9 - Sound Localisation & Sensory Interaction

Question 1

Why is sound localisation important?

Answer 1

• Helps us with survival
• Provides a perception of auditory space

Question 2

How do we localise sound in a horizontal plane?

Answer 2

Using 2 methods —> Interaural level differences (ILDs) and Interaural timing differences (ITDs).

Question 3

What are Interaural level differences (ILDs)?

Answer 3

The difference of loudness of the same sound at 2 ears, the head acts as a barrier, reflecting or absorbing sound waves.

Question 4

What determines the size of ILDs?

Answer 4

How far the sound is from the centre line.

Question 5

What are interaural time differences (ITDs)?

Answer 5

• The difference in arrival time of the same sound at the 2 ears.
• sounds from one side reach the near ear first and after a delay the far ear.

Question 6

What determines the size of ITDs?

Answer 6

How far sound is from the centreline.

Question 7

Where are the sound localisation centres located?

Answer 7

In the brainstem in collections of specialised neurones.

Question 8

Where do all neurons from the ear enter?

Answer 8

The cochlear nucleus.

Question 9

Where do neurons from the cochlear nucleus project to?

Answer 9

• The lateral superior olive (LSO)
• The medial superior olive (MSO)
• The medial nucleus of the trapezoid body (MNTB)

Question 10

What are the main centres involved in ILD and ITD detection?

Answer 10

The LSO and the MSO

Question 11

What is detected by the LSO?

Answer 11

Small differences in sound loudness (done specifically by the principle neurones)

Question 12

What input is received by the neurones in the LSO?

Answer 12

Both excitatory from the near ear and inhibitory from the far ear

Question 13

What makes the input from the far ear inhibitory?

Answer 13

The MNTB.

Question 14

How does the ILD circuit function for the left side of the head?

Answer 14

Sound from the left is louder in the left ear as excitatory input is larger than inhibitory.

Question 15

What happens as sound moves right in the ILD circuit on the left side of the head?

Answer 15

The loudness in the left decreases and increases in the right, when sound is closer to the right the output of the LSO is low

Question 16

How do both the LSOs work together?

Answer 16

The outputs are opposite but balanced as the sound moves.

Question 17

How does the ITD circuit function?

Answer 17

• Small differences in arrival time of a sound are detected by the principle neurons in the MSO.
• There are 2 excitatory inputs, one from each ear that converge onto neurons in the MSO

Question 18

When does the MSO become maximally active?

Answer 18

When both inputs act simultaneously.

Question 19

How does the ITD circuit function of the left side of the head?

Answer 19

• Sound from the left arrives at the MSO first and then sound reaches the far ear after a maximum ITD

Question 20

What happens in the MSO after the sound moves to the right?

Answer 20

The MSO output increases and sound reaches the right with less delay, the probability of simultaneous arrival also increases.

Question 21

When is the output of the left MSO maximum?

Answer 21

When the sound is close to the far ear.

Question 22

How do both the MSOs work together?

Answer 22

• The output of each MSO is highest for sounds from the far ear due to the time delay.
• The combined balanced outputs gives an accurate indication of sound position.

Question 23

How is the auditory space map modified?

Answer 23

Based on changes to the visual map.

Question 24

What is the visual map dominant for?

Answer 24

Space perception also used to realign the auditory map if there are differences between the 2.

Question 25

What are the regions in the brain where auditory and visual integration occurs?

Answer 25

• The central nucleus of the of the inferior colliculus (ICC)
• The external nucleus of the inferior colliculus (ICX)
• The optic tectum (OT)

Question 26

What does the ICC contain?

Answer 26

Neurons tuned to specific ITDs, which show little adaptive tuning to prisms.

Question 27

What does the ICX do?

Answer 27

Forms a map of the auditory space from the neurons which are a site of large scale adaptive plasticity

Question 28

What does the OT do?

Answer 28

Combines the auditory map of the ICX with a visual map of space
- The neurons here have overlapping auditory and visual receptive fields.
- A site of large scale plasticity.