Bat Echolocation and Sound Wave Principles

• Bats calculate distance and speed of their prey using echolocation

Key Concepts

• Understanding the physics of sound is crucial for echolocation
• Bats need to determine a moving moth's location
• Bats can be categorized as FM or CF based on their echolocation calls
• The mammalian auditory system processes sound
• Neurons calculate range and velocity

Overview

• A bat is similar to a baseball player, it makes calculations to intercept an object moving at high speed
• Bats hunt insects that are small and quick
• Echolocation (or biosonar) is a navigation system used by most bat species to locate objects by using sound waves

Sound

• Sound is how pressure waves are perceived through air
• Sound waves are alternating densities of gas molecules
• Vocal chords or a loudspeaker create air pressure variations
• These pressure differences vibrate the tympanic membrane in the inner ear
• Sound waves are characterized by amplitude and frequency (pitch)
• The amplitude of sound is the amount of air density change.
• The larger the density change, the greater the amplitude

Period and Frequency

• Period is the time between compressed air peaks
• Frequency is the inverse of period (1/period)
• Frequency is measured in cycles per second or Hertz (Hz)
• Complex sounds result from multiple sine waves with different amplitudes, frequencies, and phases
• Information is encoded in sound in multiple ways like Constant Frequency (CF), Amplitude Modulated (AM), and Frequency Modulated (FM) signalling
• CF signalling produces a constant tone at a specific frequency and harmonics
• Harmonics are waves with a frequency that is a positive integer multiple of the original wave, which is the fundamental frequency
• AM signals send info by amplitude variations over time
• FM signals send info by frequency change over time

Bat Calls

• Bats use constant frequency (CF), frequency modulated (FM), or both when hunting
• The Big Brown Bat (Eptesicus) mostly uses FM for echolocation
• The Horseshoe Bat (Rhinolophus) uses a combination of FM and CF
• The fruit bat Rousettus uses a rapid FM call that is more of a click
• Eptesicus and Rhinolophus calls feature harmonics, resulting from resonant properties like vocal chords and the bat's head
• While hunting, the call interval increases as the bat approaches the prey
• The bat tracks and captures the prey at the terminal stage
• Sonograms determine frequency and time
• As the bat tracks its prey, the interval between calls increases
• Eptesicus produces harmonics unlike Rhinolophus

Pulse-echo sensitivity

• An experiment can be set up with a hungry bat on a perch in front of sound reflective platforms
• Using microphones and speakers, simulated targets can be controlled
• Bats can sense a 20 nanosecond timing difference, which is a 0.1 mm detection resolution

Catching a Moth

• Distance (range) is calculated by the time difference between the call and the echo
• The formula is: Target Range = (speed of sound x pulse-echo delay) / 2
• Azimuth is calculated from binaural cues, including interaural time difference (ITD) and level difference (ILD)
• Elevation is determined by a) moving ears and comparing positions and b) the influence of the pinnae on sound patterns
• Prey size is calculated from echo amplitude and delay
• A small amplitude echo indicates a small moth, assuming a given distance
• A large amplitude echo with a long delay indicates a large prey

Doppler effect

• The Doppler effect, is how bats determine prey direction
• The Doppler effect exists when the wave's source moves relative to an observer
• There's an upward frequency shift if the source approaches and a downward shift if it recedes
• The echo from an approaching insect shifts to higher frequencies and an insect moving away shifts towards lower frequencies
• Stationary objects have no frequency change
• The formula to calculate doppler shift: fe = f (1 + 2 x flight speed / speed of sound)
  • fe = frequency of the echo
  • f = frequency of the call
• Flutter from the insect helps identify the object as prey.

Hunting Environments

• Bats using FM/Click sounds live in open areas
• CF/FM bats live in environments with lots of foliage
• FM/Click calls are good for determining range
• CF calls work well for assessing velocity and flutter
• The bat can detect return signal frequency differences, a feat not possible with FM or click
• Depending on the environment, harmonics may have a stronger return
• Bats can also hear their own harmonics, which are weak to other bats

CF and FM Combinations

• Using CF and FM together solves the "clutter problem"
• It helps distinguish prey from the surrounding vegetation
• Harmonics can provide more information with lower harmonics for distant prey and higher harmonics providing detail for close prey
• The flutter, of the prey can helps diferentiate among insect species.

The Auditory System

• Tympanic auditory systems exist in amphibians, reptiles and avian ancestors, and in mammals
• In mammals sound waves, enter the outer ear, pass travel through a narrow ear canal, and lead to the eardrum
• Incoming sound waves vibrate the eardrum, sending vibrations to three middle ear bones called the malleus, incus, and stapes
• These bones amplify sound vibrations and send them to the cochlea, a fluid-filled snail-shaped structure
• The basilar membrane separates the cochlea into upper and lower parts
• Vibrations trigger fluid ripples inside the cochlea, creating a traveling wave along the basilar membrane
• Hair cells (sensory cells) sit on top of the basilar membrane and ride the wave
• Hair cells near the wide end of the cochlea detect higher-pitched sounds
• Those closer to the center detect lower-pitched sounds
• Bat sensitivity ranges from 100 kHz to 10 kHz
• Microscopic hair-like projections (stereocilia) on the hair cells bend against an overlying structure and bend
• Bending opens pore-like channels at the tips of the stereocilia
• Chemicals rush into them cells, and create an electrical signal
• Hair cells send spikes to auditory neurons, then project to the cochlear nucleus and auditory cortex

Tonotopic Mapping

• Auditory neurons in mustached or horseshoe bats fire more in response to certain frequencies
• Little Brown bats use only FM calls and their auditory neurons are not "tuned" to specific frequencies
• Tuning curves are determined by neurons from mustached bats
• Very steep curves occur for 5 neurons around 61 kHz

Auditory Circuits

• Neurons from the basilar membrane to the cochlear nucleus are tonotopically mapped
• FM neurons project to the inferior colliculus
• Two sets of neurons project to the medial geniculate body (MGB)
• FM1 neurons (call or pulse) project with a delay, and FM2-FM4 (echo) are tuned neurons that project to the MGB with little delay
• The difference in time for FM₁ and the other FM signals reaching the MGB determines calculating range

Delay Sensitivity

• Pulse echo delay sensitive neurons are found in the MGB, which fires when there's a specific delay between pulse FM1 and echo FMx
• The IC has two cell groups which project to the MGB which then project to the FM/FM area of auditory cortex (AC)
• The MGB and AC only fire with certain pulse-echo intervals.

Velocity

• Velocity is calculated based on the Doppler shift
• Three channels of sound are analyzed in parallel (CF1, CF2, and CF3)
• The CF2 channel pulse and echo have the largest amplitude
• Velocity is acheived via the summation of two inputs
• One single pulse (CF1) or echo (CF3) doesn't fire MGB neurons
• Velocity-sensitive MGB neurons come from two CF inputs via spatial summation
• MGB neurons project tonotopically to the (CF-CF area) or range (FM-FM area)

Doppler compensation

• FM bats usually forage in open spaces
• CF/FM bats forage where foliage is dense and have evolved clutter-reducing mechanisms
• Part of the CF2 channel goes directly to the auditory cortex Doppler Shift CF processing area (DSCF)
• This region has frequency versus amplitude coordinates and represents frequencies between the CF2 resting point (~61 Hz) and 1 kHz above it
• Neurons in the DCSF region have narrow frequency tuning, centered around the dominant harmonic of bat calls (CF2), called an acoustic fovea
• CF bats' auditory systems are tuned to narrowband frequencies in the calls they produce
• The narrow tuning curve comes from lateral inhibition
• The activated neuron reduces the activity of its neighbors, which disables the spreading of signals from excited neurons

Auditory Cortex

• The auditory cortex has five subdivisions.
• The anterior and posterior parts primary cortex are tonotopically mapped and respond to a range of frequencies from 10 to 100 kHz
• The DSCF region is over-represented from 60 to 63 kHz
• The dorsal-medial area has azimuth and elevation sensitive neurons
• Their sensitivity comes relative interaural differences
• Range is calculated within the FM-FM area and the pulse-echo difference, mapped tonotopically from 1-20 ms, determines distance to the prey
• The CF/CF area has neurons sensitive to primary call frequency (CF1) relative to harmonics return (CF2/CF3) mapping differences in velocity
• The DSCF represents an "acoustic fovea" allowing for Doppler shift analysis
• This helps precise Doppler shift from flutter, distinguishing prey from plants

Explore how sine waves form sounds, focusing on amplitude modulation and harmonics. The lesson delves into bat echolocation, examining call types, hunting behaviors, and the use of sonograms. It also covers pulse-echo sensitivity, range calculation methods, and the function of delay-tuned and FM1 neurons.