Integrated development and maturation of the hearing system. A critical review article.

The understanding of development and maturation of the auditory system is essential for many reasons, including the practical aspects of auditory behaviour, testing, and teaching a hearing-impaired child to communicate effectively. The study of maturation of the auditory system is gaining increasing importance also because it should help us to interpret correctly certain aspects of auditory behaviour in infants. When studying the auditory system we should not be concerned solely with development of function in isolation; the emphasis is on integrated development. Understanding a process of integrated development enables us to understand the peculiarities of auditory behaviour in infants. Sound became the raw material of human language for good reasons. It is the only medium which made it possible to transmit efficiently complex information encoded in human language. Phylogenetically, hearing in vertebrates is a late development, and because of this, in certain unfavourable circumstances the hearing system is more vulnerable than other phylogenetically older systems. Within the auditory system, high-frequency hearing is also phylogenetically a late development, and therefore more vulnerable to certain unfavourable metabolic influences. In all species there is a 'best frequency' range, usually the one which is most vital for communication. In humans the best frequency range is the one which is most important for transmitting speech sounds. This is already noticeable in newborn and very young infants, that is, we can obtain the best reactions in the frequencies which are important for speech. Higher sensitivity for perception of patterns is already developed in newborn and young infants (and acquires a special significance). Direction detection and localisation of sound source develops gradually. It is fully developed only when the auditory pathway matures and when the function is well integrated with the maturing motor system. The ears of the young are more vulnerable to noise damage and therefore a variety of protective mechanisms exist. The quality of the sound environment of the infant is important and can be decisive for further development. It is highly desirable to detect hearing impairment as soon as possible after birth. Lack of stimulation of the auditory system, even when it functions only partially, or along a very narrow channel of communication, may lead to permanent complete loss of function of the auditory centre. But this could be prevented by early stimulation, even when it is possible only via the narrow channel of communication which fortunately is present in the great majority of cases of even severely hearing-impaired children.