Interaction Between Sensory Systems

Question 1

Why is sound localisation important?

Answer 1

Survival mechanism for both prey and predator

Question 2

What methods are used to localise sound in the horizontal plane? (2)

Answer 2

• Interaural level differences (ILDs)
• Interaural timing differences (ITDs)

Question 3

What is the ILD method for sound localisation? (3)

Answer 3

• The difference in the loudness of the same sound at the two ears
• Head acts as a barrier (reflects/absorbs sound waves)
• Size of the ILD depends on how far the sound is from the centreline

Question 4

What kind of sounds are localised using the ILD method?

Answer 4

Higher frequency sounds

Question 5

What is the ITD method for sound localisation? (3)

Answer 5

• The difference in the arrival time of the same sound at the two ears
• Sounds from one side reach the near ear first and reach the far ear after a delay
• Size of the ITD depends on how far the sound is from the centreline

Question 6

What kind of sounds are localised using the ITD method?

Answer 6

Lower frequency sounds

Question 7

When is the ILD/ITD zero?

Answer 7

When the sound is on the centreline (equal distance from both ears)

Question 8

Where in the brain are ILDs and ITDs detected?

Answer 8

Sound localisation centres in the brainstem

Question 9

What are the sound localisation centres? (4)

Answer 9

• Cochlear Nucleus (CN)
• Lateral Superior Olive (LSO)
• Medial Superior Olive (MSO)
• Medial Nucleus of the Trapezoid Body (MNTB)

Question 10

Which sound localisation centre is involved in detection of ILDs?

Answer 10

LSO

Question 11

How are ILDs detected by the sound localisation centres? (5)

Answer 11

• LSO Excitatory-Inhibitory (EI) Pathway
• Neurons from the ear enter the CN
• LSO neurons receive excitatory input from the near ear CN
• The far ear sends an excitatory input to the LSO which goes via the MNTB
• MNTB makes the far ear input to the LSO inhibitory

Question 12

What is the pathway that is used to detect ILDs?

Answer 12

The LSO Excitatory-Inhibitory (EI) Pathway

Question 13

How does the LSO Excitatory-Inhibitory (EI) Pathway work? (3)

Answer 13

Left LSO:
- When the sound is from the left, the excitatory input from the near ear is larger than the inhibitory input from the far ear so the summation of LSO inputs is very excitatory = sound on the left
- As the sound moves to the right, the excitatory input from the near ear decreases and the inhibitory input from the far ear increases
- Combined balanced output of both LSOs gives an accurate indication of where the sound is

Question 14

When is the output of the LSO the highest?

Answer 14

When the sound is on the same side of the head as the LSO

Question 15

When is ILD/ITD sound localisation the most accurate? (2)

Answer 15

• When the sound is in the centre because the outputs of both LSOs and MSOs overlap
• Rapid detection of small changes in sound position in the centre is vital for hunting

Question 16

Which sound localisation centre is involved in detection of ITDs?

Answer 16

MSO

Question 17

How are ITDs detected by the sound localisation centres? (5)

Answer 17

• The MSO Excitatory-Excitatory (EE) Pathway
• Neurons from the ear enter the CN
• MSO receives excitatory inputs from the near and far ear CN
• The MSO neurons are only maximally active when both inputs arrive simultaneously
• The neuron from the near ear is shorter than the neuron from the far ear

Question 18

What is the pathway that is used to detect ITDs?

Answer 18

The MSO Excitatory-Excitatory (EE) Pathway

Question 19

How does the MSO Excitatory-Excitatory (EE) Pathway work? (4)

Answer 19

Left MSO:
- When the sound is from the left, the excitatory input from the near ear is received by the MSO much faster than from the far ear
- Sound reaches far ear after maximum ITD and has to travel down the longer neuron
- No summation of inputs = sound on the left
- As the sound moves to the right the MSO output increases because the sound reaches the far ear with less delay so the probability of simultaneous arrival increases

Question 20

When is the output of the MSO at the maximum?

Answer 20

When the excitatory inputs from both ears reach the MSO at the same time

Question 21

What is the output of the left MSO when the sound is at the centreline? (2)

Answer 21

• Half maximum
• Sound reaches the ears at the same time so ITD is zero but the signal still has to travel down the longer neuron from the far ear so there is still a delay in arrival to the left MSO

Question 22

What is the output of the left MSO when the sound is at the right ear? (2)

Answer 22

• Maximum
• The longer nerve distance from the far ear to the left MSO is compensated by the delay (ITD) for the sound to reach the left ear so both inputs reach the left MSO at the same time

Question 23

When is the output of the MSO the highest?

Answer 23

When the sound is on the opposite side of the head

Question 24

How do the sound localisation circuits develop? (3)

Answer 24

• The pathways are formed early in development and don’t depend on sensory function
• The circuits are then calibrated to align with the visual map
• The auditory map shows adaptive plasticity that depends on sensory function and system interaction

Question 25

How were barn owls used to study interaction between the visual and auditory systems? (5)

Answer 25

- Normally owls turn their heads to directly face visual/auditory stimuli - Shifted the owls visual field 20 degrees to the left using prisms - The head orientation quickly adapts to the shifted visual field but still directly faces the auditory stimuli - After a while the auditory response shifts to align with the modified visual field - After removal of the prisms, the visual response quickly realigns but the auditory response remains shifted

Question 26

What are the key findings from the barn owl experiments? (3)

Answer 26

- The auditory space map is modified based on changes to the visual map - Suggests that the visual map is dominant for space perception and is used to realign the auditory map if they differ - The visual map rapidly adapts to changes in the visual field but the auditory map takes longer

Question 27

Where in the owl brain does auditory and visual integration occur?

Answer 27

Midbrain

Question 28

What areas of the owl midbrain are used for auditory and visual integration? (3)

Answer 28

- Central Nucleus of the Inferior Colliculus (ICC) - External Nucleus of the Inferior Colliculus (ICX) - Optic Tectum (OT)

Question 29

What are the features of the ICC? (3)

Answer 29

- ICC neurons are tuned to specific ITDs - ICC neurons are in sound frequency-specific layers - ICC neurons show little adaptive tuning to prisms

Question 30

What are the features of the ICX? (2)

Answer 30

- Projections from frequency-specific layers of the ICC converge onto ICX neurons to create a map of auditory space - ICX neurons are the site of major adaptive plasticity

Question 31

What are the features of the OT? (4)

Answer 31

- Combines the auditory map from the ICX with the visual map - OT neurons have overlapping auditory and visual receptive fields - OT feeds back to the ICX - OT neurons also show major adaptive plasticity

Question 32

How does adaptive plasticity occur in the owl midbrain? (5)

Answer 32

- ICC signals ITDs to the ICX - ICX forms the auditory space map and signals to the OT - OT sends instructive feedback to the ICX to align the visual and auditory maps - After prisms, the visual map in the OT is shifted - Feedback from the OT to the ICX realigns the auditory map

Question 33

Which neurons in the owl midbrain show adaptive plasticity? (2)

Answer 33

- ICX and OT - ICC shows little adaptive plasticity

Question 34

Why is the visual system dominant over the auditory system?

Answer 34

Visual space maps are directly represented by the photoreceptors but auditory space maps are learned with experience