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

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.

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.

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.

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.

Speech-reading, a mandatory part of speech perception, should be appropriately assessed and be considered when developing strategies for speech and language intervention.