Lecture 27: Bineural hearing and sound localisation

Question 1

Why do we have two ears?

Answer 1

• Redundancy
• Reduces the impact of noise (background)
• Sound localisation

Question 2

Describe redundancy in terms of having two ears;

Answer 2

• in case of harm

- of information (two inputs to brain to determine exactly what the noise was i.e elimination of background information

Question 3

Describe the bineural innervation of the auditory system;

Answer 3

Cochlear nucleus

• AVCN innervates both superior olive nuclei
• PVCN and DCN project to contralateral inferior colliculi
• Both superior olive project to ipsilateral inferior colliculi
• Inferior colliculi project to both MGB nuclie which project onto same side cortices.

Question 4

What information do we use to localise sound?

Answer 4

Binaural cues (Laterality) - what side the sound is on
Monaural cues

Question 5

What are the binaural cues?

Answer 5

• Interaural timing/ phase differences (ITDs)

- Interaural level differences (db- head shadow)

Question 6

What are the monaural cues?

Answer 6

Monaural (as dont require two ears to work)

• Pinna cues (head related transfer function)
• Memory
• Spectural cues for distance
• Sensory intergration (vision)
• Head movement

Question 7

What is spectural cues for distance?

Answer 7

High hz information is lost over distance due to air impedance. Therefore less info in sound indicates greater distance

Question 8

How precise is human sound localisation?

Answer 8

Can detect a change in sound source angle of 1 degree

Equivalent of:

• ITD at 10miroseconds
• ILD of 0.5db

Question 9

What is the maximum ITD?

Answer 9

0.6seconds laterally through the head

This value gets smaller till it reaches 0 at the equivalent of sound coming from straight ahead.

Question 10

What is ILD created by?

Answer 10

The head shadow, sound coming directly towards you will not have a difference but otherwise it will.

Question 11

How is the function of ITD and ILD different?

Answer 11

ITD uses low frequency sounds.

ILD uses high frequency sounds.

Question 12

Describe frequency dependancy and ITD/ILD?

Answer 12

If an ITD exists the ear that transduces the sound later (2nd ear) will also have reduced amplitude peak (db)

Higher frequencies will have greater ILD differences, while low are much the same.

However, there onset of stimulus, amplitude are the same if the noise is coming from straight ahead.

There is also no difference in ILD regardless of Hz

Question 13

What does frequency dependancy lead to?

Answer 13

Duplex theory

Question 14

What is the duplex theory?

Answer 14

ITD’s ; lower Hz (<1500Hz), Higher Hz cannot provide good phase locking

ILDs : Higher Hz (>1500Hz), Lower Hz have wavelengths that are so long there is not much head shadow. (can still use the cue if the sound source if very close)

Question 15

What is the cone of confusion?

Answer 15

It describes a plane with regard to the two ears whereby there are many points where the distance between the two ears is the same i.e could stand 45 degrees from the midline (behind or infront on the same ear) and couldnt tell where the sound was coming from as the ITD and ILD are the same.

Thus need monaural cues too

Question 16

What is one of the major monaural devices?

Answer 16

The pinna

Question 17

What does the pinna do?

Answer 17

Using the interference (Hz gaps, “notch”) generated by the sounds being reflected by the pinna into the ear drum to localise sound in the vertical plane.

Question 18

Where in the auditory pathways are “notchs” detected?

Answer 18

Coded for in the DCN. The Hz gap means the neurons dont fire and the balance of firing and non-firing that would have occurred doesnt, and this sends a signal.

We learn that a notch is associated with sound coming from a particular direction.

Sends this info to the inferior colliculus

Question 19

What concepts exist within ITDs?

Answer 19

• Delay Lines (Jeffress)

- Rate coding, Inhibition

Question 20

What are ITD’s mediated by in the auditory system?

Answer 20

The AVCN and the medial SP (superior olive)

Both AVCN innervate both MSO

Question 21

Whats inside the AVCN?

Answer 21

Bushy cells - these are innervated by multiple auditory nerve fibres. (auditory fibres are phased locked)

Bushy cells are innervated by multiple auditory nerve fibres and only respond to multiple simultaneous inputs. They are more capable of firing in phase than their inputs. (same Hz as sound) (they are locked to the phase of the Hz) and they can do it better than any single auditory nerve fibre

Question 22

What exists in the auditory system to preserves timing information?

Answer 22

Calyx of held;

Very large cup like synapse, which conserves timing information

They do this between AVCN and MSO

Question 23

What is the jeffress model?

Answer 23

Delay lines

• There are a series of synapses in the MSO.
• Each end of the series receives input from one ear.
• The neuron that is innervated simultaneously, will activate and indicate the direction of the sound source
• The neuron that is activated depends on the ITD

i.e imagine two people equal distance apart. can run at same speed. point where they meet depends on any handicap given at the start of the race.

Question 24

Is the jeffress model correct?

Answer 24

Yes in birds

Question 25

Is the jeffress model correct in mammals?

Answer 25

No, we use a rate coding system?

Question 26

What is the rate coding system and ITD?

Answer 26

So if the sound source is directly infront the ITD = 0 and the MSO neurons fire at 50%

If the sound source moves the rate of firing will either increase of decrease based on the direction of movement

Based on a combination of excitation and inhibition to change the rate of neuron firing based on which ear is leading. (sound localisation)

Question 27

Describe how ILD functions?

Answer 27

If there sound is contralateral (to that LSO) you get inhibition and if its ipsilateral you get excitation

inhibition is mediated through the MSO

The extent of inhibition is less than excitation ie decreases from base little, but can increase a lot. Thus sound localisation