Hearing #3

Question 1

What are the levels of auditory processing from SOUND ⇒ BEHAVIOUR

Answer 1

1. Sound
2. Encoding of the signal via the cochlea. 20,000 nerve signals carrying info about that sound
3. Feature analysis (about timing and intensity) via ascending pathways to PAC
4. Abstraction in the primary auditory cortex
5. Passed onto other regions of the cortex for ‘cross-modal analysis’; eg with visual and sensory areas
6. BEHAVIOUR

Question 2

Why is it that if a young childs hearing diability is left too long, then a cochlear implant may not be effective?

Answer 2

Because over time the visual system will have taken over the auditory cortex.

Question 3

What exactly is hearing loss?

Answer 3

Loss of sensitivity and frequency tuning, speech perception and hearing in noise.

Question 4

How are hair cells innervated?

Answer 4

• Mainly by afferent nerve fibres of CN VIII in inner hair cells
• Fibres travel through the internal acoustic meatus, come down through a hole in the basilar membrane and have their cell bodies within the spiral ganglion.

Question 5

What types of nerve fibres innervate the hair cells?

Answer 5

Type 1: Innervate IHC (90-95%), about 20 fibres to each hair cell

Type 2: Innervate OHC (5-10%), very sparse, distributed pattern of innervation.

Question 6

Each auditory nerve _____ is discretly tuned to a narrow band specific _____ stimulus corresponding to that location on the _____.

The basal end responds to _____ frequencies, whilst the apical end responds to far more ______ frequencies.

Answer 6

Each auditory nerve fibre is tuned to a narrow band specificfrequency stimulus corresponding to that discrete location on the cochlea

The basal end responds to higher frequencies, whilst the apical end responds to far lower frequencies.

This = tonotopic organisation of the cochlea

Question 7

What actually determines the frequency response of the cochlea?

Answer 7

Variations in stiffness of the basilar membrane.

Question 8

How does that tonotopic organisation of the cochlea organised in the brain?

Answer 8

It is the same! There is also a tonotopic organisation throughout the auditory system, that uses the same frequency/pitch detection

Question 9

What are the 2 principles that cause frequency coding in the auditory nerve?

Answer 9

1. Place principle: high frequencies
2. Volley Principle: Low frequencies

Question 10

What is the basis of the Place Principle?

Answer 10

Cochlea is a filter and is tonotopically organised so frequency can be detected spatially from base to apex → detection in a discrete place in the cortex.

Question 11

Explain the basis of the volley principle.

Answer 11

Low frequencies are temporal firing of nerve fibres in time to the frequency of the stimulus.

as stereocilia move 1000x per sec it’ll fire a 1000x per second.

BUT due to the refractory period a fibre cannot fire more then 1000x per second, so only applies for low frequencies

Question 12

How is intensity of sound coded in the auditory nerve?

Answer 12

1. Rate of firing of the individual nerve fibres increase with intensity
2. Number of fibres increases as energy spreads along the organ of corti

Question 13

What nerve takes the auditory info to the brain, and what path does it take?

Answer 13

the Vestibulocochlear Nerve

Goes through the internal acoustic meatus of the temporal bone, to the brainstem.

Goes to the coclear nucleus of the pons-medullary junction.

Question 14

Why is the binaural aspect of the auditory system so important?

Answer 14

W

Question 15

Do a step-by-step summary of the auditory pathways.

Answer 15

1. Cochlea
2. Cochlear Nucleus (pons)
3. Superior Olivary Complex (pons) 1^st binaural area
4. Inferior Colliculus (midbrain)
5. Medial Geniculate Body (thalamus)
6. Primary auditory cortex

As you go up it gets increasingly compex with more processing.

Question 16

What happens at the Cochlear Nucleus?

Answer 16

• Terminal end-point for all auditory nerve fibres.
• Neurons here respond to noise, tones, onset, offset (complex info)
• Clean-up area for biological noise of your body
• Lots of somatosensory influence.

Question 17

What happens at the Superior Olivary Complex?

Answer 17

• Major binaural hearing centre; determines interaura; timing and intensity, by comparing info from each side.
• Speech localisation and detecting speech in noise.

Question 18

What happens at the Inferior Colliculus?

Answer 18

• Major intergative centre
• Interacts with visual + somatosensory systems
• Integrate hearing and vision to enhance sound localisation

Question 19

What happens at the Medial Geniculate Body of the thalamus?

Answer 19

It radiates to the auditory cortex in heschels gyrus of the temporal lobe

• Spatial localistation and speech recognition through interaction with language centres such as Wernicke’s and Broca’s

Question 20

What happens to the tonotopic organisation of the brain with Hearing Loss?

Answer 20

The cortex’s tonotopically organisation becomes deranged with hearing loss.

The cells in the cortex that used to respond to the now absent frequencies are not being stimulated, and actually start to respond to different frequencies → over-representation of frequencies that remain!

Could be the reason of tinnitus/ hyper-acutness

Question 21

How can a deranged tonotopic map be restored?

Answer 21

Via an electrical cochlear implant, works so well because it can use electrically stimulation to create the same pattern. Shows the plasticity of the brain!

Question 22

What are the different portions of the cochlear Nucleus and what do they do?

Answer 22

A small nucleus around the pons-medullary junction.

1. Ventral Cochlear Nucleus: Extracts signals, passes those to the brainstem for sound localisation
2. Dorsal Cochlear Nucleus: similar to the cerebellum, network of pyramidal cells, huge interaction with somatosensory information complex

Already started meshing information with other sensory information to allow you to move your head to the sound, for better sound localisation.

Question 23

What types of nerve fibres are in the ventral cochlear nucleus

Answer 23

Sends signals similar to the primary auditory afferents, projects to the superior olivary complex.

Precise relay of timing/onset for sound localisation

Question 24

At all levels above the ___________, binaural responses dominate, why is this important?

Answer 24

Important for

• Extracting speech in noise
• localising sound in 3D space
• Processing speech

Question 25

Some neurons at the superior olivary complex aren't even sensitive to differences in intensity, but instead to spatial timing of the sound,

Answer 25

This is a part of how you can hear where a sound is coming from in space. As the sound takes time to travel through space, if it was coming from the right side it'll hit my right ear before the left side. Also my head provides a shadow of the sound, decreasing the intensity. The brain uses that information to determine where the sound is coming from spatially.

Question 26

What's the Coincidence detector?

Answer 26

Neurons go to the SOC, when sound arrives from my right side the neurons on the right side of the pons will be activated earlier then the left BUT if the pathway on the right is therefore longer they'll arrive at the same time. That simultaneous activation gives information about where the sound is actually coming from for **sound localisation!**