sound localisation in barn owls

Question 1

owls must be able to locate prey accurately in both the…

Answer 1

horizontal plane (azimuth)

vertical plane (elevation)

Question 2

behavioural experiments demonstrate that sound is the cue used during hunting…

Answer 2

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

Question 3

accuracy -

Answer 3

1-2 degrees in both the azimuth and elevation

needs both ears

Question 4

sensitivity -

Answer 4

most sensitive to sound coming from the front

most sensitive to high frequencies

Question 5

if one ear is plugged (decrease sound intensity in one ear)…

Answer 5

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

Question 6

the facial ruff:

Answer 6

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

Question 7

ears are arranged asymmetrically:

Answer 7

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

Question 8

inter-aural intensity differences between ears are used to…

Answer 8

determine the elevation of a sound source

Question 9

inter-aural time differences between ears are used to…

Answer 9

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

Question 10

how does owl achieve analysis of inter-aural intensity differences and inter-aural time differences…

Answer 10

auditory nerve (VIII Cranial nerve) - frequency analysis occurs in the inner ear by the auditory nerve - each fibre encodes a different frequency

Question 11

how can sensory neurons in the ear encode timing (azimuth)…

Answer 11

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

Question 12

how can sensory neurons in the ear encode intensity (elevation)…

Answer 12

elevation is encoded by changing rate of action potentials

the louder the sound the faster the action potentials fire

Question 13

cues to estimate distance:

Answer 13

signal specific + location specific

Question 14

signal specific cues to estimate distance -

Answer 14

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)

Question 15

location specific cues to estimate distance -

Answer 15

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

Question 16

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?

Answer 16

Two pathways used simultaneously in parallel to decide where the sound is coming from

• time pathway and intensity pathway

Comparisons done to get information

Question 17

To show that parallel processing is occurring using experiments…

Answer 17

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

Question 18

When injected magnocellular nucleus with anaesthetic =

Answer 18

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

Question 19

when injected into angular nucleus =

Answer 19

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

Question 20

To see how owl’s brain encodes time differences an experiment was done with a coincidence detector:

Answer 20

• 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)

Question 21

How would you explain how a neuronal circuit could both measure and encode the inter-aural time differences…

Answer 21

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)

Question 22

Elaboration of Jeffress’ model to permit the conversion of timing info into place info…

Answer 22

• 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

Question 23

Neuronal pathway: Inter-aural time differences (azimuth):

Answer 23

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

Question 24

Phase ambiguity:

Answer 24

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)

Question 25

neurones in the laminar nucleus…

Answer 25

compare the timing of inputs from the two ears and output a spatial code for azimuth

(coincidence detector)

Question 26

Neuronal pathway: Inter-aural intensity differences (elevation):

Answer 26

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

Question 27

the owl auditory system is an example of a computational map -

Answer 27

• 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

Question 28

frequency convergence:

Answer 28

used to overcome phase ambiguity - many frequencies come in with different sound periods, the period with the strongest summation indicates true delay