This chapter briefly considers the meaning and consequences of congenital blindness in order to provide a framework from which the present research can be better understood. It defines “blindness,” discusses the effects of blindness on selected aspects of cognitive organization, and discusses the development of very young blind children, culminating with a summary of research on language acquisition in this special population.

The nature of blindness and its prevalence in childhood

Most people think of blindness as the total absence of visual sensation and they imagine that a blind person is someone who experiences the world through a kind of black void. In fact, blindness cannot be viewed in terms of an absolute dichotomy between the presence and absence of visual information. The popular conception of the blind person characterizes only a very small proportion of the blind population, since the vast majority of “blind” individuals experience and utilize some visual information, though this may be limited to sensing direct light.

The generally accepted legal definition of blindness in the United States requires that central visual acuity in the better eye with best correction be no more than 20/200 Snellen. The Snellen measurement is derived from the familiar Snellen eye chart, developed in 1862.