sound localisation and sensory interaction

Question 1

why is sound localisation important?

Answer 1

provides a perception of auditory space

All the information we need to do this is taken from features of sound arriving at the two ears

For vision – the world is mapped directly onto the photoreceptors

For hearing – the receptor map is sound frequency – there is no map of auditory space - our brain must create it

Question 2

what are the two ways in which we localise sound in the horizontal plane?

Answer 2

detection of interaural level differences - ILDs (difference in sound intensity between the ears)
- As small as 1-2dB

detection of interaural timing differences (ITDs)
The difference in the arrival time of the same sound at the two ears
- As small as 10μs

Question 3

how come the ears have ILDs and where is the ILD maximal?

Answer 3

The head acts as a barrier that reflects or absorbs sound waves
Sound from one side will be louder in the near ear and quieter in the far ear

Size of the ILD depends on how far sound is from the centreline (line down the middle of head
- ILD is maximal furthest from centreline (so literally at one of the ears)
- ILDs are zero on the centreline (right in front of you or right behind)

Question 4

ILD is more used for…?

Answer 4

higher frequency sounds

Question 5

how do we get an ITD?

Answer 5

sound closest to one ear arrives their first, reaches the other ear after a delay

like ILD, it is maximal when it is furthest from the centre line etc…

Question 6

what brain areas are involved in ILD and ITD?

Answer 6

all neurons from ear enter the cochlear nucleus

form there they go to the lateral superior olive (LSO) and medial superior olive (MSO) you’ve got one of each on each side of your brainstem

ILD = at the LSO
ITD = at the MSO

Question 7

how does the LSO detect interaural level differences?

Answer 7

remember you’ve got an LSO on either side of the brainstem

when a sound occurs, each LSO receives an excitatory input from its near ear, and an inhibitory input from its far ear.
(for this circuit we assume the inputs arrive at the same time)

so one sound results in two overall outputs, one from the right LSO and one from the left. these outputs are the summation of the inhibitory and excitatory inputs the LSO receives

Question 8

explain what happens when a sound is near the right ear

Answer 8

the right LSO receives a large excitatory input (loud sound at the right ear) and a small inhibitory input (from the left ear as the sound is quiet on the left side)

so overall the right LSO summates these to give a large excitatory output (super positive). the right LSO mostly responds to sound on the right

the left LSO receives a large inhibitory input from the right ear (because the right ear detects a loud sound so would want to inhibit the left) and a very small excitatory input from the left ear as the sound is quiet on the left side

so overall the left LSO summates these inputs to give a negative, super low neuron population output

Question 9

what is the ILD value when the sound is right by the right ear?

Answer 9

the ILD is maximal, because the difference in sound between the two ears is greatest

at the left LSO, its -ve, at the right its +ve

Question 10

output of the LSO is determined by?

Answer 10

the summation of the two opposing inputs, excitatory from the near ear and inhibitory from the far one

Question 11

what happens when a sound is dead on the centreline?

Answer 11

the ILD is zero

the loudness is the same in both ears, meaning each ear receives the same inputs, a mediumish excitatory and a mediumish inhibitory

so in each LSO, the population output is about half maximal, neither the excitatory or inhibitory input dominates

so each LSO is giving the same, half-maximal output

Question 12

why is the central region the most accurate for sound localisation?

Answer 12

because there is the most overlap here between the two LSO outputs, both saying the same thing

it allows for rapid detection of small changes in sound position in the centre

Question 13

when sound is on a right diagonal (rather than directly on the right)?

Answer 13

loudness in the right ear decreases and in the left ear it increases (from when the sound was directly on the right, tho it is still louder for the right ear, the difference is just less)
so the ILD decreases, the population output of the right LSO is a bit less than maximal (tho above half-maximal that you get for sounds in the centre)

for the left LSO the population output would still be quite low (the inhibitory input is larger than the excitatory input still, as the sound is still closest to the right ear, albeit less so)

Question 14

the brain can recognise the position of a sound due to the…?

Answer 14

balanced and opposite outputs of the two LSO channels

Question 15

how is interaural time difference detected?

Answer 15

the MSO (remember you’ve got one on each side) receives two excitatory inputs, one from each ear

the overall output of an MSO is determined by whether these two excitatory inputs summate, which depends on when the inputs (from the close ear and the far ear) arrive

focusing on one MSO, for each excitatory input you must consider how far the sound has to travel to each ear, and then how long the neuron will be

Question 16

at the left MSO, what happens when the sound is directly on the left?

Answer 16

the input from the left ear arrives at the left LSO first - the sound was super close to the left ear, and the neuron form left ear to left MSO would be short

the sound reaches the right ear after the maximum time difference (ITD), and then it has to travel further down the nerve that is going from right ear to left MSO…

this means there is the largest delay possible and no summation of the two inputs, so the overall population output of the left MSO is minimal (while it would be maximal at the right MSO)

Question 17

as sound (previously directly at the left ear) moves more to the right, but is still closer to the left, what happens at the left MSO?

Answer 17

sound takes longer to reach the left ear than it did before (tho still reaches it first)

and it reaches the right ear a little quicker - so the ITD is lower

this means the probability of simultaneous arrival of the two inputs increases…

so the population output of the left MSO increases (but is still below half-maximal)

Question 18

what happens when sound is directly at the centre line (ITD)?

Answer 18

sound reaches both ears at the same time…

but the excitatory input to the MSO coming from the far ear is still delayed/arriving later due to the longer nerve it has to go along

at this point the MSO output is half-maximal

Question 19

when sound is directly at the right, what happens to the output of the left MSO?

Answer 19

the ITD is at its greatest, the inputs from the close ear and far ear arrive at the same time because the excitatory input from the left has a long distance for the sound to travel but a short nerve to go along, while the input form the right has a short distance to for the sound to travel but a long nerve to go along (balances out)

…and summate, giving maximal population output (from the left MSO, it would be minimal at the right - remember, balanced and opposite)

Question 20

the left MSO mainly responds to sound from the…?

Answer 20

the right side of the head, because the excitatory inputs at the MSO summate when the sound travels a short distance to the right ear but then down a long neuron (to get to the left MSO)

which is balanced by the sound travelling far to get to the left ear but along a short neuron input from the right ear

left MSO responding to right-side-of-head sounds is opposite to the LSO with ILDs

Question 21

left MSO when sound is on a right diagonal?

Answer 21

sound approaching the far ear means the delay gets small, there’s a bit more summation and the population output of the left MSO becomes large (more than half, less than maximal)

because the distance the sound travels becoming more for the left ear and less for the shorter for the right ear compensates for the longer neuron the input from the right ear has to travel to get to the left MSO

Question 22

summary of ITD circuit function?

Answer 22

ITDs are detected by neurons in the MSO by comparing the arrival time of excitatory inputs from both ears

The output of each MSO is highest for sounds from the opposite side of the head
- Where their two inputs arrive at the same time

Combined balanced, opposite output of the two MSOs gives an accurate indication of sound position

Question 23

what are some similarities between ITD and ILD?

Answer 23

both based on two broadly tuned channels, both have most accuracy in the centre due to overlap

the major difference is the side of the head the channels are tuned to, LSOs tuned to sounds on the same side, MSOs tuned to sounds on the opposite side

Question 24

what are two major differences between ILD and ITD?

Answer 24

the side of the head the channels are tuned to, LSOs tuned to sounds on the same side, MSOs tuned to sounds on the opposite side

ILDs – detected by the summation of excitatory and inhibitory inputs (EI)
ITDs – detected by the arrival time of two excitatory inputs (EE)

Question 25

how do the sound localisation circuits develop (two parts)?

Answer 25

how is it we know these ILDs an ITDs relate to an object’s position around us?

its genetic - the initial connections from based on genetic programmes, in early development, and on not depend on sensory function

then there’s some learning - these initial circuits/the auditory map is refined and aligned to overlay the visual map. i.e., the auditory map shows adaptive plasticity that depends on sensory system interaction

Question 26

what animal is used when investigating sound localisation circuits and why?

Answer 26

the barn owl - one of the best sound-localising animals, and using well defined circuits to detect ILDs and ITDs, which are refined via interaction with the visual system (like in mammals)

Question 27

what did Eric and Phyllis Knudsen investigate with barn owls?

Answer 27

the interaction of visual and auditory systems during development

looked at how owls learn to interpret interaural time differences (ITDs) as a location of sound in space, and how this is modified by the visual system

they artificially shifted the visual field of the owls, and wanted to see whether the auditory map would align with the visual field

Question 28

what did the Knudsens measure as an indication of where the owl thought a stimulus was located?

Answer 28

owls eyes do not move much in their sockets, and they do normally look straight at a target light or sound

so head movement was used as an accurate indication of where the owl thinks the target is located

Question 29

how did the Knusdens investigate the interaction of visual and auditory systems during development?

Answer 29

placed prism goggles on the owls that shifted the visual field, so to view something straight ahead at 0 degrees, the owl must turn its head to 20 degrees

they kept the goggles on for 49 days and tested periodically, putting the owls in a dark recording chamber, and using a coil on the owls head + a magnetic filed made by induction coils idk, to measure the head movements

when they applied visual and auditory stimuli SEPARATELY, at different vertical and horizontal positions

Question 30

what did the Knusdens find?

Answer 30

before the prisms, the head looked directly at visual and auditory stimuli, the error values were all around zero

after one day of prism wearing, the head orientation had quickly adapted to the shifted visual field for the light stimulus, but the head still directly faced the auditory stimulus

after 42 days, the head still had a shifted response for the visual stimulus, but now the auditory head shift response had aligned with the modified visual field (so sound at 0 degrees, owl turned to 20 degrees to match the shifted visual field, despite the auditory field having not been modified)

when the goggles were removed at day 49 -
the visual response quickly re aligned to the visual stimulus/back to normal BUT the auditory response remained shifted, it had adapted to the prisms (and I think would need time to realign with the visual system back to normal)

Question 31

summarise the key findings of the Knusdens’ study on barn owls

Answer 31

The auditory space map is modified based on changes to the visual map, the shifted visual field imposes the same shift on the owl’s response to the auditory stimulus
- This occurs even though the auditory field has not been modified

Visual space maps are directly represented by the photoreceptors
- Auditory space maps need to be learnt with experience
- This is why the visual inputs dominate the auditory ones for spatial location (visual is more reliable)

Question 32

why does visual input dominate auditory (kind of already said this but)

Answer 32

Sensory systems interact to improve the precision of sensory development and our perception

most reliable input dominates over other senses

Visual space maps are directly represented by the photoreceptors, so are the most reliable
- Auditory space maps need to be learnt with experience
- This is why the visual inputs dominate the auditory ones for spatial location