Barn Owl Auditory Localization Experiment

Barn Owl Auditory Prey Localization Overview

• Barn owls use sound to locate prey, even in complete darkness
• This ability relies on precise auditory processing and morphological adaptations

Auditory Scene Analysis

• The auditory system extracts relevant physical cues from complex soundscapes to identify sound sources
• Sound localization is a fundamental aspect of auditory sensing, enabling determination of a sound's origin

Barn Owl (Tyto alba)

• Nocturnal hunters, barn owls, depend exclusively on acoustic cues to find prey in darkness
• Barn owls use binaural acoustic information to hear rustling noises created by small animals and quickly find the source of the sounds
• Barn owls have a relatively narrow hearing range, 0.2 to 12 kHz, but greater sound sensitivity that is similar to a cat but greater than humans
• Optimal frequency range for sound thresholds range is between 4 and 8 kHz

Interaural Time and Level Difference

• The brain compares the amplitude and timing of sound waves when reaching each ear to calculate location
• Interaural Level Difference (ILD) measures the comparative loudness of a sound between ears
• Interaural Time Difference (ITD) measures the difference in arrival time of sound waves between ears
• Azimuth describes locations across the horizontal plane
• Elevation describes locations across the vertical plane
• Elevation in humans and other animals is determined by differences in spectral waveform
• Sounds from different elevation angles will be spectrally altered as they interact with the pinna

Morphological Adaptations

• The barn owl's specialized auditory capabilities are a result of morphology and neuronal adaptations
• The owl's facial disc is a circular paraboloid that collects and directs sound waves into its ears
• The facial ruff acts as a sound collector and amplifier, helping to direct incoming sound to the ear openings and enhance directionality
• The feathers are adjustable, changing the focal length of the sound collector for locating prey hidden under the plant or snow cover
• Auricular feathers are sound transparent with a protective function
• Reflector feathers reflect sound towards the ear openings
• The right ear is more sensitive to sounds from above, while the left ear is more sensitive to sounds from below

Head Rotation Experiments

• Owls can rotate their heads 260 degrees due to high degree of oclulory mobility
• Head rotation moves in the direction of the perceived sound
• Scientists measure owl head rotation in response to sound stimulus by placing a detection coil on the head, and placing the owl on a platform with an induction coil
• A current is generates as the head turns and is relative to the position of the head
• Presenting sounds at varying elevations and azimuths can determine where the owl orients its head, using a second speaker as a reference
• Increases or decreases in sound level affect the owl's attention, testing how it perceives direction
• Soft earplugs minimally reduce sound, while hard plugs reduce more sound, comparing head location to the actual sound source allows perception of sound to be measured
• Plugging one ear causes the perceived source to shift away from the plugged ear
• Interaural level difference (ILD) primarily determines perceived elevation
• Interaural time difference (ITD) primarily determines perceived azimuth
• The localization of elevation is frequency-dependent; horizontal alignment yields no elevation difference at low frequencies; vertical alignment with facial ruff allows both ears to reflect high frequency sounds

Detecting Azimuth: ITD

• Crests of sound waves reach closer ear
• Sound travels at 340 m/second, the distance between owl ears is 0.1 meter, so a maximum arrival time difference is 2.9x10^-4 seconds, or 290 microseconds
• For sounds in range of detection (~3kHz), the owl detects time differences of 0.3 milliseconds
• Binaural detection occurs in two ways
  • Disparities with onset/offset times are less likely to be confused with echoes
  • Ongoing disparities in soundwaves allow for repeated measures and greater precision
• Ongoing temporal disparity is adjusted experimentally by manipulating the phase of incoming sound, which causes owl to move left or right

ILD and ITD

• Spatial map determined by ILD and IDT accounts for azimuth and elevatation
• A brief sound registers in the left ear, then the mouse is positioned left and below

Bird Brain Auditory Circuitry

• The Inferior Colliculus (IC) is a main processor through which nearly all auditory info passes
• The avian homolog for IC is mesencephalicus lateralis dorsolis (MLd)
• Focus is on avian brain anatomy and function
• The nucleus magnocellularis (NM) and angular nucleus (NA) receive auditory information along the 8th cranial nerve
• Two pathways from these nuclei carry to the IC pathways
• The pathways are bilateral and transmit information to the premotor cortex

ILD Pathway (Elevation)

• The ILD pathway is used in an extremely simplified perspective
• Sound intensity reported by firing rates of NA neurons to posterior portion of the dorsal nucleus of the lateral lemniscus (LLDp), and then into the core inferior colliculus (IC)
• Reciprocal inhibition occurs between both LLDp regions
• The amount of LLDp cell will be altered by amount of inhibitory synapse

ITD Pathway (Azimuth)

• Eighth nerve auditory information innervates nucleus magnocellularis (NM), which projects to nucleus laminaris (NL) along the interaural time difference (ITD) pathway
• A temporal cross-correlation function can be used to find how sound relates to timing
• The Jeffress Model has two sets of stimulus locked spike train signals
• Model has suggested that a systematic spatial delay exists with coincidence detectors
• Sensory info travels from hair cells from the ear and sends signals ipsilaterally from the nucleus magnocellularis (NM)
• Nucleus laminaris contains coincidence detections that receive auditory input from the left or right ear
• Sounds from the azimuth corresponds to stimulation from the nucleus laminaris
• A neural map of auditory space is formed

ITD Tuning

• There is evidence that neurons in the nucleus laminaris form a neural map
• An electrode guided through nucleus laminaris varying in distance measures firing rate of neuron as a ITD function
• Neurons dorsally that are tuned towards ITD fire with sounds from the left, ventrally neurons fire with sounds from the right
• Different ITD are sensitive to neurons

Challenges to Jeffress Model

• It was computationally tested and there was a problem with it
• Where the contralateral pathway spikes will not be received at the same time

Temporal Disparity Adjustment

• Ipsilateral and contralateral pathways need to synchronize at the same time
  • If ipsilateral is slower (thinner or internodal distance) and contralateral pathway is faster, spikes can arrive at the same time

Additional Challenge to Jeffress Model

• Neurons cannot function at such a high rate
  • Individual auditory neurons lack ability to follow high frequency
  • phase/time locking is where neuron fires in sync with stimulus phase

Spatial Maps

• Electrodes are commonly implanted into the IC to test the correlation between auditory signal and what the owl hears
• Head direction is monitored while doing this test
• Specific neurons will respond to certain signals

Receptive Fields

• Responses of a neuron along the horizontal (azimuth) and elevation planes
• A receptive field is the spatial region where a neuron fires above the background noise
• After testing these locations we can map the area
• The space is usually ovoid
• Sounds outside that reduce internal firing which allows the owl to focus on the sound

Visual Displacement Plasticity

• IC projects to the external IC (ICX), which links to the optic tectum (OT), which processes primary visuals for fish, reptiles, and birds
• There is a strong connection between how auditory is interpreted by the brain and the visuals/environment
• Owls with visual input offsets will move their head in response to the change
• Owls with that offset after a month had visuals on par with auditory
• There is plasticity in OT connections
• OT neurons have horizontal shift in the visual field

This quiz focuses on the auditory localization mechanisms in barn owls. It explores how owls use interaural time differences and other cues to locate sound sources. Key aspects include the role of earplugs, speaker positioning, and the owl's facial ruff.