Hearing 2

• Innervation of organ of Corti


  • 3,500 IHCs
  • 30,000 fibres (~95% to IHC)
  • 12,000 OHCs in 3-5 rows
• Auditory nerve fibres


  • Each fibre responds to limited range of frequency
  • As sound made louder, fibre responds to wider range of frequencies
  • Best frequency

  
• Sounds with levels and frequencies within the shaded area drive the fibre above spontaneous rate
• Coding frequency by place


  • Basilar membrane
  • Direction of travelling wave

  
• Auditory pathway in human

  1. Cochlea

  2. Spiral ganglion

  3. Ventral cochlear nucleus

  4. Dorsal cochlear nucleus

  5. Superior olive

  6. Lateral lemniscus

  7. Inferior colliculus

  8. Medial Geniculate Body (Thalamus)

  9. Auditory cortex

  
• Processing sites (nuclei) in the auditory pathway


  • Auditory nerve
  • Cochlear nucleus - (first synapse of nerve fibres)
  • Superior olivary complex - (interaction of information from two ears – see localisation later)
  • Inferior colliculus (midbrain centre)
  • Medial geniculate body (thalamic auditory nucleus)
  • Auditory cortex (in temporal lobe)

  
• Heschl's gyrus


  Primary auditory cortex

  
• Place code


  • Auditory system keeps track of where information originated on basilar membrane
  • All along the auditory pathway sound frequency is represented topographically
  • Sound frequency is mapped within processing centres and on surface of auditory cortex
  • Tonotopy (Tonotopic representation) – mapping of sound frequency

  
• Time code


  • Firing of action potentials in auditory nerve fibres synchronise to peaks in sound waveform
  • Phase locking
  • Time between action potentials tells us the frequency of the sound
  • Only works for low frequency sounds (below ~800kHz) but includes frequencies for speech and music

  
• Sound localisation


  • Position of sound not represented on basilar membrane
  • Calculate position of sound by comparing inputs from two ears
  • Ears separated by solid mass (the head!)
  • Sounds in different positions give rise to differences in timing and intensity (loudness) between the ears - interaural differences
  • Brain extracts these differences to localise sound

  
• Interaural time differences


  • Difference in onset time for transient (impulsive) sounds
  • Difference in time of arrival at the ears
  • Auditory system can detect time differences of ~10 μs for repeated clicks (c.f. action potential ~1ms)
  • Also localise continuous tones
  • Detects ongoing phase difference between the ears
  • But only for low frequencies at ~800Hz

  
• Time difference coded in streams of action potentials
• Interaural time differences between left and right ears extracted in superior olivary nuclei – groups of cells preferring different time differences, i.e. sound positions

  
• Interaural intensity differences (IID)


  • Head casts a sound shadow for high frequencies
  • Brain detects difference in sound level between the ears
  • For frequencies above 1600 Hz the dimensions of the head are greater than the wavelength of sounds. An unambiguous determination of the input direction based on interaural phase alone is not possible at these frequencies. However, the interaural level differences become larger, and these level differences are evaluated by the auditory system.

  
• Information from the two ears converges in the superior olivary complex

  
• Neurons extract difference in sound intensity between the ears

  
• Used to compute position of sound

  
• Localisation of sound - summary


  • The difference in the arrival time of the sound wave at each ear is called the interaural time difference (ITD). Each ear has different coordinates in space.
  • Interaural Level Difference (ILD/IID) corresponds to the difference of the intensity of the sound at each ear. However, this cue depends heavily on the frequency component of the sound. When it is below 1600 Hz, the ILD is almost non-existant. In contrast, for frequencies over 1600 Hz, ILD is a useful cue.
  • Distance: A moving sound source has a variable intensity level. If it becomes closer, the intensity level increases, if it gets further away the intensity decreases. This cue can only be used in a non-reverberant environment (such as an anechoic chamber). In a reverberant context, the distribution of sound waves, and as a result their respective intensity levels, is dependent on the rebound properties of the room.

  
• Two ears not just for localising


  • Binaural hearing also aids signal detection in noisy environments
  • Cocktail party effect - Ability to focus on one voice in a noisy room
  • Sound of interest easier to detect when originates from a different location to the competing noise

  
• Adding noise (to other ear) makes signal easier to detect!
• Detection of signal improved when signal and noise originate from different positions in space.
• Clinical audiogram


  • Hearing level (how different from normal)
  • Degree of hearing loss

  
• Cochlear implant


  • For patients with no cochlear function: electrical activation of cochlear nerve fibres by electrode array inserted in cochlea
  • Microphone and processor: converts sound to electrical pulses
  • Electrode array in cochlea activates nerve fibres
  • Connecting wires
  • Induction coil transmits signals across scalp and skull to implanted receiver