the direction of the verb's movement (inflection). Another aspect of typology that has been studied in sign languages is their systems for cardinal numbers. Typologically significant differences have been found between sign languages. Acquisition Children who are exposed to a sign language from birth will acquire it, just as hearing children acquire their native spoken language. The Critical Period hypothesis suggests that language, spoken or signed, is more easily acquired as a child at a young age versus an adult because of the plasticity of the child's brain. In a study done at the University of McGill, they found that American Sign Language users who acquired the language natively (from birth) performed better when asked to copy videos of ASL sentences than ASL users who acquired the language later in life. They also found that there are differences in the grammatical morphology of ASL sentences between the two groups, all suggesting that there is a very important critical period in learning signed languages. The acquisition of non-manual features follows an interesting pattern: When a word that always has a particular non-manual feature associated with it (such as a wh-question word) is learned, the non-manual aspects are attached to the word but don't have the flexibility associated with adult use. At a certain point, the non-manual features are dropped and the word is produced with no facial expression. After a few months, the non-manuals reappear, this time being used the way adult signers would use them. Written forms Sign languages do not have a traditional or formal written form. Many deaf people do not see a need to write their own language. Several ways to represent sign languages in written form have been developed. Stokoe notation, devised by Dr. William Stokoe for his 1965 Dictionary of American Sign Language, is an abstract phonemic notation system. Designed specifically for representing the use of the hands, it has no way of expressing facial expression or other non-manual features of sign languages. However, his was designed for research, particularly in a dictionary, not for general use. The Hamburg Notation System (HamNoSys), developed in the early 1990s, is a detailed phonetic system, not designed for any one sign language, and intended as a transcription system for researchers rather than as a practical script. David J. Peterson has attempted to create a phonetic transcription system for signing that is ASCII-friendly known as the Sign Language International