Lecture 27: Bineural hearing and sound localisation Flashcards
Why do we have two ears?
- Redundancy
- Reduces the impact of noise (background)
- Sound localisation
Describe redundancy in terms of having two ears;
- in case of harm
- of information (two inputs to brain to determine exactly what the noise was i.e elimination of background information
Describe the bineural innervation of the auditory system;
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.
What information do we use to localise sound?
Binaural cues (Laterality) - what side the sound is on Monaural cues
What are the binaural cues?
- Interaural timing/ phase differences (ITDs)
- Interaural level differences (db- head shadow)
What are the monaural cues?
Monaural (as dont require two ears to work)
- Pinna cues (head related transfer function)
- Memory
- Spectural cues for distance
- Sensory intergration (vision)
- Head movement
What is spectural cues for distance?
High hz information is lost over distance due to air impedance. Therefore less info in sound indicates greater distance
How precise is human sound localisation?
Can detect a change in sound source angle of 1 degree
Equivalent of:
- ITD at 10miroseconds
- ILD of 0.5db
What is the maximum ITD?
0.6seconds laterally through the head
This value gets smaller till it reaches 0 at the equivalent of sound coming from straight ahead.
What is ILD created by?
The head shadow, sound coming directly towards you will not have a difference but otherwise it will.
How is the function of ITD and ILD different?
ITD uses low frequency sounds.
ILD uses high frequency sounds.
Describe frequency dependancy and ITD/ILD?
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
What does frequency dependancy lead to?
Duplex theory
What is the duplex theory?
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)
What is the cone of confusion?
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
What is one of the major monaural devices?
The pinna
What does the pinna do?
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.
Where in the auditory pathways are “notchs” detected?
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
What concepts exist within ITDs?
- Delay Lines (Jeffress)
- Rate coding, Inhibition
What are ITD’s mediated by in the auditory system?
The AVCN and the medial SP (superior olive)
Both AVCN innervate both MSO
Whats inside the AVCN?
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
What exists in the auditory system to preserves timing information?
Calyx of held;
Very large cup like synapse, which conserves timing information
They do this between AVCN and MSO
What is the jeffress model?
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.
Is the jeffress model correct?
Yes in birds
Is the jeffress model correct in mammals?
No, we use a rate coding system?
What is the rate coding system and ITD?
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)
Describe how ILD functions?
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
