Lip Reading - Process

Process

In everyday conversation, people with normal vision, hearing and social skills sub-consciously use information from the lips and face to aid aural comprehension and most fluent speakers of a language are able to speechread to some extent (see McGurk effect). This is because each speech sound (phoneme) has a particular facial and mouth position (viseme), although many phonemes share the same viseme and thus are impossible to distinguish from visual information alone. When a normal person speaks, the tongue moves in at least three places (tip, middle and back), and the soft palate rises and falls. All of these articulatory gestures are phonetically significant, changing the speech sound produced in important ways, but are invisible to the lip reader.

Consequently, sounds whose place of articulation is deep inside the mouth or throat are not detectable, such as glottal consonants. Voiced and unvoiced pairs look identical, such as and, and, and, and, and and (in American English); likewise for nasalisation. It has been estimated that only 30% to 40% of sounds in the English language are distinguishable from sight alone; the phrase "where there's life, there's hope" looks identical to "where's the lavender soap" in most English dialects. Author Henry Kisor titled his book What's That Pig Outdoors?: A Memoir of Deafness in reference to mishearing the question, "What's that big loud noise?" He used this example in the book to discuss the shortcomings of speechreading.