On the other hand, the study of blind children's development constitutes a unique opportunity to study the effect of vision on development. Thus, congenitally blind children without added handicaps allow us to study how lack of vision affects human development. In this regard, congenitally blind children provide us with what can be called a natural experiment. For this reason, the study of blind children's development will help us to get a better understanding of more general issues surrounding developmental processes. Lewis and Collis (1997b) have suggested a number of areas which can be illuminated by the findings of the studies of children with disabilities. These may help scholars to elucidate whether certain achievements are a prerequisite for later achievements, as well as interdependencies
between developmental processes. For example, the issue of the cogmtlve underpinnings of a first language may be tested with data from research with blind children, who, in general, show a delay in their early cognitive development (see Chapter 2). As Lewis and Collis (1997b) point out, the fact that two different behaviours emerge at the same time in normally developing children does not imply that there exists a relationship of dependency between them. The results of the studies of children with disabilities may help us to view things from a different perspective. These results may also be useful to readjust the importance given to certain aspects of psychological functioning. Traditionally, normal children have been seen as a benchmark of achievement and as exemplars of what areas may be of interest in development. This approach disregards or overlooks important phenomena which may arise more clearly in children with disabilities. Concerning vision in particular, Developmental Psychology has traditionally given visual information an important role in development to the detriment of other aspects of development, such as language and the information it provides to human beings. The study of blind children's development may contribute to these issues being reconsidered. We need to have bi-directional influences in our understanding of development from research with normal, sighted children and from research with children with disability/impairments such as blindness.