interaction between sensory systems Flashcards
how do we localise sounds
1) Interaural level differences (ILDs)
2) Interaural timing differences (ITDs)
What are Interaural level differences (ILDs)
- The difference in the loudness (level) of the same sound at the two ears (ILD)
- The head acts as a barrier, reflecting or absorbing sound waves
- Size of ILD depends on how far sound is from the centreline
What are Interaural timing differences (ILDs)
- The difference in the arrival time of the same sound at the two ears
- Sounds from one side reach the near ear first, and after a delay get to the far ear
- Size of ITD depends on how far sound is from the centreline
are ILD or ITD more common for high freq sounds
Interaural level differences (ILDs)
are ILD or ITD more common for low freq sounds
Interaural timing differences (ITDs)
how do we localise sound
By detecting the ILD or ITD we know where the sound source is
where are sound localisation centres
brainstem
how does sound travel from ear -> brainstem
neurones from ear -> neurones in cochlear nucleus → lateral superior olive (LSO)/medial superior olive (MSO)/ and medial nucleus of trapezoid body (MNTB)
how does sound travel from ear -> brainstem
neurones from ear -> neurones in cochlear nucleus → lateral superior olive (LSO)/medial superior olive (MSO)/ and medial nucleus of trapezoid body (MNTB)
main centres involved in ILD and ITD
lateral superior olive
medial superior olive
medial nucleus of trapezoid body
how are small differences in loudness detected
- detected in the LSO by the principal neurones
1. The LSO neurones receive an excitatory input from near ear (+) and an indirect inhibitory input from far ear (–)
2. excitatory input that crosses the midline to the MNTB on the same side as the LSO
3. MNTB makes the input from the far ear inhibitory
role of lateral superior olive (LSO)
involved in measuring difference in sound intensity between ears
role of medial nucleus of trapezoid body (MNTB)
major source of inhibition in auditory brainstem circuitry
how does the LSO detect volume differences between ears
- ear closest to sound = excitatory input to LSO > inhibitory input to MNTB
- output of LSO is determined by summation of opposing inputs
how do the LSOs work together?
- each LSO receives positive input from near ear and negative input from far ear
- overlap of 2 LSO outputs = accuracy of localisation
how do we detect interaural level differences
- the two LSOs act as channels tuned to sounds from each side of head
- each channel is made up of neurones tuned to different ILDs
function of ITD/MSO EE pathway
→ detection of interaural timing differences
how does the ITD/MSO EE pathway work
- small differences in sounds arrival time are detected in medial superior olive (MSO) by principle neurones
- 2 excitatory inputs (each ear) converge on MSO
- MSO becomes max active when BOTH inputs arrive at same time
- because neurones are different lengths, activity from far ear takes longer to reach MSO than near ear
why do we get ITDs
ITDs result because the ears are physically separated in space by the head. Therefore, the direction-dependent differences in path lengths that sound must travel to reach each ear from the source will generate different times of arrival of the sound at the two ears
how do MSOs work together
- for sound from left
- L = MSO is low, R = MSO is high
- output of each MSO is highest for sounds from far ear
- due to time delay required for coincidence
how are ITDs encoded
ITDs are encoded by neurones in MSO that compare coincident arrival of excitatory inputs from both ears
difference between ILD and ITD
left = LSO (sound from left), left MSO (sound from right)
sounds in HZ plane are localised using which 2 methods
- interaural level differences (ILDs) = used for high freq sounds
- interaural timing differences (ITDs) = used for low frequency sounds
where and how is ILD encoded
lateral superior olive
encoded by summation of excitatory and inhibitory inputs (EI)
where and how is ITD encoded
- represented in MSO
- encoded by coincidence of 2 excitatory inputs (EE)
are ILDs and ITDs adaptable or fixed based on genetic programmes?
both
- The initial connections are formed based on genetic programmes
- auditory map shows adaptive plasticity that depends on sensory function and sensory system interaction
how did they conduct vision as sound localisation calibration in barn owls
- The owls visual field was artificially shifted and they looked at the effect this had on their ability to localise sounds
- assessed ability to localise stimuli using head movements
- They artificially shifted the owls visual field by 20o to the left using prisms in front of the eyes
- To look at the target at 0o the head position will be 20o to the right of it
- The owls hearing was not artificially modified at all
The Measurement of Owl Head Orientation to Visual or Auditory Stimuli
- owls were put into a dark recording chamber
- head orientation was measured from the search coil on top of the owls head
- The position of the light and sound stimulus were varied horizontally and vertically
- A Shifted Visual Map Imposes the Same Shift on the Auditory Map
