Development of hearing. Part III. Postnatal development.

As humans, we hear the way we do because of at least three major forces. The first is phylogeny, the evolutionary changes in the auditory system since its beginnings. The second is embryology, the formation of the system in each individual. Finally, there is the interaction between the biologically determined mechanism we are born with and the environment. This series of articles reviews each aspect in turn, so that we may have a fuller appreciation of how it is we come to hear the way we do. Part I examined how the auditory system evolved, and Part II outlined the sequence of ear formation in prenatal life. In this concluding article, we examine the development of auditory perception from when we first hear (in utero) through 12 months of age, which is when speech usually appears. Humans seem to come "programmed" to perform all the processes necessary to learn the language of their environment. Interaction with the environment hones our abilities to process sound and thereby optimizes language acquisition and production.