1. By Jesse Gumbiner September 19th, 2013
Bat Echolocation
By Jesse Gumbiner
September 19th, 2013
Source: Chapter 2 of Behavioral Neurobiology by Thomas J. Carew

3. Bat Facts 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

4. Early Research Lazzaro Spallazani (1794) Donald Griffin (1938)
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

5. Echolocation
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

6. Sonograms

7. Features of Echolocation
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

8. Features of Ecolocation (Cont.)
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

9. 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

10. Doppler Shift (Cont.) 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

11. Doppler Shift (Cont.)

12. Doppler Shift and Flutter
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

13. Precision
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

14. Three Stages of Hunting
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 pulses per second
Necessary because of closing distance, need to detect minute variations

15. Neural Echolocation: Inner Ear
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

16. Attenuation
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

17. Encoding Time 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

18. Harmonics Each call has up to three harmonics Three combos: H1-H2

19. Auditory Cortex 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

20. DSCF Area 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

21. Harmonics (Again)
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

22. Conclusion
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?