Expressive language disorder is a communication disorder in which there are difficulties with verbal and written expression. It is a specific language impairment characterized by an ability to use expressive spoken language that is markedly below the appropriate level for the mental age, but with a language comprehension that is within normal limits. There can be problems with vocabulary, producing complex sentences, and remembering words, and there may or may not be abnormalities in articulation. Careful diagnosis is also important because "atypical language development can be a secondary characteristic of other physical and developmental problems that may first manifest as language problems". Models of language production Willem Levelt outlined the currently accepted theory of speech production. Words are produced after the concept waiting to be produced is conceptualized, related words are selected, encoded and the sound waves of speech are produced. Association with language networks There is also a lot of debate about whether specific language impairments, such as expressive language disorder, are caused by deficits in grammar or by a deficit in processing language information. However, an alternative hypothesis to the cause of SLIs has been posited, called the Procedural Deficit Hypothesis. The Procedural Deficit Hypothesis opines that we can explain language impairments due to abnormal development of brain structures that are involved in procedural memory, our memories that remember how to perform different cognitive and motor tasks. The procedural memory system is associated with basal-ganglia circuits in the frontal lobe and further investigation of this particular hypothesis could aid in the development of a clinical neurological picture of what the underlying causes of SLI are. Scientific studies of speech and language Some of the earliest neuroscientific discoveries were related to the discovery that damage to certain areas of the brain related to impairments in language, such as the discovery of Wernicke's area and Broca's area. Lesions in these parts of the brain impair language comprehension and language production, respectively. Paul Broca was the first to note that the left hemisphere of the brain appeared to be localized for language function, particularly for right handed patients. Modern neuroscientific research has verified this, though language may be lateralizaed to the right hemisphere in some right-handed individuals. Developmental verbal dyspraxia In 1990, it was reported that the several generations of the KE family had developmental verbal dyspraxia and orofacial praxis that were inherited in a typical autosomal dominant pattern. Further analysis