Communication disorders in speakers of tone languages: etiological bases and clinical considerations.

Lexical tones are a phonetic contrast necessary for conveying meaning in a majority of the world's languages. Various hearing, speech, and language disorders affect the ability to perceive or produce lexical tones, thereby seriously impairing individuals' communicative abilities. The number of tone language speakers is increasing, even in otherwise English-speaking nations, yet insufficient emphasis has been placed on clinical assessment and rehabilitation of lexical tone disorders. The similarities and dissimilarities between lexical tones and other speech sounds make a richer scientific understanding of their physiological bases paramount to more effective remediation of speech and language disorders in general. Here we discuss the cognitive and biological bases of lexical tones, emphasizing the neural structures and networks that support their acquisition, perception, and cognitive representation. We present emerging research on lexical tone learning in the context of the clinical disorders of hearing, speech, and language that this body of research will help to address.