Timing code for pitch
•
ISO is a network of the national standards institutes of 156
countries, on the basis of one member per country, with a
Central Secretariat in Geneva, Switzerland, that
coordinates the system.
• Because "International Organization for Standardization"
would have different abbreviations in different languages
("IOS" in English, "OIN" in French for Organisation
internationale de normalisation), it was decided at the
outset to use a word derived from the Greek isos, meaning
"equal".
• Therefore, whatever the country, whatever the language,
the short form of the organization's name is always ISO.
3/30/2006
1
• How would such neurons be able to signal
higher frequencies (because, as we know,
people can hear frequencies of up to 20,000
Hz)?
3/30/2006
Volley Principle
Volley Principle
• Wever suggested that while one neuron
alone could not carry the temporal code for
a 20,000 Hz tone, 20 neurons, with
staggered firing rates, could.
• Each neuron would respond on average to
every 20th cycle of the pure tone, and the
pooled neural responses would jointly
contain the information that a 20,000 Hz
tone was being presented.
• As we’ve discussed, a simple version of the
timing code can't work, but if neurons work
together according to the volley principle,
it's possible to produce a timing code even
for higher frequencies.
3/30/2006
3
3/30/2006
Characteristics of auditory nerve
responses to sound
4
Phase Locking
• Phase locking: an observed phenomenon
(in support of the volley principle) where
neurons fire in synchrony with the phase of
a stimulus.
• Auditory nerve fibers sensitive to a particular
frequency range fire at the same part (phase) of
every cycle of a sound in that range. This is called
phase-locking.
3/30/2006
2
5
3/30/2006
6
1
Phase Locking
Summary: What is the neural code
for both pitch and loudness?
• No individual neuron could fire at each peak, but a bunch
of phase-locked neurons working together can produce a
burst of activity at each peak, and so the firing frequency
of a collection of neurons can indeed mimic the frequency
of the stimulus.
3/30/2006
• Pitch depends on place code & temporal
code.
• Loudness depends on firing rates & number
of neurons.
• How do these 4 neural codes co-exist?
7
8
Responses of each of 2 bundles of auditory
nerve fibers connected to 2 different positions
along the basilar membrane
Four general classes of sounds:
•
•
•
•
3/30/2006
(1) low freq, low intensity
(2) low freq, high intensity
(3) high freq, low intensity
(4) high freq, high intensity
• Each bundle corresponds to a critical band.
• Each sound evokes a different pattern of firing in
the auditory nerve
– Pitch
• determined by place code (which are firing) and by temporal
code (fire in bursts that phase lock to stimulus frequency)
– Loudness
• determined by firing rate (more spikes per burst for louder
sound) an by number of neurons (at high intensity, get some
spikes from both positions)
3/30/2006
9
3/30/2006
Critical Bands
The Central Auditory System
• Critical bands correspond to a pooling along
the basilar membrane
– the width in terms of frequency corresponds to
an estimate of the physical length, along the
membrane, over which auditory nerve signals
are pooled
3/30/2006
10
11
• Each auditory
nerve sends
information to
the cochlear
nucleus.
• From there,
projections
diverge to many
different
pathways.
3/30/2006
12
2
The Central
Auditory System
Each set of auditory pathways has a
specialized function
• There are many
parallel pathways
in the auditory
brainstem.
• The binaural
system receives
input from both
ears.
• The monaural
system receives
input from one ear
only.
3/30/2006
13
How does your auditory system keep
track of all the auditory events in the
environment (what caused the
sounds, where, how many)?
• For example, when you are listening to
music you can listen analytically (breaking
things down) or holistically.
3/30/2006
14
Spatial localization of sounds
• Interaural intensity differences:
– If a sound is played at a position off to the right
side, sound intensities will be slightly different
in the two ears
• 1) paths are of different length because sound has to
travel past the head to get to the left ear and sound
intensity decreases with distance (1 over the square
of the distance)
• 2) head interferes with the sound-wave, casting the
auditory equivalent of a shadow on the far ear
• Interaural time differences:
– Sound from the right arrives at right ear first
because its closer
3/30/2006
15
3/30/2006
16
3/30/2006
18
How does auditory system respond
selectively to such short timing
differences?
• Lloyd Jeffries, a psychophysicist, proposed a
theory in the early 1950s that MSO neurons act as
coincidence detectors.
• Coincident spikes arriving from the two ears
evoke a response in the MSO neuron.
• Inputs from the two ears are delayed by various
amounts relative to one another by the relative
length of the axons.
• For this to work, the timing of individual spikes
must be very precise, and it is, at least for low
frequency tones because of phase locking.
3/30/2006
17
3
Each MSO neuron is tuned to a specific ITD
• This neuron responds best at an ITD of zero and less
well at progressively greater ITDs.
• It responds again when the two sounds are an entire
cycle out of phase.
3/30/2006
19
Articulators + Source Energy =
Speech Sounds
• Speech:
– Vocal cords open slowly and close quickly
– Airflow pulses to produce a buzz (a waveform with a
characteristic period and, hence, frequency)
– The complex buzz can be decomposed into constituent
sinusoidal frequencies (each an integer multiple of the
original—fundamental—frequency)
– Original source spectrum filtered by vocal tract; precise
effect depends on articulator position
3/30/2006
20
Vocal Folds
3/30/2006
21
3/30/2006
22
How do we look at speech?
• Fourier transformations create
spectrograms
3/30/2006
23
4