Review of visual speech perception by hearing and hearing-impaired people: clinical implications.

BACKGROUND: Speech perception is often considered specific to the auditory modality, despite convincing evidence that speech processing is bimodal. The theoretical and clinical roles of speech-reading for speech perception, however, have received little attention in speech-language therapy. AIMS: The role of speech-read information for speech perception is evaluated by considering evidence from hearing infants and adults, people with speech disorders, and those born profoundly hearing impaired. METHODS & PROCEDURES: Research studies are evaluated for evidence on lip-reading for speech perception: the mandatory role of speech-reading for hearing adults' perception of the McGurk effect and hearing infants' awareness of the congruence between lip movements and speech sounds; brain neuroimaging studies of speech-read and heard speech perception; the speech-reading abilities of people with disordered speech; and the phonological coding abilities of people with profound pre-lingual hearing loss. Theories of multimodal speech perception are explained. MAIN CONTRIBUTIONS: Five pieces of evidence indicate that speech-reading is an integral part of speech processing. Hearing people's perception of speech is influenced by speech-read cues, and those speech-read cues cannot be ignored. Infants are aware that lip movements and speech sounds match from soon after birth and by four months of age have language specific speech-reading skills. Studies of brain activity show that the brain processes speech-read and heard speech similarly. Some children and adults with speech disorders are reported to rely less on speech-read cues than people without speech disorders, and children who are profoundly hearing impaired from birth have concepts of rhyme, match homophones, and can repeat and spell nonsense words. CONCLUSIONS & IMPLICATIONS: Speech-reading, a mandatory part of speech perception, should be appropriately assessed and be considered when developing strategies for speech and language intervention.