

radial afferent (blue)lateral efferent (pink)

Auditory nerverate-intensity functions

log amplitude (dB SPL)

100

80

60

40

20

sp

ikes

/ s

eco

nd

30 60 90

high spontaneous rate

low spontaneous rate

saturation

many

few

Phase Locking of Inner Hair Cells

Auditory nerve connected to inner hair cell tends to fire

at the same phase of the stimulating waveform.

Phase-locking

pres

sure

-1

-0.5

0

0.5

1

0 0.2 0.4 0.6 0.8 1

Inter-spike IntervalsResponse to Low Frequency tones

time (t)

volta

ge

Response to High Frequency tones > 5kHz

Random intervals

time (t)

2 periods 1 periodnerve spike

volta

ge

Innervs

OuterHair Cells

Inner vs Outer Hair Cells

Inner Hair Cells Outer Hair Cells

Sensory Motor

Afferent nerves Efferent nerves

Single row c.3 rows

OHC movement

PassiveNo OHC movement

ActiveWith OHC movement

OHC activity

• Increases sensitivity (lowers thresholds)

• Increases selectivity (reduces bandwidth of auditory filter)

• Gives ear a logarithmic (non-linear) amplitude response

• Produce Oto-acoustic emissions

OHCs are relatively more active for quiet sounds than forloud sounds.

They only amplify sounds that have the characteristicfrequency of their place.

Conductive vs Sensori-neural deafness

Conductive Sensori-neural Sensori-neural

Origin Middle-ear Cochlea (IHCs) Cochlea (OHCs)

Thresholds Raised Raised Raised

Filter bandwidths Normal Normal Increased

Loudness growth Normal Normal Increased (Recruitment

Becomes linear, soNo combination tonesOr two-tone suppression

Mostly a combination ofOHC and IHC damage

Auditory nerve frequency-threshold curves

log frequency

log

am

pli

tud

e (d

B S

PL

)

100

80

60

40

20

Characteristic Frequency

Normal Threshold

Normal bandwidth

Auditory tuning curves

log frequency

log

am

pli

tud

e (d

B S

PL

) 100

80

60

40

20Characteristic Frequency

Normal Threshold

Abnormal Threshold

Normal bandwidth

Normal bandwidth

Healthy ear

Inner hair-celldamage

Outer-hair cell damage

log frequency

log

am

pli

tud

e (d

B S

PL

) 100

80

60

40

20

Characteristic Frequency

Normal Threshold

Abnormal Threshold

Normal bandwidth

Abnormal bandwidth

BM becomes linear without OHCs(furosemide injection)

Amplification greater and tuningmore selective at low levels

Robles, L. and Ruggero, M. A. (2001). "Mechanics of the mammalian cochlea," Physiological Review 81, 1305-1352.

Normal auditory non-linearities

• Normal loudness growth (follows Weber’s Law)

• Combination tones

• Two-tone suppression

• Oto-acoustic emissions