Lip reading
Lip reading, also known as speechreading, is a technique of understanding speech by visually interpreting the movements of the lips, face and tongue when normal sound is not available. It relies also on information provided by the context, knowledge of the language, and any residual hearing. Although lip reading is used most extensively by deaf and hard-of-hearing people, most people with normal hearing process some speech information from sight of the moving mouth. 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. 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. Visemes can be captured as still images, but speech unfolds in time.