<P> A model must simply account for the means by which a process takes place . This is sometimes done in reference to changes in the brain that may correspond to changes in behavior over the course of the development . Computational accounts of development often use either symbolic, connectionist (neural network), or dynamical systems models to explain the mechanisms of development . </P> <P> Cognitive development is primarily concerned with the ways that infants and children acquire, develop, and use internal mental capabilities such as: problem solving, memory, and language . Major topics in cognitive development are the study of language acquisition and the development of perceptual and motor skills . Piaget was one of the influential early psychologists to study the development of cognitive abilities . His theory suggests that development proceeds through a set of stages from infancy to adulthood and that there is an end point or goal . </P> <P> Other accounts, such as that of Lev Vygotsky, have suggested that development does not progress through stages, but rather that the developmental process that begins at birth and continues until death is too complex for such structure and finality . Rather, from this viewpoint, developmental processes proceed more continuously . Thus, development should be analyzed, instead of treated as a product to obtain . </P> <P> K. Warner Schaie has expanded the study of cognitive development into adulthood . Rather than being stable from adolescence, Schaie sees adults as progressing in the application of their cognitive abilities . </P>

Two ways in which researchers study development over time are