Transcript Bat Echolocation - (canvas.brown.edu).

By Jesse Gumbiner
September 19 th , 2013
Source: Chapter 2 of Behavioral
Neurobiology by Thomas J. Carew
 Mammals, one of the most diverse orders
 Two categories:
 Megachiropta – Large bats with big eyes, small ears, and
usually no echolocation
 Microchiropta – Smaller bats with poor eyesight and
large ears for echolocation
 Varied climates
 Temperate bats’ diet consists of mainly insects
 Primarily nocturnal
 Lazzaro Spallazani (1794)
 Found that bats could navigate fine when blindfolded
but could not when their ears were plugged
 Conclusion: Bats use hearing to detect objects/movement
 Donald Griffin (1938)
 Discovered high-energy ultrasonic pulses emitted from
flying bats
 Called it echolocation
 Noted that pulses were faster when closer to objects
 Also inhibited when mouths were covered
 Tested discriminatory ability: Worms vs. Plastic disks
 High-pitched sounds emitted by bat, echoes used to
detect objects and movement
 Two kinds of ultrasound signals:
 FM (Frequency-Modulated) Sweep
 Also known as Broadband Signal
 Extremely short pulse (<5ms)
 Across wide range of frequencies (100Hz-25KHz)
 CF (Constant-Frequency) Pulse
 Longer in duration (5-30ms)
 Third type uses combo of the two, CF-FM pulses
 Distance: Measured by delay between pulse and echo
 Usually FM pulses used to sweep broadly across a wide
frequency range and deliver a precise distance
 Simmons: recorded bat cries and played them back to
bat at different delay times
 Bats can discriminate delays as minor as 60 nanoseconds
 Distances of 10-15mm
 Subtended Angle (angular size)
 Determined by loudness of the echo
 Helps bat figure out absolute size of objects
 Absolute Size
 Computed from distance (delay) and angle (volume)
 Ex: small amplitude, short delay = small close object
 Azimuth
 Uses binaural cues in brain
 Elevation
 Bats can move their ears, compare echo amplitudes with
ears in different positions
 Velocity
 Doppler shift
 Crucial for hunting moving prey, must determine both
absolute speed and relative speed of prey
 Frequency of a sound changes pitch depending on
movement
 Ex: train moving past you, higher as it approaches you,
lower as it leaves
 Bat’s echo returns at a higher frequency than it emits =
bat getting closer to target
 Bat’s echo returns at lower frequency than call = target
getting further away
 CF signals used because of ongoing analysis
 Acoustic Fovea and Doppler Shift Concentration
 Extreme sensitivity to sounds at specific frequency of CF
pulses
 Ex: Rhinolophus bat extremely sensitive to 83KHz
(frequency of it’s CF call)
 However, returning echo will not be 83KHz because of
doppler shift
 Constantly adjusts frequency of call while flying so that
returning echo is always 83KHz
 Also helps keep bat’s call outside sensitive range so it’s not as
loud as the echoes that return, reducing noise
 Flutter = movement of prey’s wings
 Acoustic glint (strong echo) returned when wing is at
right angle, weaker echo when it’s at a different angle
 Allows bat to sense the flutter of wings during an
extended CF pulse
 Extremely subtle: can
discriminate wingbeat
speeds of 35/s
 Simmons showed that bats could detect jitters with
echo-delay changes as small as 10 nanoseconds (a few
millionths of a second)
 Distances of 2 micrometers
 Way more precision than needed for simple distances
 Hypothesized to be used for scanning acoustic texture of
objects, “feeling” physical characteristics of an object
 Can sense shape and form
 Search stage
 Bat hangs motionless emitting low-repetition pulses
 In dense areas, uses FM In open areas uses CF
 Can sense any movement within 5 meters of it
 Approx. 10 pulses per second (slow)
 Approach stage
 Bat takes off after target, and increases its pulses
 50 pulses per second
 FM pulses used to increase range and help with flight
 Terminal stage
 Bat closes in and captures prey
 Brief sudden increase to 100-200 pulses per second
 Necessary because of closing distance, need to detect minute
variations
 Basilar Membrane and hair cells like humans
 Bats that use CF pulses (Doppler) have a specialized
thickening of the basilar membrane at the place where
their echo’s frequency will be processed
 Frequency depends on the species
 Correlates with Acoustic Fovea
 Even at first stage of auditory processing, Bat is
amplifying echoes for analysis
 Also includes more neurons responding b/c of larger
basilar membrane area
 Bats emitted call is many times louder than the echoes
but it must sense the echo, not the call
 FM bats use two strategies
 Contract inner ear muscles briefly (5-10ms) during call
 Higher auditory neurons stop auditory signals from call
 CF bats need to overlap calls with echoes though
 Remember: CF bats emit different frequencies than
echoes
 Ear is enhanced to hear sounds at frequency of their
echoes, relatively deaf to lower frequency of call
 CF-FM bats use combo of attenuation strategies
 Inferior colliculus
 Can sense small temporal differences in auditory input
 Different neurons have different responses to delay
between call and echo
 Highly tuned to only one frequency with FM sweep
 Low threshold for firing action potential
 Critical for determining distance
 High number of neurons attuned to CF frequency
 Continues enhancement of echoes in midbrain as well as
inner ear
 Each call has up to
three harmonics
 Three combos:
H1-H2
H1-H3
H1-H4
 FM-FM Area (Distance coding)
 Neurons only respond to an FM call followed by its echo
 Each neuron specializes in particular delay length
 Different neurons for each call-to-harmonic relationship
 FM-FM neurons organized in columns by delay time
 CF-CF Area (Velocity coding)
 CF1-CF2 and CF1-CF3
 Responded extremely well to combination of call
frequency and one harmonic frequency
 Did not respond to call or echo alone
 Dual frequency coordinate system to map velocity
 Occupies 30% of auditory cortex
 Codes for Doppler shifted CF signal frequency
 Auditory cortex personalized for each bat
 Organized in columns as well, considered essential for
frequency discrimination and ability to process minute
physical attributes
 Why do bats emit harmonics when a single frequency
would suffice for echolocation? One theory:
 Bats have to deal not only with prey, but with other bats
in their colony (air traffic control)
 Bats use first harmonic (so quiet only they can hear it)
 Cortex only stimulated by combination of harmonics
 Harmonic is amplified for that bat but not others
 Dual password for activation of neurons (H1-H2, not just H1)
 Side note: Moths have coevolved to hear ultrasonic bat
calls
 Some have even adapted to emit their own calls
 Other species use methods besides vision as their
primary perception
 Bats perceive the world in incredible detail through a
combination of actions (calls) and sensations (echoes)
 Possible evidence for ecological approach?
 Questions?