The Auditory System

• What is the purpose of the Peripheral Auditory System?

  • to collect complex sound (sound waves from different sources)and break it down into constituent frequencies - extract multiple different features of that sound source such as frequency, intensity, localisation
  • auditory objects/streams = different building blocks that belong together are grouped together to separate them from other sounds so any appropriate meaning can be extracted
  • Sound = longitudinal waves (air molecules travel back and forth , parallel to wave's propagation)

  *amplitude of vibration

  *frequency of vibration
• 3 Components of the Ear:

  • External Ear = pinna (cartilaginous flap on the side of the head) and the ear canal (external auditory meatus - 2.5cm long in an adult human)
  • Middle Ear = Tympanic  membrane (eardrum) , ossicles (middle ear bones such as malleus, incus and stapes/footplate MIS) and middle ear muscles
  • Inner Ear = the cochlea (auditory sense organ), vestibular components, oval window (connected to middle ear), round window

  
• Functions of the External Ear:

  • Sound collection - pinna funnels sound waves into the ear canal
  • Directionality - shape of the pinna (the different ridges and dips etc) and 2 ears distinguish which direction a sound is coming from
  • Protection - 

  *ear canal contains hairs and earwax (cerumen) glands which protect and keep it clean 

  *length of the canal also means that the delicate middle and inner ear are physically separated from the surface of the head.
• Functions of the Middle Ear:

  • Impedance matching – efficient transfer of acoustic energy from the air to the fluid filled cochlea (without the middle ear 99.9% of the sound energy would simply bounce back)
  • Transmission of sound - through ossicles means that only one end of the fluid-filled tube of the cochlea is pushed and so the fluid is free to move in the tube (if both ends were pushed at the same time it wouldn't move)
  • Protection – ear muscles contract in response to loud sounds and a persons own vocalisation (middle ear reflex - stiffens the middle ear to protect inner ear)
• Functions of the Inner Ear:

  • Transduction – mechanical vibrations covered into nerve impulses in the cochlear branch of CN VIII
  • Processing – separation of sounds into different frequencies; amplification of vibrations to improve sensitivity; representation of different levels of sound in nerve firing rate

  *rate of vibration of the stapes is governed by the different frequencies that are present in the incoming sound

  • Two windows allow movement of the fluid within the cochlea
• Localisation of Sound:

  • uses binaural hearing (both ears) to tell where a sound is coming from on a horizontal plane
  • Interaural time difference (ITD) – distance b/w 2 ears (23cm) takes sound longer to arrive at the farthest ear (0-0.7ms) depending on the position of the source
  • Interaural intensity difference (IID) – higher freqs unable to diffract around head to further ear, leaving a "shadow effect" (sounds can be 20 dB less intense)
  • The auditory system is able to carry this precise timing information associated with ITD/IID to brainstem (superior olivary complex)
• The Auditory Pathway:

  • bilateral input to the auditory system - binaural hearing
  • cochlea performs peripheral processing - separation of frequency components
  • Cochlear nucleus in brainstem only receives direct input from the ear on that side (IPSILATERAL)
  • Sound from both ears is first combined at superior olivary complex where ITD and ILD cues are analysed to localise the sound source
  • At higher stages (inferior colliculus , medial geniculate body, auditory cortex) the auditory system starts to show links to other sensory systems - like the visual system.
• Types of Hearing Loss: CONDUCTIVE

  • occurs when there is a disruption of the transmission of acoustic energy from the external environment to the cochlea

  *blockage or deformation of the external ear

  *perforation or scarring of the ear-drum (tympanic membrane)

  *fusing/separation of the ossicles

  *blockage of the eustachian tube leading to changes in middle ear pressure

  *fluid within the middle ear cavity

  ! disruption to external and middle ear components only
• Types of Hearing Loss: Central

  • damage to the auditory centres in the brain

  *can occur following a stroke

  *can be very varied in their cause and effect

  *can be unilateral or bilateral
• Types of Hearing Loss: SENSORINEURAL

  • occur when there is a change in the structure or function of the cochlea and/or auditory nerve (INNER EAR ONLY)

  *damage/loss of outer hair cells, which reduces sensitivity of higher frequency sounds = typically noise-induced or age-related hearing loss

  *damage/loss of inner hair cells or their synapses - prevents detection of the movement of the basilar membrane so no signal can get to the auditory nerve

  *damage to the auditory nerve - damage to neurones/synapses , myelin sheath, tumours on vestibular branch of CN VIII
• Types of Hearing Loss: age-related hearing loss (presbyacusis)

  • At higher frequencies the sensitivity of the ear is less - higher frequency conversational speech are inaudible and others are much quieter
  • All the yellow area above the red line = inaudible
  • Those below but close to the red line = much quieter than normal and will require more effort listening and more difficult to distinguish from any background noise

  *increasing frequency sound = ch, sh, p, h , g, k, f, s, th

  • A flat loss of 60 dB HL will mean that all components of conversational speech are inaudible