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

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

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

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

Sonograms

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

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

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

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

Doppler Shift (Cont.)

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

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

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 100-200 pulses per second Necessary because of closing distance, need to detect minute variations

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

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

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

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

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

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

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

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?