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. 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 desynchronized and tend to show different looking patterns for familiar