There is no data to show any difference in development between children living in cities and those in the countryside, between those living in Africa and those living among the skyscrapers of New York. However, studies on environmental influence are invaluable in showing just how importance this is with regard to development. The nervous system is formed quite early. Within the first six years of life, it has already reached an advanced stage of development. The first years of life are therefore vital. Some crucial aspects of development, such as speech, eyesight and hearing, occur at critical times in the life of a child. If, during these critical periods, the child does not experience stimulation of certain functions, then they may be lost. This is why it is considered necessary now, in cases of severe deafness, to take action using implants or prosthetics during the very first years of life.