Intensity representation 2

1
Intensity representation 2

• Mechanisms underlying the development of
  intensity representation

2
What needs explaining?

• Absolute sensitivity improves dramatically during
  infancy and then more slowly throughout
  childhood.
  
• Intensity discrimination and detection in noise
  are immature until 5 or 6 years of age.
  
• Temporal tasks that involve intensity resolution
  mature over the same time period.

3
What should we be looking for?

• Factors that influence absolute sensitivity
• Conductive efficiency
• Cochlear sensitivity
• Neural transmission
• Physiological noise
• Factors that influence intensity coding
• Growth of response with increasing intensity
• Variability in neural response

4
Absolute sensitivity

• Development of conductive efficiency

5
Development of external and middle ear
6
Development of pinna
7
Predictions about function of external and middle
ear

• Increasing ear canal length and pinna size will
  lead to a decrease in the resonant frequency of
  the external ear, as well as changes in spectral
  shape.
• Changes in the middle ear cavity size, and
  possible structural changes in the middle ear
  will lead to changes in the efficiency of the
  middle ear.

8
Transfer function of the external ear
4k
8k
16k
9
Middle ear impedance level
10
Absorption level into middle ear
11
Compare to threshold development
12
Absolute sensitivity

• Cochlear sensitivity?

13
Click-evoked ABR threshold over age (preterm
infants)
14
Tone-burst evoked ABR thresholds (newborns)
15
Tone-burst evoked ABR thresholds (infants)
16
Absolute sensitivity

• Neural transmission

17
Neural transmission continues to develop during
infancy
18
Three-month-olds absolute threshold is
correlated with I-V interval
19
Factors contributing to absolute threshold
development

• Conductive efficiency--all the way to
  adolescence, though effect is small after
  infancy
  
• Probably not cochlear maturity-- at least after
  term birth
  
• Neural transmission-- brainstem in early infancy
  other parts of the brain later?

20
Growth of cochlear microphonic with increasing
intensity (rats)
21
Growth of ABR Wave V amplitude with increasing
intensity (infants)
22
What else do we need to know?

• Cochlear sensitivity
• Growth of neural response with age
• Older ages
• More central parts of the nervous system

23
Conclusions

• Conductive development can account for a large
  part of absolute threshold development during
  infancy and childhood.
  
• Brainstem transmission accounts for additional
  immaturity of absolute threshold during later
  infancy.
  
• Although there are hints that neural response
  grows with increasing intensity differently in
  infants, the existing data do not support any
  conclusions about sensory bases of immature
  intensity processing beyond early infancy.