Lecture 4: Muessel - Echolocation in Bats Flashcards
Why do bats emit a sonar?
In order to form an image of their environment as the echo returns with distortions based on the environment
- Useful where there is emitted light
How accurate is echolocation?
Very accurate- can detect positions of mesh 80um thick
Used for prey pursuit and capture
Describe some of the peripheral mechanisms possessed by the bat?
A tragus in the ears to provide elevation information
Enlarged ears
Nose leaves which funnels sounds produced by nostrils
- Morphology varies between species
How is the sonar emitted?
Sound funnelled into a cone
Nose leaf controls the width (azimuth)
Precise vocals
How is the sonar received
Selective to different frequencies from different directions
Different ear shape –> different selectivity
How do bats filter the noise?
Pinnae of ear collects and funnels sound
Sound information is converted into pressure waves by vibration of ear drum at equal frequencies
Information carried to cochlea
Vibrates along basilar membrane
What information can be generated from echos?
Distance of object
Angular size + amplitude - loudness (size of producer)
Direction
- elevation, provided by tragus
- azimuth (anywhere in 60 degrees in front), calculated using intensity differences
What are the two types of ultrasound calls?
Frequency Modulated (FM) and Constant Frequency (CF)
Describe Frequency Modulated (FM) calls?
Calls start at a high frequency and end at a low frequency (calls sweeps through a range)
Short in duration (
Describe Constant Frequency (CF) calls?
Long duration calls (5-30ms) which are emitted as a pulse at the same frequency until a FM sweep at the end
How do calls change with distance to target
Closer to target = increased repetition
What are harmonics?
Integer multiplications of the calls fundamental frequency
Fundamental = 25Hz Harmonics = 50, 75, 100
- This is not seen as different notes just decreasing sound quality
- 2nd and 3rd harmonic have more energy than fundamental (1st harmonic call)
What is the benefits of FM calls?
Able to detect the distance of the target?
- Use each echo from the different frequencies
- Good for short range (hence end of CF)
What are the benefits of CF calls?
Detecting: velocity of the target, relative velocity, flutter or wing beat
Sensitive analysis as any distortion is due to environment
Increased range of call
Produced around acoustic fovea
What is Doppler Shift?
Detecting how the target is moving relative to the reciever
- Coming towards = higher frequencies
- Moving away = lower frequencies
What is the benefit of being able to detect flutter or wing beat?
Causes a modulation in amplitude and/or frequency due to change in wing angle
Able to identify species- predator or prey
What is an acoustic fovea?
Area in the basilar membrane where it is most sensitive due to increase concentration of neurons.
CF component doesnt match –> avoid deafening
Adaptations of call (Doppler shift) ensures echos return in fovea
What differences are there in ecology between the different types of echolocation?
Exclusive FM bats: take prey from surfraces, use high frequencies to avoid interference, can distinguish prey up to 1 cm accuracy.
Predominantely CF calls: forage in open space or ambush from a perch, use low frequency at a distance and change to FM in pursuit
Explain the neural pathways involved in echolocation
Ear –> Auditory Nerve –> Cochlear nucleus (Basilar membrane) –> midbrain inferior colliculus –> forebrain auditory cortex
What is the purpose of the basilar membrane?
Neuronal processing
Vibration of the ear drum causes a stimulation of hair cells which then cause an excitation of the auditory nerve
Frequency is encoded by position along membrane
What link does basilar membrane have to acoustic fovea?
Overexpression of receptors at the acoustic fovea in order to ensure maximal vibration.
Area likely to be thickened and lengthened.
What are the three important areas in the auditory cortex?
FM-FM area
CF-CF area
DSCF area
What is the purpose of the FM-FM area?
compares the fundamental call (FM1) to higher harmonic (FM2). Compares delays between call and echo
Cells are systematically arranged with parameters mapped
What is the purpose of the CF-CF area?
Comparing CF1 to Echo in combination to analyse velocity information
What is the purpose of the DSCF area?
Analyses delays due to wing beat and flutter.
Makes up 30% of auditory cortex
Involves maps: frequency radial, amplitude in columns
What are 2 problems with echolocation in bats?
Distinguishing cross talk
- calls and echo have no relevance without the other
- fundamental harmonic is heard through skull tissue
Not to deafen each other:
- sensitivity is reduced when call is preduced
- FM species contract inner ear, CF species emit call outside of acoustic fovea.
