Hearing

Hearing

inner ear

basilar membrane

tectorial membranehairs

ovalwindow

roundwindow

middleear

stirrup

anvil

hammer

outer ear

eardrum

Outer ear:

• Mechanical protection of the middle ear• Diffracts and focuses sound waves (pinna)• The ear canal acts as a resonator (3-5 kHz enhancement)• The end of the canal has an eardrum which vibrates with

sound

inner earmiddleear

outer ear

eardrum

Middle ear:

Converts impedance of the air to the impedance of the cochlear liquidZAIR:ZLIQ = 1:4000 99.9% loss of energy if no impedance match

Protects inner earReactions to intense sounds (but rather slow 60-120 ms reaction time)

Low-pass filter 15 dB/oct from 1 kHz

Characteristic acoustic impedance of a tube filled with gas or fluid

Z0= c/A, -the density of the mediumc-the velocity of soundA-cross-sectional area of the tube

air outsidesalty liquid (cochlear fluid) inside

inner earmiddleear

anvil

hammer

outer ear

eardrum

stirrup

Inner ear:

Mechanical frequency analysis of the incoming soundConverts mechanical movements to electrical pulses

Changes in acoustic pressure => movement of bones in middle ear=> movement of membrane on oval window => vibrations in the cochlear liquid=> vibrations of basilar membrane

Cochlea inner earmiddle

earouter ear

oval window

round window

0rgan of Cortibasilar membrane

tectorial membranehairs

Basilar membrane as a mechanical frequency analyzer

0.05 mm 0.5 mmstiff

basal end

pliableapicalend

500 Hz

100 Hz

Cochlea as frequency analyzer

How selective is the basilar membrane ?

Frequency response

input

system

output

Ratio of output to input

outp

ut/i

nput

frequency

• Movement of the basilar membrane in dead animal observed by a microscope

• von Bekesy 1960

Selectivity very different after the death of the animal!

Cochlea is most likely an active system with a positive feedback loop that accounts for the high cochlear sensitivity.

“fresh” animal

“tired” animal

dead animal(von Bekesy)

• small piece of radioactive material glued on basilar membrane

• Doppler shift in emitted -rays indicates amplitude of the membrane vibrations

Nonlinear system!(curves vary with intensity)

Code for the brain

1. Sensory neurons produce spikes

2. Spike rate increases with an increase in the stimulus intensity (here it was a weight on a muscle)

Adaptation: after a while, the firing rate decreases even when the stimulus intensity stays the same

Action potential in a brain cell of a fly exposed to visual scenes

time [ms]

0 150

Shapes of five individual action potential (spikes)

Stimulus at t=0 (sudden change of the scene that fly sees)

From movements to electrical pulses

― The basilar membrane contains ~15,000-20,000 hair cells (sensory cells)

― Inner hair cells transduce vibration into electrical signal and send them to the brain

―Outer hair cells receive signals from the brain, which could change mechanical properties of the organ of Corti

inner earmiddleear

outer ear

organ of Corti

basilar membrane movements => bending of hair cells => electrical pulses

innerhair cells

~ 40 hairs/cell ~ 140 hairs/cell

outer hair cells

auditory nerve fiber

auditory nerve fiber

tectorialmembrane

basilarmembrane

tunn

el o

f co

rti

inner hair cells – informationouter hair cells – govern cochlear mechanics ?

Intracellular voltage as a function of stimulus pressure (600 Hz sinusoid)

inner hair cell outer hair cell

0in

out

one-way rectifier

electrode

Intracellular voltage changes in an inner hair cell for different frequencies of stimulation

electrodeelectrode

?

Spikes on the auditory nerve are in phase with the signal

Only in one half of the cycle• One-way rectification

Period histogram

where the spike appears with respect to the waveform

Coding of the stimulus intensity

sound level [dB]

threshold of firing

Tuning curves

Reverse correlation technique

Bandwidths of tuning curves increase with frequency(frequency resolution decreases with frequency)

Place Theory of Hearing

Tones of certain frequencies excite certain areas of the cochlea that are connected to certain auditory fibres.

• the fibres are distributed tonotopically (by their best frequencies) in the auditory nerve

• this tonotopical organization is preserved throughout the higher areas of hearing all the way to the brain

signal

BP1BP2

BPn

BRAIN

bank of cochlear band-pass filters

Place theory of peripheral auditory processing

Bandwidths of tuning curves increase with frequency(frequency resolution decreases with frequency)

sound level [dB]

Firi

ng o

f th

e au

dito

ry n

erve

characteristic frequency

band

wid

th

firing rate depends on sound intensity

time [s]01.2

fre

qu

en

cy [

kHz] 5

0

Response in brain of fly to a change of the scene

Response of hearing periphery to a change in acoustic scene (switching on and off a tone)

Response of horseshoe crab’s visual neuron to change in light

Two-tone suppression(lateral inhibition)in

tens

ity

frequency

tone elicits certain response (firing rate)

second tone in the + area increases the firing rate

second tone in the – area decreases the firing rate

“on center”(“off surround”)

responds to increase in light intensity

“off center”(“on surround”)

responds to decrease in light intensity

Sensitivity of visual neuron (retinal ganglion cell) of a frog to changing size of a dot

bright dot

dark dot

2-dimensional “receptive field” in vision

Receptive field on your skin