Neurobiology of Specific Language Impairment
This review summarizes what is known about the neurobiology of specific language impairment. Despite its name, specific language impairment is frequently not specific. It is common to find associated impairments in motor skills, cognitive function, attention, and reading in children who meet criteria for specific language impairment. There is evidence that limitation in phonologic working memory may be a core deficit in specific language impairment. Both genetic and environmental factors have been shown to be important etiologic factors in specific language impairment. Structural neuroimaging studies suggest that atypical patterns of asymmetry of language cortex, white-matter abnormalities, and cortical dysplasia may be associated with specific language impairment. Abnormalities in the later stages of auditory processing have been demonstrated using auditory event-related potentials. Functional neuroimaging may cast further light on the neurobiology of specific language impairment and serve as a means of developing and evaluating therapy. A better understanding of the neurobiology of specific language impairment is critical for the rational development of therapeutic strategies to treat this common disorder.

The process of acquiring language is complex and not yet fully understood. Language is a recent evolutionary phenomenon. It is likely that the linguistic computational abilities that underlie the complexity of human language have evolved over the past 6 million years. In most children, the potential for language is present at birth. However, its eventual development represents a dynamic interaction between the developing brain and a child's environment. Most children acquire language through environmental exposure to language without any special assistance. However, for some children with otherwise normal development, the acquisition of language proves disproportionately difficult. Despite relatively normal nonlinguistic cognitive function and normal hearing, their language development is impaired. These children are considered to have specific language impairment.

With an estimated prevalence of 7.4% in kindergarten children, specific language impairment is a common childhood developmental disorder. It potentially leads to ongoing impairment in an individual's communication, academic, and social skills. It thus represents a major public health problem. Unlike children with global developmental delay, it is very uncommon to identify a cause of language impairment among children who meet criteria for specific language impairment. Despite the high prevalence of specific language impairment, its causes and biologic basis are, at present, incompletely understood.

This review aims to summarize what is known about the biologic causes of specific language impairment. Problems defining specific language impairment are discussed first. Any theoretic framework that attempts to address the causes of specific language impairment needs to account for the characteristic pattern of language deficits seen among children with specific language impairment and for impairments seen in other neurologic functions. There is evidence that children with specific language impairment have limitations in verbal working memory and difficulties processing rapidly changing sounds. There is also evidence that the neurophysiology of language processing is different in children with specific language impairment than in children with normal language. Studies reporting that children with specific language impairment have brains that are morphologically different from normally developing children are reviewed. Finally, theories of the factors limiting language processing in specific language impairment are discussed.