Lecture 9 - sound localisation and sensory information

Question 1

why sound localisation is important

Answer 1

there is no map of auditory space, brain has to create the map using auditory information

Question 2

how we localise sounds

Answer 2

1. interaural level differences (ILDs)
2. Interaural timing differences (ITDs)

Question 3

what is interaural level differences (ILDs)

Answer 3

• difference in loudness of the same sound at the two ears, as small as 1-2dB
• (high frequency sounds)

Question 4

what is interaural timing differences (ITDs)

Answer 4

• difference in arrival time of the same sound at the two ears, as small as micro seconds
• (lower frequency sounds)

Question 5

what does both ILD and ITD depend on?

Answer 5

depends on how far sound is from the centerline

Question 6

where are the sound localisation areas?

Answer 6

brainstem

Question 7

where do all neurons from the ear enter?

Answer 7

Cochlear Nucleus (CN)

Question 8

where does neurons from the cochlear nucleus go to ? (3 things)

Answer 8

• Lateral Superior Olive (LSO)
• Medial Superior Olive (MSO)
• Medial Nucleus of the Trapezoid Body (MNTB)
• one of each centre on both sides

Question 9

what are the main centers involved in ILD and ITD

Answer 9

LSO and MSO

Question 10

detection of interaural level difference (ILDs) mechansism

Answer 10

• detected in LSO by principal neurons
• LSO neurons receive excitatory input from near ear, indirect inhibitory from far ear
• begins as excitatory input that crosses midline to MNTB on same side as LSO
• MNTB makes input from far ear inhibitory

Question 11

ILD circuit function: the LSO

Answer 11

• ILD circuit for the left side of the head
• sound from left - louder in left ear
• excitatory input larger than inhibitory
  -summation of inputs is very excitatory
• in left position is near maximal

Question 12

what happens as the sound moves right (LSO)

Answer 12

• loudness in left ear decreases and increase in the right
• excitatory input reduces
• inhibitory input increases

Question 13

when sound is close to right ear (LSO)

Answer 13

• when sound is close to right ear the output of LSO is very low - tuned to sound from the left
• output of the LSO is determined by summation of two opposing inputs

Question 14

ILD function: how both LSOs work together

Answer 14

• each LSO receives an excitatory input from near ear and inhibitory input from far ear
• outputs are opposite but balanced as sound moves
• output of each is higher for sounds from same side of the head
• most overlap of LSOs outputs when sound is in central region for sound localisation
• rapidly detect small changes in sound position

Question 15

detection of interaural timing differences (ITDs)

Answer 15

• detected in MSO by principal neurons
• two excitatory inputs - one from each ear - converge on neurons in MSO

Question 16

when does MSO become maximally active?

Answer 16

when both inputs arrive simultaneously. circuit detects timing rather than level
- activity from far ear takes longer to reach the MSO then the input from the near ear

Question 17

ITD circuit function: the MSO

Answer 17

• for left side of the head
• excitatory inputs converging on left MSO
• sound from left - left input arrives at MSO first
• sound reaches far ear after a maximum ITD. Has to travel further down long nerve
• largest delay - no summation of two inputs
• population output of the left MSO is minimal

Question 18

as sound moves right (MSO)

Answer 18

• MSO output increases
• sound reaches right ear with less delay (ITD)
• probability of simultaneous arrival

Question 19

sound at the right ear (MSO0

Answer 19

• output of left MSO maximum for sound close to far ear - tuned sound from right
• the longer nerve distance is compensated by the delay (ITD) for sound to reach the left ear
• here both MSO inputs arrive at same time - full coincidence of both excitatory inputs

Question 20

ITD circuit function: how both MSOs work together

Answer 20

• for a sound from the left, left MSO very low and right MSO very high
• output of each MSO is highest for sounds from the far ear. due to the time delay required for coincidence
• outputs are balanced as sound moves for sound position
• overlap at centre ensures accuracy of sound localisation

Question 21

comparing the two mechanisms used for sound localisation

Answer 21

• methods for detecting ILDs and ITDs are very similar
• both are based on two broadly tuned channels
• major difference is the side of the head the channels are tuned to
• left LSO-sound from the left
• left MSO - sound from the right

Question 22

how sound localisation circuits develop

Answer 22

1. circuits evolved and formed during early development and do not depend on sensory function
2. initial circuits calibrated using alignment with visual map and depends on sensory function and sensory interaction

Question 23

regions of owl midbrain where auditory and visual integration occurs

Answer 23

• central nucleus of inferior colliculus
• external nucleus of inferior colliculus
• optic tectum

Question 24

central nucleus of inferior colliculus

Answer 24

• contains neurons tuned to specific ITDs
• the ITDs are in sound frequency-specific layers
• these neurons show little adaptive tuning to prisms

Question 25

external nucleus of inferior colliculus

Answer 25

• projections from frequency-specific layers of the ICC converge on neurons of ICX
• map of auditory space is formed by these neurons
• ICX neurons are a site or large-scale adaptive plasticity

Question 26

optic tectum

Answer 26

• combines the auditory map of ICX with a visual map of space
• neurons have overlapping auditory and visual receptive fields
• there are also feedback projections from OT to the ICX
• OT neurons are also a site for large -scale plasticity

Question 27

how adaptive plasticity occurs in owl midbrain

Answer 27

• auditory map in ICX is aligned with visual map in the OT with instructive feedback
• after prisms: instructive feedback from OT realigns the auditory map to match the shifted visual map