Hearing
outer ear
middle
ear
inner ear
hammer
eardrum
anvil
stirrup
oval
window
hairs
tectorial membrane
basilar membrane
round
window
outer
ear
middle
ear
inner 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
Characteristic acoustic impedance of a
tube filled with gas or fluid
outer
ear
middle
ear
inner ear
hammer
Z0= rc/A,
r-the density of the medium
c-the velocity of sound
A-cross-sectional area of the tube
eardrum
anvil
stirrup
air outside
salty liquid (cochlear fluid) inside
Middle ear:
Converts impedance of the air to the impedance of the cochlear liquid
ZAIR:ZLIQ = 1:4000 99.9% loss of energy if no impedance match
Protects inner ear
Reactions to intense sounds (but rather slow 60-120 ms reaction time)
Low-pass filter 15 dB/oct from 1 kHz
middle
ear
outer
ear
inner ear
Cochlea
oval window
0rgan of Corti
basilar membrane
tectorial membrane
hairs
round window
Inner ear:
Mechanical frequency analysis of the incoming sound
Converts 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
Basilar membrane as a mechanical frequency analyzer
0.05 mm
pliable
apical
end
stiff
basal end
500 Hz
100 Hz
0.5 mm
Cochlea as frequency analyzer
How selective is the basilar membrane ?
Frequency response
input
output
system
output/input
Ratio of output to input
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!
dead animal
(von Bekesy)
Cochlea is most likely an
active system with a positive
feedback loop that accounts
for the high cochlear
sensitivity.
“tired” animal
“fresh” animal
• small piece of
radioactive material
glued on basilar
membrane
• Doppler shift in emitted
g-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
0
150
time [ms]
Shapes of five individual
action potential (spikes)
Stimulus at t=0 (sudden change of the scene that fly sees)
From movements to electrical pulses
outer ear
― 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
middle
ear
inner ear
organ of Corti
basilar membrane movements => bending of hair cells => electrical pulses
~ 40 hairs/cell
~ 140 hairs/cell
tunnel of corti
tectorial
membrane
inner
hair cells
auditory nerve
fiber
basilar
membrane
outer hair cells
auditory nerve
fiber
inner hair cells – information
outer hair cells – govern cochlear mechanics ?
one-way
rectifier
Intracellular voltage as a function of stimulus
pressure (600 Hz sinusoid)
out
inner hair cell
in
0
electrode
outer hair cell
Intracellular voltage changes in an inner hair
cell for different frequencies of stimulation
electrode
electrode
?
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
threshold of firing
sound level [dB]
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
Place theory of peripheral auditory processing
BP1
BP2
BRAIN
signal
BPn
firing rate depends on sound intensity
Firing of the auditory nerve
bank of cochlear band-pass filters
sound level [dB]
bandwidth
Bandwidths of tuning curves increase with frequency
(frequency resolution decreases with frequency)
characteristic frequency
frequency [kHz]
5
0
0
time [s]
1.2
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)
intensity
tone elicits certain
response (firing rate)
second tone in the + area
increases the firing rate
second tone in the – area
decreases the firing rate
frequency
Sensitivity of visual neuron (retinal ganglion cell)
of a frog to changing size of a dot
bright dot
dark dot
“on center”
(“off surround”)
responds to increase
in light intensity
“off center”
(“on surround”)
responds to decrease
in light intensity
2-dimensional “receptive field” in vision
Receptive field
on your skin