Language as a species-specific ability in humans is intrinsic to the development of knowledge and understanding. It endows us with a capacity unique to the human species to structure and organize experiences through the manipulation of categorical and abstractconcepts. With increasing internalization of language comes the capacity for disengagement from the environment so that concepts exist independently of their immediate context, as internal repre-sentations. Through this capacity to organize and form internal rep-resentations of experiences, a stable construction of reality is made possible (Bunowski and Bellugi, 1970; Guidano and Liotti, 1983).
Language is thus an integral part of mentation. Language is also central to modes of communication. Both of these aspects of language have been subjects of study for many disciplines, including philosophy, psycholinguistics, psychology and cogni-tive sciences. Furthermore, the disconnection of language from other cognitive processes and the disruptions in its communica-tive functions have been subjects of investigation in the fields of neurology, neurosurgery, neurophysiology and neuropsychology. Multidisciplinary efforts have led to a burgeoning body of re-search on language in normal and brain-damaged individuals, and the converging findings have greatly advanced our understanding of language as a complex human ability.
Regardless of the discipline within which the study of lan-guage is attempted, its essential features are common. Language is governed by a set of rules that link its various components. The ba-sic units of a sound-based language (e.g., as opposed to a sign-based language) are classified in terms of phonemes, morphemes, lexi-con, syntax, semantics, prosody and discourse. Phonemes are the smallest units of sound; morphemes are the smallest meaningful word units that when combined form words; syntax refers to the relational features by which words are combined, that is, grammar; lexicon refers to the words or vocabulary of a language; semantics refers to the meaning of words and sentences; prosody means the inflection and rhythm of utterances; and discourse involves the combination of sentences within any given context and constitutes narratives (Damasio and Damasio, 1992).