Audition 2

What is the auditory pathway?

• cochlear nucleus - medulla (brainstem)

• superior olivary complex - pons (brainstem)

• nuclei of lateral lemniscus - pons (brainstem)

• inferior colliculus - midbrain (brainstem)

• medial geniculate body/nucleus? - thalamus

What forms the axons that travel through the auditory nerve and connect hair cells to cochlear nucleus?

spiral ganglion neurons

What do IHCs and OHCs connect to respectively?

• type 1 AN fibres

• type 2 AN fibres

What are type 1 fibres like?

• thick, myelinated, fast signal transmission

• many more

• more specific connections

  • each inner hair cells innervated by 10-20 type 1 fibres

What are type 2 fibres like?

• unmyelinated and slow

• more minor role

• less specific

  • share with around 10 other hair cells

What do different types of AN fibres differ in?

their thresholds - they start firing at different intensities as they differ in sensitivty and dynamic range

Can a single auditory fibre encode the entire range of audible stimuli?

no, multiple help to process and encode different sound intensities, dynamic range

What can type 1 fibres be subdivided into?

• high, medium and low spontaneous rate fibres

• high SR most sensitive

What is rate code?

AN fibres increase their firing rate as a function of sound intensity

What is frequency tuning?

discharge rate depends on the amount of acoustic energy at or near the neuron’s characterisitc frequency - dependent on basilar membrane mechanics

Is tonotopy maintained in the auditory nerve?

yes

What is phase locking?

discharges of auditory nerve fibres to low frequency sounds are not random, they occur at particular times

What does phase locking cause to occur then? (2)

• spike time intervals with phase locking encode temporal features of the stimulus

• sets upper limit for high frequency sounds

The nerve fibre is stochastic. What does that mean?

• Each individual is not particularly informative about the stimulus

• Combined spikes from a number can determine lots about the temporal distribution

What does the fact that there is virtually no AC response for freq above 3kHz set?

an upper limit to the temporal resolution of the temporal patterns that can be encoded by phase locking

Summary of the auditory nerve?

Where does the base vs apex of the basilar membrane project to in the cochlear nucleus?

base - medial CN

apex - lateral CN

What are the different cell types in the cochlear nucleus?

• pyramidal cell

• octopus cell

• globular bushy cell

• multipolar cell

• spherical bushy cell

What do bushy cells do?

• in anteroventral cochlear nucleus

• preserve temporal information contained in phase locking and project to the superior olivary nuclei

What do multipolar and pyramidal cells do?

• in posteroventral cochlear nucleus (MP)

• in dorsal cochlear nucelus (pyr)

• may use lateral inhibition to extract spacial contrast and project to inferior colliculus

What do octopus cells do?

• in posteroventral cochlear nucleus

• encode temporal pattern information across many AN fibres or encode sound intensity

• project to inferior colliculus or lateral lemniscus

Which structure has the first stage of binaural convergence?

superior olivary nuclei

What input does the medial superior olive receive?

excitatory input from both sides

What input does the lateral superior olive recieve?

• excitatory input from the ipsilateral side

• inhibitory input from the contralateral sde via the medial nucleus of trapezoid body

What are interaural time differences?

• sounds arrive earlier in the near ear

• neuron fires maximally if it receives inputs from both ears at the same time

• if slight difference - won’t peak so neural code will tell brain not coming from same distance

What are interaural level differences?

• sounds are louder in the near ear

• so inhibition from contralateral ear helps to understand where sound localised from?

Are interaural level differences frequency dependent?

yes - at higher sound frequencies ILDs tend to become larger, more complex and hence potentially more informative

What is the LSO sensitive to?

• interaural level differences

• respond best to sounds that are louder in the ipsilateral ear

What is the MSO sensitive to?

• interaural time delays

• receive direct exitatory input from both ears therefore fire strongly only when the inputs are temporarily co-incident

What is the proposed model for ITDs?

Jeffress delay line and coincidence detector model

Who suggests that the model does not provide accurate descriptions for mammals (only birds)?

McAlpine and Grothe (2003)

General structure and function of inferior colliculus?

• collects and integrates input from all auditory brainstem nuclei

• obligatory relay for all ascending auditory information

• most inputs from other hemisphere so neurons strongly excited by soudns presented in contralateral ear

• strong connection between IC hemispheres allows for further binaural interaction

Is there tonotopy in the IC?

yes

What is thought to be integrated at the IC?

non-acoustic inputs

What is the structure of the medial geniculate body?

• 3 major nuclei

  • ventral MGB

  • medial MGB

  • dorsal MGB

What does the ventral part receive?

input mostly from the central nucelus of the IC - tonotopically organised

What does the dorsal and medial parts receive?

more input from the shell of the IC but also from non-auditory structures (strongly modulated by other sensory systems)

Is the primary auditory cortex tonotopically organised?

yes but breaks down in A2

What do higher cortical areas integrate?

• acoustic information with contextual information to make sense of acoustic input

• increasingly complex response properties in auditory cortical neurons

What can some neurons do - Eliades and Wang (2008)?

• signal mismatches between the expected sensory feedback from a vocalisation and the actual sensory feedbaack, a type of error signal that may be crucial for fine tuning our vocal production

What may also control flow of information in subcortical structures?

descending projections from L5 and L6 that target the thalamus, IC and brainstem

What are cortical circuits like?

highly plastic - can readily change their response properties as a result of new experiences suggesting cortex essential for perceptual learning

Basic description of AN?

encodes stimulus intensity by discharge rate and uses tonotopy and phase locking to encode stimulus frequency and temporal patterns