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Intensity representation 2
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  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.