Auditory I Conclusions
Central auditory pathways mediate sound identification.
Neurons are tuned for specific auditory features,
defined by auditory tuning curves and STRFs.
More complex features are represented higher in the pathway.
Auditory neurons are tuned to different aspects of speech.
For human auditory cortex, groups of neurons
(electrodes) respond to subsets of phonemes, intonation,
arguing that different aspects of speech are processed separately
at this stage
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Brain-wide activity when listening to a story
Semantic Content (Meaning) of language is distributed broadly
Jack Gallant, UC Berkeley
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Bat echolocation: Using echoes from
self-generated sounds to find objects
Navigation around obstacles in darkness was studied starting in 1790s. What
sensory signals the bats were using was mysterious.
Active Localization. Localizing objects by emitting energy (sound) that echoes off the
target, and analyzing the echoes to determine distance and direction. Used by bats,
dolphins & whales, oilbirds, cave swiftlet, shrew, catfish.
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Bats are moth catchers
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Bats are moth catchers
Dr. Cynthia Moss, John Hopkins Neural Systems and Behavior Lab
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Sounds produced by bats
CF-FM bats
constant
frequency
(CF) component
frequencymodulated
(FM) sweep
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Echoes contain information about the target
Bats emit calls at ~ 30kHz (CF1)
Bats auditory system
Over-represents CF2 of echo
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Bat cochlea is tuned to returning echo
Each bat species is most sensitive to one frequency band, which usually
corresponds to the second harmonic of the returning echo. This is the acoustic
fovea.
-- expanded cochlear area
-- very sharp frequency tuning by single neurons
-- most sensitive detection
Mustached bat
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CF2 echo representation in the auditory cortex
Detect rapid frequency modulations
= insect flutter
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Echoes contain information about the target
Echo delay = time delay between call and echo
Measure of target distance
echo delay
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Measurement of distance by echo delay
Bat calls are directional
Target distance (m) = c (340 m/s) × t (echo delay, s)
2
1 meter distance = 6 ms pulse-echo delay
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Auditory cortical areas for echolocation
Detects echo delays
Luo, after Suga 1989
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Echo-delay tuning in FM-FM area
Compare FM1 call to FM2 echo
These are combination-tuned
neurons: they respond only to
a specific combination of an
FM1 call, plus an FM2 echo at
a specific delay.
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Echo-delay tuning in FM-FM area
Topographic map of call-echo delay
Different
Delay times
Map of target distance!!!
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Tuning for echo delay in FM-FM area
Hypothetical model for echo delay tuning
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Echo-delay tuning in FM-FM area
Echo delay tuning is synthesized between inferior colliculus and FM-FM area
cochlea
each dot
is a spike
each line
is a spike
cochlear nucleus
inferior colliculus
MGN thalamus
Auditory cortex
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Echoes contain information about the target
Echo delay = time delay between call and echo
Measure of target distance
Doppler shift = frequency shift between call and echo
Measure of relative velocity of target
Doppler shift
echo delay
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Doppler shift and target velocity
f (echo) = f (call) * ( c +/- Bat velocity) / ( c +/- Prey velocity)
+ Bat velocity toward prey
+ Prey velocity is moving away from Bat
c = 340m/s
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Doppler shift and target velocity
Doppler shift in returning frequency
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Auditory cortical areas for echolocation
Detects
Doppler
shift
Auditory fovea
Luo, after Suga 1989
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Doppler shift tuning in CF-CF area
A map of Doppler shift, and target velocity
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Doppler shift tuning in CF-CF area
CF1
CF3
CF1
Target
velocity
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Echolocation:
Brain specializations to detect prey
1. Acoustic fovea expands representation of echo
• Detect fine fluctuations of frequency
2. FM-FM neurons receive emitted FM call and FM echo
• They fire only when there is a specific time delay between the signals
• Delay lines may allow coincident detection of call and echo
• Map of different target distances
• FM signals well-suited for time delay because discrete, separated in space,
and use a sweep of frequencies to compare call and echo
3. CF-CF neurons receive emitted CF call and CF echo
• They fire only for a specific frequency combination of CF call and CF echo
• Echoes have different frequencies because of Doppler effect
moving targets have different frequency than stationary targets
• Map of different target velocities
• CF signals well-suited for frequency changes because compare two frequencies
present for a long time
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Why are there any moths left?
Moths have coevolved with bats, and have developed ears that can hear ultrasound.
Moths can often hear the loud ultrasound pulses
before bats can hear the returning echoes, and react
with escape behavior
Some moths emit ultrasonic sounds to confuse bats
(jam radar)
Humans use ultrasound as insect repellent
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