Intensity representation 2 Mechanisms underlying the development of intensity representation
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.
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
Absolute sensitivity Development of conductive efficiency
Development of external and middle ear
Development of pinna
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.
Transfer function of the external ear 4k8k 16k
Middle ear impedance level
Absorption level into middle ear
Compare to threshold development
Absolute sensitivity Cochlear sensitivity?
Click-evoked ABR threshold over age (preterm infants)
Tone-burst evoked ABR thresholds (newborns)
Tone-burst evoked ABR thresholds (infants)
Absolute sensitivity Neural transmission
Neural transmission continues to develop during infancy
Three-month-olds’ absolute threshold is correlated with I-V interval
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?
Growth of cochlear microphonic with increasing intensity (rats)
Growth of ABR Wave V amplitude with increasing intensity (infants)
What else do we need to know? Cochlear sensitivity Growth of neural response with age Older ages More central parts of the nervous system
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.