Transcript lecture 10

INTRODUCTION TO HEARING
WHAT IS SOUND?
amplitude
Intensity
measured
in
decibels.
SAME PITCH
DIFFERENT
TIMBRE
this line shows the
quietest sounds you
can hear. At sound
pressure levels below
this line you can't hear
the sound.
(loudest)
100
80
sound level 60
in decibels
40
(dB)
20
0
(quietest)
(low)
can hear
can't hear
10
100
1000
10,000
Frequency (Hz)
(high)
EQUAL LOUDNESS CURVES
(loudest)
100
80
sound level
in decibels 60
40
(dB)
20
0
(quietest)
(low)
*
*
All points on
this curve
All points on
have the
this
curve
All
same points on
have
the
this
curve
perceived
same
have the
loudness
as
perceived
thesame
standard
loudness
as
perceived
(*)
the
standard
loudness
as
(*)the standard
(*)
*
10
100
1000
10,000
Frequency (Hz)
(high)
SUMMARY
Sound is variation in pressure
Frequency, pitch
Intensity (SPL, “sound pressure level”), loudness, decibels
Timbre, harmonics
Equal loudness
Hearing thresholds
THE EAR
THE EAR
malleus
incus
stapes
}
little bones
(ossicles)
leverage= 1.3 x
helicotrema
}
pinna
tympanic membrane oval window
(external ear) (ear drum)
OUTER EAR
MIDDLE
EAR
area
decrease
17 x
INNER EAR
this section coiled up
in the actual ear (cochlea)
Amplification:
Bones 1.3x
Area 17 x
Total = 22.1x
Density:
Air: 1.2 kg/m3
Water: 1000 kg/m3
Ratio:
top tube
middle tube
bottom tube
tectorial membrane
basilar membrane
hair cells
vibrations pass down the top tube to the
end (the HELICOTREMA) and then
back down the bottom tube.
LESS STIFF
STIFF
oval window
vibrates most to
high frequencies
(around 10 kHz)
HIGH
FREQUENCIES
helicotrema
vibrates most to
middle
frequencies
(around 1 kHz)
vibrates most to
low frequencies
(down to around
27 Hz)
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 varies
2 - one place most active for a given frequency
3 - Tonotopic code; coded as place
PERIODICITY THEORY
(how they are firing, temporal code)
1 - sound coded as pattern
vibrates most to
high frequencies
(around 10 kHz)
vibrates most to
middle
frequencies
(around 1 kHz)
vibrates most to
low frequencies
(down to around
27 Hz)
MODEL OF THE BASILAR MEMBRANE
Varies in stiffness…
RESONANCE
Traveling wave:
WHERE THE WAVE HAS ITS
HIGHEST AMPLITUDE
DEPENDS ON ITS FREQUENCY
Evidence for place
-- physiology
(basilar membrane)
(cells tuned for frequencies)
-- masking
Evidence for periodicity
-- multiple cells could do it
-- phase locking of cells
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
MASKING
Evidence for place
-- physiology
(basilar membrane)
(cells tuned for frequencies)
-- masking
Evidence for periodicity
-- multiple cells could do it
-- phase locking of cells
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
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 for place
-- physiology
(basilar membrane)
(cells tuned for frequencies)
-- masking
Evidence for periodicity
-- multiple cells could do it
-- phase locking of cells
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
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 for place
-- physiology
(basilar membrane)
(cells tuned for frequencies)
-- masking
Evidence for periodicity
-- multiple cells could do it
-- phase locking of cells
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
Place theory
sound coded as place
Periodicity theory
sound coded as pattern
Duplicity
below 1-4 kHz, coded by periodicity
above 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
The Auditory
System
Auditory cortex
Auditory thalamus
Superior colliculus
Inferior colliculus
cochlea
Cochlear nucleus
Superior olive
The Auditory
System
(cortical route)
cortex
thalamus
Inferior colliculus
Cochlear nucleus
The Auditory
System
(sub-cortical route)
Superior colliculus
Inferior colliculus
Cochlear nucleus
Superior olive
The Auditory
System
primary
auditory
cortex
Cells in primary auditory cortex:
-- tonotopically arranged
-- respond to more complex features (eg
howler monkey calls)
The Superior
Colliculus
front
a
left
a
f
c
b
g
d
back
b
d
a
c
right
AUDITORY LOCALIZATION
Auditory localization
1 inter-aural time of arrival differences
-- circle of confusion
2 inter-aural intensity differences
3 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