sound localisation in barn owls Flashcards
owls must be able to locate prey accurately in both the…
horizontal plane (azimuth)
vertical plane (elevation)
behavioural experiments demonstrate that sound is the cue used during hunting…
owl placed on a stand, a target speaker moves along a track (azimuth) ad up and down (elevation) - the experimenter can choose where the sound is coming from
magnetic coils track head movement
accuracy -
1-2 degrees in both the azimuth and elevation
needs both ears
sensitivity -
most sensitive to sound coming from the front
most sensitive to high frequencies
if one ear is plugged (decrease sound intensity in one ear)…
when left ear plugged = owl struck above target
when right ear plugged = owl struck below target
when a hard plug was used to prevent all sound the owl struck further from the target
the facial ruff:
articular feathers = acoustically translucent - let sound pass through to reflector feathers
reflector feathers = collect sound - send it to ears
asymmetrical trough - right side points up to collect sound above the horizon and vice versa
ears are arranged asymmetrically:
right ear is slightly below the plane of eyes = focused upwards - collects sounds above the plane of the horizon
left ear is slightly above the plane of eyes = focused downwards - collects sounds blow the plane of the horizon
inter-aural intensity differences between ears are used to…
determine the elevation of a sound source
inter-aural time differences between ears are used to…
determine the azimuth of a sound source
- onset/offset disparity - sound from right heard in right ear first, time delay before it is heard by the left ear
- ongoing disparity - differences in sound patterns are analysed = determines the azimuth
in experiments - elimination of ongoing disparity meant owl couldn’t find target
how does owl achieve analysis of inter-aural intensity differences and inter-aural time differences…
auditory nerve (VIII Cranial nerve) - frequency analysis occurs in the inner ear by the auditory nerve - each fibre encodes a different frequency
how can sensory neurons in the ear encode timing (azimuth)…
azimuth is encoded by timing of the action potentials = phase locking
- each fibre fires at a particular phase angle of the sinusoidal input signal
- phase locking = if neurones fire faster (increase in loudness of sound) doesn’t influence place of phase locking on the sound wave
how can sensory neurons in the ear encode intensity (elevation)…
elevation is encoded by changing rate of action potentials
the louder the sound the faster the action potentials fire
cues to estimate distance:
signal specific + location specific
signal specific cues to estimate distance -
bird has to know something about the thing making the sound… can detect:
- overall amplitude of direct sound and reverberation
- frequency spectrum of direct sound and reverberation (higher frequencies are attenuated in the atmosphere)
location specific cues to estimate distance -
bird doesn’t need to know anything about the thing making sound but needs to know their environment - get info about distance based on ques in environment (needs both ears to do this type of distance estimation):
- off axis reflection - e.g. noise bouncing off building
- near axis reflection
- elevation of direct sound
How does the brain of the owl take trains of sensory action potentials that signal time and intensity and use these to generate a head movement that is aimed towards the source of the sound?
Two pathways used simultaneously in parallel to decide where the sound is coming from
- time pathway and intensity pathway
Comparisons done to get information
To show that parallel processing is occurring using experiments…
Recording electrode = record from external nucleus of the inferior colliculus and inject with anaesthetic
Magnocellular nucleus effects azimuth
Angular nucleus effects elevation
- When it wore off the owl could detect properly again
When injected magnocellular nucleus with anaesthetic =
can not find target on horizon
- Receives bilateral inputs (compares timing between left and right ears - Inter-aural time differences)
- Azimuth = horizontal plane = encoded by timing of action potentials
when injected into angular nucleus =
couldn’t find the elevation of the target
- Intensity differences between ears are used to compute elevation of sound source - Inter-aural intensity differences
- Elevation = vertical plane = encoded by rate of action potentials
To see how owl’s brain encodes time differences an experiment was done with a coincidence detector:
- sound wave hits right ear first then time delay then left ear, owls brain detects difference - if both inputs from both ears reach coincidence detector at same time = maximum output (but if delay lines were same length inputs wouldn’t reach at same time = weak input)
How would you explain how a neuronal circuit could both measure and encode the inter-aural time differences…
Jeffress’ model = correct model
- if sound source comes from directly in front = noise hit left and right ear simultaneously - all nerve endings fire and if both inputs reach coincidence detector at same time = maximum output
- Delay lines = act to synchronise input signals so that they strongly activate an output neurone (coincidence detector)
Elaboration of Jeffress’ model to permit the conversion of timing info into place info…
- Multiple coincidence detectors - All fibres fire on coincidence detects and delay lines mean that information can reach one of the coincidence detectors at the same time
- Timing information from which coincidence detector fires maximally helps the owl detect where the noise is coming from
- Relay station = magnocellular nucleus
- coincidence detectors= laminar nucleus
Neuronal pathway: Inter-aural time differences (azimuth):
turning timing info into place codes:
‣ Each coincident detector differs in their place in the neuronal circuit
‣ its distinct identity arises from its unique place in the array
‣ its essential that this order be preserved in its projection to higher auditory processing centres: a spatial MAP
Phase ambiguity:
if mice makes noise but then moves one cycle on sound wave away to a different place = cannot differentiate
(Firing at same time but off by one cycle - but still get maximum firing because hitting coincidence detector at same time = confusing for the brain)
neurones in the laminar nucleus…
compare the timing of inputs from the two ears and output a spatial code for azimuth
(coincidence detector)
Neuronal pathway: Inter-aural intensity differences (elevation):
within the posterior lateral lemniscal…
- the ventral neurons fire most strongly when the ear on that is stimulated most loudly
- the dorsal neurons fire when the ear on the other side is stimulated most loudly
the owl auditory system is an example of a computational map -
- Transform the representation of information (sound) into a place code
- Rapid processing: computations are pre-set and executed in parallel
- Simplify connectivity for processing and utilization
- A common mapped representation allows the nervous system to employ a single strategy for reading the information
- Enable fine tuning in a manner not possible for non-topographic coding
frequency convergence:
used to overcome phase ambiguity - many frequencies come in with different sound periods, the period with the strongest summation indicates true delay
