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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