Auditory Cortex

Question 1

Outline the pathway for auditory processing

Answer 1

• first central synapse is in the cochlear nuclei of the medulla
• complex synaptic network in brainstem
• projection to primary auditory cortex via thalamus(medial geniculate nucleus MGN also called medial geniculate body MBG. or complex)
  ◦ lateral geniculate nucleus for vision

Question 2

What mediates the sound source localisation?

Answer 2

Network of brainstem nuclei

• organisation of brainstem auditory nuclei suggests complex processing
• ascending projections of auditory brainstem (above cochlear nuclei) show high degree of bilateral connectivity
• best understood function is sound localisation

Question 3

What does spatial localisation depend on?

Answer 3

◦ interaural time differences
◦ interaural intensity differences
‣ requires comparison of sounds from the two ears

Question 4

Describe the interaural time differences for sound source localisation

Answer 4

Jeffries Model

• medial superior olivary (MSO) receives inputs from left and right AV cochlear nucleus
• longest interaural time differences around 700 microseconds
• psychophysics indicates differences can be detected as short as 10 microseconds
• time differences used for computing location for sound frequencies up to 3kHz
• assume there is a delay in transmission to the medial superior olivary upon arrival time of sound
• neurons will be tonotopically arranged but are selective to diff interaural time delays
• sound reaches left ear first and activate cochlear then reaches dorsal cochlear nucleus
• same thing happens from other ear, but reaches ear slightly later (if sound further away from the right ear)
• post synaptic neurons are coincidence detectors
  ◦ generate spike when presynaptic AP arrive simultaneously from left and right ear
• AP will reach neuron A via neuron 1 of left ear before neuron 5 of right ear
  ◦ results in matchpoint

Question 5

What does interaural time difference require?

Answer 5

Precise spike timing

• AN afferents can phase lock in low-mid frequency range
• class of cells in anteroventral cochlear nucleus (bushy cell) show even stronger phase locking
• AVCN orjects to MSO potentially providing precise timing information