Lecture 20: Hearing 3: Neural Processing of Sound

Question 1

Which cranial nerve is responsible for the neural processing of sound?

Answer 1

8th cranial nerve- vestibulocochlear nerve.

Question 2

Type 1 spiral ganglion innervates _________

Type 2 spiral ganglion innervates ________

Answer 2

1: IHC (90-95% of nerve fibres)
2: OHC (5-10% nerve fibres)

Question 3

Briefly describe the distribution of the audtiroy nerve fibres

Answer 3

Like the organ of Corti, auditory nerve fibres are sharply tuned to different frequencies defined by location along the cochlea.

Base of the cochlea is high frequency area (consonants)

Apex of the cochlea is the low frequency area (vowels)

Cells and n.fibres are tuned to different frequencies.

specific n.f is responsive to a specific range of frequencies.

Question 4

Describe the frequency coding method in the auditory nerve

Answer 4

Coding of frequency comes by 2 methods:

1) Place Principle (Spatial Coding) (Left Fig)

Cochlea is a filter and is tonotopically organised so that frequency is detected by spatial representation from base to apex.

• Variations in stiffness of the basilar membrane (mechanical properties) determine its frequency response.
• This principle detects both low and high frequencies.

Same tonotopic organisation throughout auditory system underpins f_requency/pitch detection_

2) Volley Principle (Temporal Coding) (Right Fig)

L_ow frequencies_ (in particiular) are detected by temporal firing of nerve fibres in time to the frequency of the stimulus (frequency of stimulus matches frequency of nerve firing e.g. if the freq is 1000Hz, then the stereocillary fibres will be depolarised 1000 times per second).

• Frequency analysis determined by neural response locked to stimulus cycle (phase-locked)
• This only detects low frequencies (<1-2kHz), cannot respond to high frequencies due to limitation of refractory period

Question 5

_______ of __________determines the frequency response

Answer 5

Variations in stiffness of the basilar membrane determines frequency response

Question 6

(Physical appearance)

Base of the cochlea is ______ and ______

Apex of the cochlea is _______ and ______

(physical characteristics of the cochlea as you move from the base to the apex)

Answer 6

Base of the cochlea is narrow and stiff

Aplex of the cochlea is wide and floppy

Question 7

How is the intensity coded in the auditory nerve?

Answer 7

Rate of firing of individual nerve fibres increases with sound intensity

Number of fibres increases as energy spreads along organ of Corti

As intensity increases, the rate of firing increases

Question 8

Contralateral pathway is _____________

Answer 8

Excitatory

Question 9

Ipsilateral pathway is ___________-

Answer 9

inhibitory

Question 10

Name the pathways from the cochlea to the auditory cortex

Answer 10

Cochear

Cochlear nucleus

Superior Olivary complex

Inferior colliculus

Medial Geniculate Body of thalamus

Auditory cortex

Tonotopic representation of sound is presented throughout the system

Question 11

Processing of Sound in the Central Auditory System is Binaural. Why is this improtant?

Answer 11

If someone has unilateral hearing loss, they will find it hard to locate sound and not get lost behind background noise

The 2 systems cross over and fuse together. You are using both ears.

Question 12

What does the cochlear nucleus do?

Answer 12

(first relay centre)

Auditory nerve fibres terminate.

1) Relay Nucleus: Some nerual integration to extract features of sound (e.g. onset and offset, noise vs tones)

2) Somatosensory influence (source of tinnitus?).

This is important for ‘cleaning” the response and removing biological noise (e.g. heart and respiratory noise)

Question 13

What does the Superior Olivary Complex do?

Answer 13

Superior Olivary Complex (first Binaural Centre- Respond to Both Ears)

• Major binaural hearing + integrated centre. It is important for determining interaural timing and intensity (compare difference from each ear)
• Role in s_ound localisation_ and detecting speech in noise (extract info directly from diff locations)

Question 14

What does the Inferior Colliculus do?

Answer 14

Major i_ntegrative centre_. Neurons are very complex.

• I_nteraction with somatosensory_ and visual systems
• Integrate hearing and vision enhance sound localisation

Question 15

How does the Mediate Geniculate Body of Thalamus contribute to the processing of sound?

Answer 15

Medial Geniculate Body of Thalamus Radiates to Auditory Cortex In heschel’s gyrus of temporal lobe

Spatial localisation and speech recognition through interaction with language centres such as Wernike’s and Broca’s areas.

Question 16

What sort of processing of sound occurs at:

1) The cochlear level
2) Brainstem/cortex

Answer 16

At the cochlear level: Extraction of

1) frequency, 2) intensity and 3) timing

At neurons of brainstem and cortex:

1) Tones and more complex sounds (e.g. noise, transients, time-varying sounds)

Question 17

The cells in the primary auditory cortex is organised _______

Answer 17

Tonotopically

(500Hz medially and ~18.000 Hz laterally)

Question 18

Describe the Plasticity of Auditory Systems

Answer 18

Auditory systems show high degree of plasticity. Cortex tonotopic organisation becomes deranged with hearing loss

With high frequency hearing loss, cortex responding to high freq no longer stimulated. This results in over-representation of low frequency in adutiroy cortex (reorganisation occurs)

E.g. Insertion and electrical tonotpic activation of cochlea with cochlear implant restores tonotopic map (to a certain extent). This may underpin success of implant due to aduitory system’s plasticity.

Question 19

BRIEFLY describe the different parts of the cochlear nucleus

Answer 19

The CN has 3 main divisions

• Dorsal cochlear nucleus (DCN) is dominate part of human CN, projects directly to midbrain (inferior colliculus). It processes and integrates info.
• Ventral cochlear nucleus (anterior and posterior) (VCN) projects to superior olivery complex. It is important for sound localisation.

Question 20

Describe the Dorsal Cochlear Nucleus

Answer 20

DCN pyramidal/fusiform cell layer has input from auditory nerve, and integrate with other somatosensory systems such as vestibular system, trigeminal nervous system, descending spinal motor path.

Important to speech (movement of face to speak, turn head to sound source). Source of tinnitus (integrate with trigeminal)?

Question 21

Describe the Ventral Cochlear Nucleus (anterior and posterior)

Answer 21

VCN bushy cells has similar responses to type 1 primary auditory afferents:

• Project to superior olivary complex
• Precise relay of timing information about onset of sound essential for measuring “inter-ear” differences in sound arrival for localisation (integration with somatosensory and visual systems.

Question 22

Why is Binaural Auditory Response so important? ** EXAM?

Answer 22

At all levels above cochlear nucleus, binaural (2 ears) responses dominate. This is very important to:

• Extracting speech in noise (visualise speech, compare response from both ears to extract),
• Localising sound in 3D space,
• Processing speech.

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We are more sensitive to differences in intensity between ears.

• In this example, bigger physiological response from binaural input than from single ears.

Nerves (spatial neurons) are sensitive to differences in time of arrival of sound between ears

• In this example, neurones in SOC are maximally responsive to sounds coming more from the left ear (just off midline).
• Underlies perception of sound in space

Question 23

Sound localisation occurs through ______ and ______

Answer 23

Sound localisation through detection of interaural time and intensity

Question 24

Describe the Processesing of Interaural Timing Cues

Answer 24

Processing Interaural Timing Cues (Coincidence Detector)

Cells in superior olivary complex receive simultaneous excitatory input from both ears, excited more with certain interaural time delays. Hence sound localisation!

• Some neurons are slightly longer than others. When they get to central point, if signal is from one ear, it is going to travel further down the neuron from that ear than the other side.
• Difference in path length due to time delays cause signals meet at a certain point to produce a stimulus.