Lecture 20: Hearing 3: Neural Processing of Sound Flashcards
Which cranial nerve is responsible for the neural processing of sound?
8th cranial nerve- vestibulocochlear nerve.
Type 1 spiral ganglion innervates _________
Type 2 spiral ganglion innervates ________
1: IHC (90-95% of nerve fibres)
2: OHC (5-10% nerve fibres)
Briefly describe the distribution of the audtiroy nerve fibres
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.
Describe the frequency coding method in the auditory nerve
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
_______ of __________determines the frequency response
Variations in stiffness of the basilar membrane determines frequency response
(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)
Base of the cochlea is narrow and stiff
Aplex of the cochlea is wide and floppy
How is the intensity coded in the auditory nerve?
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
Contralateral pathway is _____________
Excitatory
Ipsilateral pathway is ___________-
inhibitory
Name the pathways from the cochlea to the auditory cortex
Cochear
Cochlear nucleus
Superior Olivary complex
Inferior colliculus
Medial Geniculate Body of thalamus
Auditory cortex
Tonotopic representation of sound is presented throughout the system
Processing of Sound in the Central Auditory System is Binaural. Why is this improtant?
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.
What does the cochlear nucleus do?
(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)
What does the Superior Olivary Complex do?
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)
What does the Inferior Colliculus do?
Major i_ntegrative centre_. Neurons are very complex.
- I_nteraction with somatosensory_ and visual systems
- Integrate hearing and vision enhance sound localisation
How does the Mediate Geniculate Body of Thalamus contribute to the processing of sound?
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.
What sort of processing of sound occurs at:
1) The cochlear level
2) Brainstem/cortex
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)
The cells in the primary auditory cortex is organised _______
Tonotopically
(500Hz medially and ~18.000 Hz laterally)
Describe the Plasticity of Auditory Systems
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.
BRIEFLY describe the different parts of the cochlear nucleus
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.
Describe the Dorsal Cochlear Nucleus
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)?
Describe the Ventral Cochlear Nucleus (anterior and posterior)
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.
Why is Binaural Auditory Response so important? ** EXAM?
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.
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
Sound localisation occurs through ______ and ______
Sound localisation through detection of interaural time and intensity
Describe the Processesing of Interaural Timing Cues
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.
