Lip reading, also known as speechreading, is a technique of understanding a limited range of speech by visually interpreting the movements of the lips, face and tongue without sound. Estimates of the range of lip reading vary, with some figures as low as 30% because lip reading relies on context, language knowledge, and any residual hearing. Although lip reading is used most extensively by deaf and hard-of-hearing people, most people with normal hearing process can infer some speech information by observing a speaker's mouth.
Process
Although speech perception is considered to be an auditory skill, it is intrinsically multimodal, since producing speech requires the speaker to make movements of the lips, teeth and tongue which are often visible in face-to-face communication. Information from the lips and face supports aural comprehension and most fluent listeners of a language are sensitive to seen speech actions (see McGurk effect). The extent to which people make use of seen speech actions varies with the visibility of the speech action and the knowledge and skill of the perceiver.
Phonemes and visemes The phoneme is the smallest detectable unit of sound in a language that serves to distinguish words from one another. /pit/ and /pik/ differ by one phoneme and refer to different concepts. Spoken English has about 44 phonemes. For lip reading, the number of visually distinctive units - visemes - is much smaller, thus several phonemes map onto a few visemes. This is because many phonemes are produced within the mouth and throat, and are hard to see. These include glottal consonants and most gestures of the tongue. Voiced and unvoiced pairs look identical, such as [p] and [b], [k] and [g], [t] and [d], [f] and [v], and [s] and [z]; likewise for nasalisation (e.g. [m] vs. [b]). Homophenes are words that look similar when lip read, but which contain different phonemes. Because there are about three times as many phonemes as visemes in English, it is often claimed that only 30% of speech can be lip read. Homophenes are a crucial source of mis-lip reading.
Coarticulation Visemes can be captured as still images, but speech unfolds in time. The smooth articulation of speech sounds in sequence can mean that mouth patterns may be 'shaped' by an adjacent phoneme: the 'th' sound in 'tooth' and in 'teeth' appears very different because of the vocalic context. This coarticulation affects lip-reading 'beyond the viseme'.
Speech interpretation While visemes offer a useful starting point for understanding lipreading, spoken distinctions within a viseme can be distinguished and can help support identification. Moreover, the statistical distribution of phonemes within the lexicon of a language is uneven. While there are clusters of words which are phonemically similar to each other ('lexical neighbors', such as spit/sip/sit/stick...etc.), others are unlike all other words: they are 'unique' in terms of the distribution of their phonemes ('umbrella' may be an example). Skilled users of the language bring this knowledge to bear when interpreting speech, so it is generally harder to identify a heard word with many lexical neighbors than one with few neighbors. Applying this insight to seen speech, some words in the language can be unambiguously lip-read even when they contain few visemes - simply because no other words could possibly 'fit'.
Variation in readability and skill Many factors affect the visibility of a speaking face, including illumination, movement of the head/camera, frame-rate of the moving image and distance from the viewer (see e.g.). Head movement that accompanies normal speech can also improve lip-reading, independently of oral actions. However, when lip-reading connected speech, the viewer's knowledge of the spoken language, familiarity with the speaker and style of speech, and the context of the lip-read material are as important as the visibility of the speaker. While most hearing people are sensitive to seen speech, there is great variability in individual speechreading skill. Good lipreaders are often more accurate than poor lipreaders at identifying phonemes from visual speech.
A simple visemic measure of 'lipreadability' has been questioned by some researchers. The 'phoneme equivalence class' measure takes into account the statistical structure of the lexicon and can also accommodate individual differences in lip-reading ability.Feld J1, Sommers M 2011 There Goes the Neighborhood: Lipreading and the Structure of the Mental Lexicon. Speech Commun. Feb;53(2):220-228 In line with this, excellent lipreading is often associated with more broad-based cognitive skills including general language proficiency, executive function and working memory.
Lipreading and language learning in hearing infants and children
The first few months
Seeing the mouth plays a role in the very young infant's early sensitivity to speech, and prepares them to become speakers at 1 – 2 years. In order to imitate, a baby must learn to shape their lips in accordance with the sounds they hear; seeing the speaker may help them to do this. Newborns imitate adult mouth movements such as sticking out the tongue or opening the mouth, which could be a precursor to further imitation and later language learning. Infants are disturbed when audiovisual speech of a familiar speaker is desynchronizedDodd B.1976 Lip reading in infants: attention to speech presented in- and out-of-synchrony. Cognitive Psychology Oct;11(4):478-84 and tend to show different looking patterns for familiar than for unfamiliar faces when matched to (recorded) voices. Infants are sensitive to McGurk illusions months before they have learned to speak. These studies and many more point to a role for vision in the development of sensitivity to (auditory) speech in the first half-year of life.
The next six months; a role in learning a native language
Until around six months of age, most hearing infants are sensitive to a wide range of speech gestures - including ones that can be seen on the mouth - which may or may not later be part of the phonology of their native language. But in the second six months of life, the hearing infant shows perceptual narrowing for the phonetic structure of their own language - and may lose the early sensitivity to mouth patterns that are not useful. The speech sounds /v/ and /b/ which are visemically distinctive in English but not in Castilian Spanish are accurately distinguished in Spanish-exposed and English-exposed babies up to the age of around 6 months. However, older Spanish-exposed infants lose the ability to 'see' this distinction, while it is retained for English-exposed infants. Such studies suggest that rather than hearing and vision developing in independent ways in infancy, multimodal processing is the rule, not the exception, in (language) development of the infant brain.
Early language production: one to two years
Given the many studies indicating a role for vision in the development of language in the pre-lingual infant, the effects of congenital blindness on language development are surprisingly small. 18-month-olds learn new words more readily when they hear them, and do not learn them when they are shown the speech movements without hearing.Havy, M., Foroud, A., Fais, L., & Werker, J.F. (in press; online January 26, 2017). The role of auditory and visual speech in word-learning at 18 months and in adulthood. Child Development. (Pre-print version) However, children blind from birth can confuse /m/ and /n/ in their own early production of English words – a confusion rarely seen in sighted hearing children, since /m/ and /n/ are visibly distinctive, but auditorially confusable.Mills, A.E. 1987 The development of phonology in the blind child. In B.Dodd & R.Campbell(Eds) Hearing by Eye: the psychology of lipreading, Hove UK, Lawrence Erlbaum Associates The role of vision in children aged 1–2 years may be less critical to the production of their native language, since, by that age, they have attained the skills they need to identify and imitate speech sounds. However, hearing a non-native language can shift the child's attention to visual and auditory engagement by way of lipreading and listening in order to process, understand and produce speech.
In childhood Studies with pre-lingual infants and children use indirect, non-verbal measures to indicate sensitivity to seen speech. Explicit lip-reading can be reliably tested in hearing preschoolers by asking them to 'say aloud what I say silently'. In school-age children, lipreading of familiar closed-set words such as number words can be readily elicited.Dodd B. 1987 The acquisition of lipreading skills by normally hearing children. In B.Dodd & R.Campbell (Eds) Hearing by Eye, Erlbaum NJ pp163-176 Individual differences in lip-reading skill, as tested by asking the child to 'speak the word that you lip-read', or by matching a lip-read utterance to a picture, show a relationship between lip-reading skill and age.
In hearing adults: lifespan considerations While lip-reading silent speech poses a challenge for most hearing people, adding sight of the speaker to heard speech improves speech processing under many conditions. The mechanisms for this, and the precise ways in which lip-reading helps, are topics of current research. Seeing the speaker helps at all levels of speech processing from phonetic feature discrimination to interpretation of pragmatic utterances. The positive effects of adding vision to heard speech are greater in noisy than quiet environments, where by making speech perception easier, seeing the speaker can free up cognitive resources, enabling deeper processing of speech content.
As hearing becomes less reliable in old-age, people may tend to rely more on lip-reading, and are encouraged to do so. However, greater reliance on lip-reading may not always make good the effects of age-related hearing loss. Cognitive decline in aging may be preceded by and/or associated with measurable hearing loss. Thus lipreading may not always be able to fully compensate for the combined hearing and cognitive age-related decrements.
In specific (hearing) populations
A number of studies report anomalies of lipreading in populations with distinctive developmental disorders. Autism: People with autism may show reduced lipreading abilities and reduced reliance on vision in audiovisual speech perception. This may be associated with gaze-to-the-face anomalies in these people. Williams syndrome: People with Williams syndrome show some deficits in speechreading which may be independent of their visuo-spatial difficulties. Specific Language Impairment: Children with SLI are also reported to show reduced lipreading sensitivity, as are people with dyslexia.
Deafness Debate has raged for hundreds of years over the role of lip-reading ('oralism') compared with other communication methods (most recently, total communication) in the education of deaf people. The extent to which one or other approach is beneficial depends on a range of factors, including level of hearing loss of the deaf person, age of hearing loss, parental involvement and parental language(s). Then there is a question concerning the aims of the deaf person and their community and carers. Is the aim of education to enhance communication generally, to develop sign language as a first language, or to develop skills in the spoken language of the hearing community? Researchers now focus on which aspects of language and communication may be best delivered by what means and in which contexts, given the hearing status of the child and her family, and their educational plans. Bimodal bilingualism (proficiency in both speech and sign language) is one dominant current approach in language education for the deaf child.