An enormous amount of research and theorizing has been produced in an attempt to describe and explain the course of cognitive development. The major approaches to cognitive development stem from the work of Piaget, from the information-processing theorists of cognitive science, and from Vygotsky. Piagetian theory emphasizes the individual child as the virtually independent constructor of his or her own development, an emphasis that undervalues the contribution of other people to cognitive development and excludes teaching and cultural influences. The development of Parallel Distributed Processing or Connectionism promises progress with some of the cognitive areas that are less logical and sequential, such as the structure and acquisition of concepts. Cognitive development is the individual construction of an internal mental model of external reality. Becoming literate is a central part of cognitive development for several reasons: literacy involves many linguistic, perceptual, attentional, memory, and cognitive skills.