Lecture 15 Hearing II Flashcards
Central auditory nerve processing activity allows for the perception of…….
These functions (culminate in perception of subtle speech nuances) allow for…
Loudness (intensity), pitch(frequency) and sound direction.
Social communication and sound localisation
The central auditory pathway
Auditory pathways compared to visual pathways and why
Spiral ganglia -> auditory complex (temporal lobe) -> hearing, speech, language centres overlap.
More complex than visual -> more relay nuclei from peripheral organ (cochlea) to cortex, several alternative pathway from cochlea to auditory cortex
Describe the complicated central auditory pathway
{Draw mindmap for this with card 4}
Spiral ganglion (1° auditory neurons) -> auditory nerve -> ventral cochlear nucleus [dorsal cochlear nucleus] -> superior olivary complex (1st relay station) -> ipsilateral superior olivary complex [contralateral superior olivary complex] -> lateral lemniscus -> inferior colliculus (midbrain) -> medial geniculate nucleus (thalamus) -> auditory complex (final & complex sound analysis - determine sound complexities)
-> contact superior colliculus : integration of visual & auditory info
Three features of central auditory pathways and examples
1. Other projections and brain stem nuclei contribute to auditory pathways.
2. Extensive feedback in auditory pathway
3. Auditory nuclei in brain stem receive input from both ears
- DCN - straight to inferior colliculus. Inferior colliculus receives input from DCN and VCN
- Superior olivary complex: neurons send efferent innervation to stimulate OHC
Auditory cortex: send info to MGN & inferior colliculus - Starts from superior olivary complex
Analysing sound frequency in auditory nerve
- increased intensity means
- characteristic frequency is
- increased firing rate
- highest response to a certain frequency. (That frequency has most number of spikes due to higher firing rate)
Stimulus intensity depends on
The firing rate of a given neuron and the number of neurons firing.
(More movement of basilar membrane, more sensory hair cells activated, more neurons employed, higher intensity)
- higher frequency - higher basilar membrane movement
- lower frequency - lower basilar membrane movement
Frequency encoding at
- very low frequency
- intermediate frequency
- high frequency
- phase locking (response of each neuron at every cycle. AP fired at a consistent part of the wave.)
- volley principle (phase locking + tonotopy ) [response phase locked not at every cycle but is constant - ex. Every 4 cycles]
- rely exclusively on tonotopy (AP fired randomly, irregularly. Cannot analyse sound frequency).
Sound localisation is important…..
How many ears are needed for sound localisation on the horizontal plane compared to the vertical plane?
…to know where the sound is coming from
Horizontal: both ears , Vertical: one ear enough
What are the two processes used for localising sound on the horizontal plane?
Explain the processes
Interaural time delay (low frequency - 20-2000Hz).
- analysed by specialised neurons in SON.
- experienced when sound comes at you at angle between 45° and 90°.
- sound from right: reach right ear, after delay left ear.
- sound in front no delay
Interaural intensity difference (high frequency - 2-20kHz)
- sound shadow cast on area opposite from where sound comes from. (The ear (sound shadow) -> softer sound)
- high frequency sound so no tell difference of sound waves
How is Interaural time delay analysed in the SON
Binaural neurons in SON (1st relay station) receive info from both ears -> analyse time delay with low frequency sound.
delay time and interaural time delay
If sound is coming from the left ear, how is this sound analysed by the SON for interaural time delay?
Activity initiated in left cochlear nucleus -> synaptic activity sent to SON by auditory nerve.
Sound reach right ear -> synaptic activity stimulated in right cochlear nucleus. First impulse (left ear) travelled further.
Both impulse will the reach the same neuron in SON -> sum of synaptic potential generates AP -> signal that sound from left.
What are the neurons that are responsive to high frequency sounds in the superior olivary nucleus?
What are their characteristics?
- Type excitatory- excitatory neuron (EE): respond (highest firing rate) with excitation from contralateral and ipsilateral ear.
- Type excitatory- inhibitory neuron (EI): excited by sound from contralateral ear & inhibited by sound from ipsilateral ear.
-sensitive to intensity, present in auditory cortex and pathways, anatomical basis of sound localisation processes on horizontal plane
How is sound localized on the vertical plane?
The sound difference between direct and reflected sound is analysed. Each difference is analyzed on vertical plane and know if sound above or below
External ear : bump & ridges - reflect sound
The primary / secondary auditory cortex
Function and features
- most complex sound analysis occurs with highest level of frequency and intensity tuning (neurons in really nuclei [auditory pathway] analyse sound info [intensity, frequency, localisation]. Higher the relay nucleus, more complex analysis)
- high frequency sounds located medial, low frequency sounds located lateral
What is the ice cube model
Shows how complex sounds are analyzed by 1°A cortex.
