Bat Echolocation: Sounds and Hearing - PDF

How do we “see” with our ears? Auditory scene analysis Bat echolocation Brazilian free tail bats leaving Bracken Cave  A bat eats a mealworm tossed in the air MW3 is scooped up by the bat’s tail (B3) MW3 Me...

How do we “see” with our ears? Auditory scene analysis Bat echolocation Brazilian free tail bats leaving Bracken Cave  A bat eats a mealworm tossed in the air MW3 is scooped up by the bat’s tail (B3) MW3 Mealworm (MW) tossed into the air Bat Sonar: echolocation Stages of pursuit and capture Inter-call interval decreases as bat nears prey What are sound waves?  Sound Perception of pressure waves of air – alternating compression and thinning (rarefaction) of air Compression propagates – Analogous to waves generated by throwing a rock into a pond Sound Waves http://www.mediacollege.com/audio/01/sound-waves.html  Sound Waves Period = time between compression peaks Frequency (pitch) = 1 / period – Measured in hertz (hz)  Sound Waves Amplitude (loudness) = amount of compression and thinning – Size of change in density of air molecules amplitude  Sine Waves High frequency Shorter wavelength (l) Low frequency Longer wavelength (l) wavelength f=1/l Amplitude Sounds are Multiple Sine Waves Phase Phase = time between peaks of different sine waves Perceivable frequencies 20 Hz – 20 kHz  AM, FM, and CF signals Constant Frequency Time  Bat sonogram: ultrasonic signals FM: frequency modulation CF: constant frequency  Bat sonogram: ultrasonic signals Note harmonics & Increased call rate (house bat) (horseshoe bat) FM: frequency modulation CF: constant frequency  Experiment: How do we know bats use sonar? Bats trained to choose target Play back chirps with varying delay calls don’t reflect “Fool” the bat by altering pulse-echo delay Information needed to catch a moth 1. Distance (range) 2. Relative velocity 3. Size of the moth 4. Azimuth (left/right on horizon) 5. Elevation Information from the echo: 1. Loudness 2. Delay 3. Direction 4. Doppler shift Distance (range): determined from echo delay  CF-FM versus FM bats FM/Click good for range, CF good for velocity and flutter  Harmonics Note: CF duration decreases First harmonic may not be strongest  Harmonics Distant prey: lower harmonics stronger - less attenuation Close prey: higher harmonics stronger - finer detail Bat hears its own first harmonic (weak to other bats) Auditory system overview Ossicles Auditory Nerve Cochlea Tympanic Membrane Auditory system overview Tonotopic organization: a. basilar membrane b. cochlear nucleus c. auditory cortex Resonant Hair Cells Auditory system overview Auditory cortex Thalamus (MGN) Inferior colliculus Cochlear nucleus Cochlea Auditory neurons: Frequency Sensitivity Note that the little brown bat (FM) has broad frequency sensitivity Frequency Sensitivity CF2 harmonic Mustached bat (CF-FM): Different auditory neurons are tuned to different frequencies Calculating Range: Pulse/echo time difference  Determining Range: FM/FM time difference Integration site IC: Two types of cells FM1 = pulse FMx = echo Cochlea  Delay-sensitive neurons in MGB* Pulse echo delay sensitive neurons found in MGB - Fire with specific delay between pulse FM1 and echo FMx Arise from two groups of cells in IC - Pulse FM1 & Echo FMx FM1 response in MGB later than echo harmonics MGB neurons project to FM/FM area of auditory cortex (map delays) IC FM1 AC MGB FM3 *Also found in IC  Conduction Times MGB 1 ms FMx A 1.3 ms FM1 MGB 1 ms FMx B 4 ms FM1 MGB 1 ms FMx C 15 ms FM1 Spatial-temporal summation A B A+B Pulse/Echo Coincidence detection P + Echo Echo P E E P Threshold Pulse  Pulse/Echo Coincidence detection Mechanisms that delay the contribution of the pulse to summation Echo Echo Echo MGB AC (-) Pulse Pulse Pulse 1. Delay line 2. Differential 3. Inhibitory rebound Conduction Velocity Auditory system overview Auditory cortex Thalamus (MGN) Inferior colliculus Cochlear nucleus Cochlea Auditory Cortex FM-FM cells Threshold sound pressure Lateral inhibition sharpens sensitivity  Prey movement and flutter Doppler shift: Change in frequency of a wave for an observer moving relative to the source of the wave Velocity detection Doppler-shifted CF signals  Calculation of target velocity IC CF1 MGB AC CF3 Velocity-sensitive MGB neurons arise from two CF neurons For example, CF1 + CF3  Calculation of target velocity IC MGB MGB Firing Rate CF1 CF1 and CFX CF3 CF inputs CF1 or CFX alone Single pulse or echo alone can’t fire MGB neurons. Velocity-sensitive MGB neurons arise from integration of two CF inputs For example, pulse CF1 + echo CF3  IC 30 kHz MGB CF1 A 60.2 kHz CF2 30 kHz MGB CF1 B 61.8 kHz CF2 Auditory Cortex CF1/CF2 cell Threshold sound pressure CF1 CF2 Frequency/threshold ranges required for firing CF1/CF2 cell  Chapter 4: Bat echolocation 1. Bat auditory system – inner ear – C to CN to IC to MGB to Cortex – tonotopic maps 2. Computing Range a. FM/FM b. Pulse/echo summation in MGB – delay line, conduction velocity, rebound inhibition 3.Computing Relative velocity a. CF1/CF2 or CF1/CF3 b. Summation in MGB