Perceptual Development: Hearing

Sound

All sounds can be described in terms of frequency, level, and changes in frequency or level over time. Frequency is the acoustic dimension that gives rise to the percept of pitch. Level is the acoustic dimension that gives rise to the percept of loudness. Changes in frequency and level over time help to identify sound sources and to determine which sounds are being produced by the same source, among other things. The ear represents a sound's spectrum, the level of sound at each frequency, as well as its waveform, the sound amplitude over time. This representation must be faithfully conveyed through the auditory nervous system for sound to be accurately perceived.

Development of the Ear

The inner ear, specifically the cochlea, is responsible for converting sound into a neural response that represents the spectrum and waveform of sound. The cochlea begins to respond to sound—for the most part sounds generated by the mother—late in the second trimester of gestation. By the last month of gestation, every indication is that the cochlea functions much as it does in adults. Numerous studies have shown that newborn infants recognize their mother's voice and other things about the sounds to which they were exposed prenatally.

For sound to get into the inner ear postnatally, however, it must be conducted through the external and middle ears. The external ear includes the pinna and the ear canal. The external ear not only leads sound into the head but also shapes the spectrum of sound. The result is that frequencies in a particular range—between 2000 and 5000 hertz (Hz) in adult males—will be delivered to the middle ear at higher levels than are other frequencies. Infants' smaller pinnas and shorter ear canals will tend to resonate at higher frequencies than the adult ear.

The middle ear contains the eardrum and the ossicles, the small bones that connect the eardrum to the inner ear. It makes it possible for airborne sound to stimulate the structures in the fluid-filled inner ear. The efficiency with which sound is conducted through the middle ear is reduced in infants. In newborns, the combined effect of the immature external and middle ears is a small reduction in sound conduction of frequencies below 2000 Hz and a larger reduction in sound conduction in frequencies above 2000 Hz. High-frequency sound conduction improves dramatically during infancy, but conductive efficiency is not completely mature until 10 to 11 years of age.

Auditory Neural Development

Immature transmission of information through the auditory brain stem also limits hearing during infancy. It appears that auditory neural transmission is inefficient early in infancy, resulting in less information about sound reaching higher levels of the brain. Information is also transmitted more slowly and less synchronously in the immature auditory brain stem. Neural responses at higher levels continue to develop well into childhood, although the role that these play in hearing development is not clear.

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