9 - Sound Localisation & Sensory Interaction Flashcards
Why is sound localisation important?
- Helps us with survival
- Provides a perception of auditory space
How do we localise sound in a horizontal plane?
Using 2 methods —> Interaural level differences (ILDs) and Interaural timing differences (ITDs).
What are Interaural level differences (ILDs)?
The difference of loudness of the same sound at 2 ears, the head acts as a barrier, reflecting or absorbing sound waves.
What determines the size of ILDs?
How far the sound is from the centre line.
What are interaural time differences (ITDs)?
- 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.
What determines the size of ITDs?
How far sound is from the centreline.
Where are the sound localisation centres located?
In the brainstem in collections of specialised neurones.
Where do all neurons from the ear enter?
The cochlear nucleus.
Where do neurons from the cochlear nucleus project to?
- The lateral superior olive (LSO)
- The medial superior olive (MSO)
- The medial nucleus of the trapezoid body (MNTB)
What are the main centres involved in ILD and ITD detection?
The LSO and the MSO
What is detected by the LSO?
Small differences in sound loudness (done specifically by the principle neurones)
What input is received by the neurones in the LSO?
Both excitatory from the near ear and inhibitory from the far ear
What makes the input from the far ear inhibitory?
The MNTB.
How does the ILD circuit function for the left side of the head?
Sound from the left is louder in the left ear as excitatory input is larger than inhibitory.
What happens as sound moves right in the ILD circuit on the left side of the head?
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
How do both the LSOs work together?
The outputs are opposite but balanced as the sound moves.
How does the ITD circuit function?
- 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
When does the MSO become maximally active?
When both inputs act simultaneously.
How does the ITD circuit function of the left side of the head?
- Sound from the left arrives at the MSO first and then sound reaches the far ear after a maximum ITD
What happens in the MSO after the sound moves to the right?
The MSO output increases and sound reaches the right with less delay, the probability of simultaneous arrival also increases.
When is the output of the left MSO maximum?
When the sound is close to the far ear.
How do both the MSOs work together?
- 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.
How is the auditory space map modified?
Based on changes to the visual map.
What is the visual map dominant for?
Space perception also used to realign the auditory map if there are differences between the 2.
What are the regions in the brain where auditory and visual integration occurs?
- The central nucleus of the of the inferior colliculus (ICC)
- The external nucleus of the inferior colliculus (ICX)
- The optic tectum (OT)
What does the ICC contain?
Neurons tuned to specific ITDs, which show little adaptive tuning to prisms.
What does the ICX do?
Forms a map of the auditory space from the neurons which are a site of large scale adaptive plasticity
What does the OT do?
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.
