Intensity representation 2Mechanisms 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 sensitivityDevelopment of conductive

efficiency

Development of external and middle ear

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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

4k 8k16k

Middle ear impedance level

Absorption level into middle ear

Compare to threshold development

Absolute sensitivityCochlear sensitivity?

Click-evoked ABR threshold over age (preterm infants)

Tone-burst evoked ABR thresholds (newborns)

Tone-burst evoked ABR thresholds (infants)

Absolute sensitivityNeural 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.