How the Environment Shapes Language in the Brain
Sign language is similar to spoken language in fundamental ways: their linguistic structure is similar, they are produced and comprehended in similar ways, and the language regions of the brain’s left hemisphere are responsible for language in both modalities. However, in a series of studies we have found that these statements are true only if the child experiences language used in the environment from birth. Because deaf children cannot hear the language spoken around them, and cannot see sign language when it is absent from the environment, they often experience language for the first time at older ages well past infancy when their hearing peers have already mastered language.
Using this unique situation, we have discovered that linguistic stimulation during early life is necessary for the human language capacity to develop fully. The longer the child matures without language, the more atypical linguistic functioning and brain language processing become in adulthood. Thus, the universal human ability to learn language and the ability of the traditional language regions of the brain to process language crucially depend upon the timing of linguistic experience in early human development.