lecture 24 - nervous system 4: hearing & the brain

Question 1

what are attached to the hair cells in the organ of corti?

Answer 1

nerve fibres

Question 2

what are the 2 types of nerve fires attached to hair fibres?

Answer 2

type 1 - associated with inner hair cells (95%)
• there are a number of type 1 going into each inner hair cell
• carry info to the brain

type 2 - associated with outer hair cells (5%)
• 1 nerve fibre goes to a number of outer hair cells
• spiral up the cochlea and make contact with neighbouring cells
• function is unknown

Question 3

processing nuclei in the auditory pathway

Answer 3

1) auditory nerve
2) cochlea nucleus - first synapse in the central auditory pathway
3) superior olivary complex - site of interaction of inputs from 2 ears
4) inferior colliculus - midbrain centre where different brainstem pathways converge
5) medial geniculate body - thalamic auditory nucleus
6) auditory cortex - in the temporal lobe of the cerebral cortex

Question 4

where is the auditory cortex in the human brain?

Answer 4

along the upper bank of superior temporal gyrus

• Heschl’s gyrus

Question 5

what are the 2 ways to code sound frequency in the brain?

Answer 5

place code

time code

Question 6

place code

Answer 6

throughout the auditory pathway sound frequency is mapped within the processing centres and on surface of auditory cortex

tonotopic organisation

Question 7

place code: tonotopic representation of sound frequency

Answer 7

high frequency hair cells up towards the apex and low frequency hair cells down towards the base

the nerve fibre is selective for a particular range of frequencies

nerve fibres terminate in an orderly fashion in the cochlea nucleus

nerve cells show the same frequency selectivity as the nerve fibres

Question 8

time code

Answer 8

phase-locking

firing of APs in auditory nerve fibres synchronise with peaks of sound waveform

time between APs tells us about the frequency of sound

only occurs for low frequency sounds

Question 9

what is phase-locking?

Answer 9

synchronisation of AP firing to peaks in waveform of low frequency sounds

Question 10

how does phase-locking occur?

Answer 10

when a sound is at peak form, there is greater probability for AP than when at the trough

time between AP = a period (t)

frequency = 1/t

Question 11

how does sound localisation work?

Answer 11

uses place and time code

need to know where the sound is coming from

Question 12

what are the 2 mechanisms of sound localisation?

Answer 12

1st - recognising here the sound is in vertical plane (below/above or infant/behind)
• depends how waves interact with outer ear

2nd - horizontal plane (left/right)
• differences in timing and intensity of the sound in 2 ears

Question 13

cues for sound localisation in horizontal plane

Answer 13

sounds in different positions in space lead to timing and intensity differences at the ears as they are not in the same place - separated by the head

this are termed inter-aural

brain detects these differences to localise sound

Question 14

inter-aural time cues for sound localisation

Answer 14

for all sounds
• differences in sound onset time between the 2 ears

for longer, continuous sounds
• differences in phase of sound between ears

Question 15

what happens when a low frequency sound comes from 1 side of the head?

Answer 15

the sound reaches the left ear in 1 phase and the the right ear at a different phase

due to the difference in distance the sound has to travel

Question 16

what happens when a high frequency sound comes from 1 side of the head?

Answer 16

the brain cannot tell the difference between the 2 peaks

phase information becomes ambiguous

Question 17

the cochlea implant

Answer 17

for patients with no cochlea function

electrical activation of cochlear nerve fibres by electrode array inserted in cochlea