Biologic Development of the Auditory System

Question 1

When do we first “hear”?

Answer 1

• Anatomic connections of the cochlea to brain are complete ~10-20 weeks before birth
• ABR waveforms can be measured in 15-week premature neonates
• Startle reflexes documented at 24-25 weeks gestational age

Question 2

When is auditory development complete?

Answer 2

• Simple patterns of neural activity in the primary auditory cortex: 1-2 years
• More complex auditory processing: Late puberty
• Higher levels of cortex are always plastic
  - Auditory memory, speech-language regions, speech motor area
  - Does development of the auditory system end?

Question 3

Describe the early growth of the auditory periphery.

Answer 3

• Inner and outer hair cells differentiate in the otocyst at ~8-10 weeks
• Tectorial membrane develops and matures much later
  - Therefore, low threshold signal detection and cochlear frequency selectivity are not evident until late stages of gestation

Question 4

Describe “hearing” development.

Answer 4

• Cochlear frequency sensitivity not present until 30-35 weeks gestation due to development of tectorial membrane
  
  • Mid-frequency region is the first to develop and connect with the brainstem
  • Development of more apical and basal areas follow
  • 6 months after birth, high- and low-frequency thresholds continue to improve

Question 5

Describe the developmental plasticity of the binaural hearing system.

Answer 5

• Studies of unilateral cochlear ablation have provided useful info re: auditory neuroplasticity
• Typically, the medial and lateral nuclei of the superior olive and the medial nucleus of the trapezoidal body receive input from both cochleas
  - Following ablation of one cochlea, the nuclei receive two inputs originating from the same side
  
  • This significantly changes the response properties of neurons in the brainstem and midbrain

Question 6

Describe asymmetric input during infancy.

Answer 6

• Congenital unilateral hearing loss
• Unilateral cochlear implant and contralateral profound HL
• Unilateral conductive HL (e.g. chronic OM)
  
  • May result in impaired auditory function and delays
• Bilateral HL but unilateral amplification

Question 7

Describe auditory connections at the level of the cortex.

Answer 7

• System plasticity is greater at the cortical level than in more peripheral areas
  - More pathways of connectivity including projections to multimodal

Question 8

Describe clinical implications of auditory system plasticity in the case of HL.

Answer 8

• SNHL leads to the development of isofrequency regions (may play a role in tinnitus and loudness recruitment)
• Early Hearing Loss Detection and Intervention
  - A child that goes undiagnosed may face problems in language development
  
  • Highlights importance of intervention during the critical period of language development