Human developmental psychobiology seeks to link emerging psychological and behavioral functions with their underlying neural mechanisms. The development of perception, cognition, emotion and motor performance involves the interaction of environmental experience with the maturing brain (see Greenough, chapter 5). However, maturational changes in the neural substrates that underlie these psychological phenomena re as yet not well defined. There is substantially more information on the facts of psychological development han on their biological underpinnings. Although in principle the relations between brain maturation and psychological development can be addressed at many levels in experimental animals, these investigations are limited in the range of cognitive processes that can be examined. Specifically, language and the cognitive operations it supports can only be studied in humans (see Bellugi, chapter 6).