Bat Echolocation Flashcards
Biosonar =
image forming system using reflected sound instead of light
- Active sense: animal emits pulse of sound
- Sound is distorted by the environment and bounces back producing an echo
- Echo is detected and interpreted
- Used for navigation and forage
Behavioural ability -Discovering echolocation:
• Spallanzani & Justine 1794 -
‣ Blinded bats still avoid obstacles
‣ Candle wax in ears: bats crash into obstacles
• Griffin & Pierce 1938 -
‣ Instrument detected high frequency signals
‣ Plug ears or taping mouth prevents navigation
‣ Ultrasound is attenuated rapidly in the atmosphere he postulated it could be used for navigation and prey detection
Peripheral mechanisms:
- some bats have specialised flap-like structures within the pinnae provide additional elevation info
- ears enlarged to detect faint echoes
- elaborate nose leaves to funnel sound in species which call through nostrils
emitting:
‣ Sounds propagates as a sphere
‣ Funnel sound into cone
• Mouth (moustached bat)
• Nose (horseshoe bat)
‣ Width of sound beam altered by altering nose leaf of seba’s short tailed bat
‣ Precise vocal control: motor control of breathing muscles and larynx regulate frequency and patterning
receiving:
‣ pinnea collect and funnel - design dictates which frequencies bat can detect
‣ Amplification: decrease after pinnea removed
Sensory filtering mechanism:
Sound pressure waves:
• outer ear:
◦ Pinnae - collect and funnel - design dictates which frequencies are detected
◦ Ear drum vibrates at same frequency as sound wave
• Middle ear: transmitted via ossicles
• Inner ear:
◦ oval window or cochlea
◦ Cochlea houses the basilar membrane - hair cells
How does the bat know distance of object?
time delay of the echo
How does bat know angular size of object?
amplitude (loudness) of the echos
How does bat know target (absolute) size of object?
combine distance and amplitude
◦ Small amplitude + very short delay = tiny object
◦ small amplitude + very long delay = larger object
How does the bat know the direction of the object?
◦ Elevation:
‣ move ears independently ; compare echo amplitude (like Owls)
‣ tragus provide additional elevation info through complex reflections
◦ Azimuth:
‣ auditory sensitivity 60o cone in front; detection worse than humans.
‣ possibly use intensity difference (not time delay like owls)
How does the bat know the velocity of an object?
Doppler shift analysis of echoes’ frequencies
Behavioural Ability… two types of ultrasonic signals:
• Frequency modulated (FM) sweeps:
◦ Broadband - sweep high to low over wide ranges of frequencies
◦ Short pulse < 5ms
• Constant frequency (CF):
◦ single frequency component
◦ long pulses of 5-30ms
many bats use a combination of FM and CF pulses
As bats approach an object they…
increase repetition rate and decrease duration
Harmonics:
there are harmonics in the calls in addition to the fundamental frequency
◦ Perceived not as separate notes but rather as the quality of sound
◦ Most energy of the call is in 2nd or 3rd harmonic, not 1st
◦ Extremely important role in prey detection
FM signals:
- distance to target
- Each frequency within sweep provides a single point at which the bat can make a pulse-echo determination
- Broadband = high resolution of time delay
- But energy is spread out: less energy = travels less far
- Effective over short distances
CF signals:
- Target velocity (relative velocity of bat with respect to prey)
- Insect flutter or wing beat
- One frequency, energy more concentrated = propagates further
- Due to Doppler shift
- effective over long distances
Doppler shift:
Alteration of sound frequency when source moves relative to receiver (or other way round!):
• Speed of object advancing adds to speed of sound
(higher frequency = higher pitch)
• Speed of object retreating subtracts from speed of sound
(lower frequency = lower pitch)
Doppler shift allows bats to figure two important parameters of prey:
1) It allows the bat to figure relative velocity
2) Flutter analysis: Detection of wing beat
‣ Produce tiny Doppler shifts as wings move forward and back
‣ Thus frequency and amplitude of echo are modulated as angle of wings change = acoustic glints
‣ Relative oscillation of wings specific to species
When Bat has targeted prey and it emits pulses:
◦ Echo returns at a higher frequency than the emitted call = approaching the target
◦ Echo returns at a lower frequency than the emitted call = going away from the target
Why are CF signals useful for Doppler Shift analysis?
- Long CF pulses (10-100ms) allow sensitive analysis within a single frequency
- CF pulses have more energy = increased range
- CF bats are extremely sensitive to a very narrow range of frequencies around the CF component of their emitted call = acoustic fovea (analogous to the retinal fovea of visual system)
- Due to a higher representation of the CF frequency within the auditory system from cochlea to cortex
- Problem: the CF component of the echo doesn’t match the acoustic fovea
Doppler shift compensation:
- Mechanism to keep echo within acoustic fovea
* The bat adapts the frequency of its call in order to allow the echo to return within its acoustic fovea
Neural pathway:
Ear -> Auditory nerve to hindbrain cochlear nucleus -> Midbrain inferior colliculus -> Forebrain auditory cortex
Midbrain inferior colliculus:
First level of integration -
Interneurons sensitive to specific call-echo delays
Basilar membrane:
◦ first stage of neural processing for sound in bats and there is specialisation for echo analysis
◦ vibrates to sound hitting the eardrum
◦ stimulate hair cells -> excite primary auditory neurons -> CNS
◦ these neurons encode all aspects of sound
◦ Frequency of the sound is encoded by place on the basilar membrane that is maximally vibrated by the sound
◦ Amplitude is encoded by rate of action potentials
Auditory cortex has 3 important areas that each process particular tasks for analysis of calls:
- FM – FM area: distance coding
- CF - CF area: velocity coding
- DSCF area
FM – FM area of the auditory cortex:
Distance coding:
◦ Neurones respond to delay between call and echo in combination
◦ Compare the fundamental with a higher harmonic
◦ Cells arranged systematically, parameters mapped across regions
‣ Each neurone tuned to particular delay and amplitude
CF -CF area of auditory cortex:
Velocity coding:
◦ Neurones respond a CF call and echo in combination: pairing of CF1 call and CF2/CF3 echo
◦ Represent precise Doppler shifts and encode specific velocity
◦ Cells arranged systematically, parameters mapped across regions
DSCF area of auditory cortex:
◦ 30% of auditory cortex volume
◦ Second harmonic Doppler shifted echoes only
How do bats in colonies avoid cross talk?
- Fundamental harmonic less than 1%
- Fundamental harmonic transmitted to bat’s inner ear via skull tissue
- Other bats only hear higher harmonics
- FM-FM and CF-CF need FM1/CF1 plus echoes of higher harmonics
How do bats not deafen themselves? or swamp the echo?
They briefly and dramatically reduce their auditory sensitivity just during the emission of its call
◦ FM species:
‣ Contract inner ear muscles
‣ Further attenuated at the nucleus of the lateral limniscus
◦ CF species: Doppler shift:
‣ Call is lower than echo
‣ Echo kept in fovea by compensation so call falls outside fovea
‣ Call is underrepresented
‣ Bat has low sensitivity to these frequencies.
