Linguistics and neuroscience studies show that the stages of language acquisition from birth to age 5 and beyond in visual-spatial modality (sign language) and vocal-auditory modality (speech language) are not only in the same order but also on the same timeline.
In this period, babies produce coos and goos -- both vocally and manually.
In this stage, baby learns to coordinate her/his motor skills.
Baby produces "vocal play" as well as "manual play". Babies from different cultures or linguistic environments and even deaf babies babble non-distinguished sounds.
Canonical babbling is when babies produced reduplicated syllables.
In speech (vocal babbling), babies produce syllabic consonant-vowel repetitions. E.g. "dadadada", "babababa" and other meaningless sounds.
Babies, who are exposed to signlan, also do manual babbling, producing syllabic hold-movement repetitions. E.g. opening and closing hand repeatedly.
Babies develop some fine motor skills, such as pointing, waving, and picking up small objects.
At this stage, pointing with index finger also began to emerge as early as 9-10 months or as late as 12 months. Pointing is a non-linguistic though communicative gesture apart from babbling and language.
At this "sophisticated babbling" stage, babies of English speaking parents babble constonant-vowel-constonant (CVC). On the other hand, babies of ASL-speaking families babble with more varied patterns.
From this stage, babies develop toward their first words at average 12 months. Although, at this stage a baby may utter a first referential word or a few first words, using simple handshapes or simple sounds.
Vocabulary development begins to gradually increase from the first birthday.
Below highlights the milestones of a language development in sign language (American Sign Language) from 12 to 24 months old.
Referential one-word utterances begin to gradually expand at this stage. These one-words are not conventional adult words but babies can convey them consistently to express meaning.
For example in English speech, the baby says "mu" every time she/he is offered a bottle of milk. Or, the baby utters "open handshape tapping on torso" to express for a bath.
Like English-speaking people, ASL-speaking adults use regular ASL words with babies. They don't adapt nor simplify these ASL words.
Infants will adapt some difficult handshapes themselves. For examples, they may use an index finger instead of the "J" handshape for "juice". As they develop physically, they would evolve their handshapes to regular handshapes of the ASL words. Parents should continue to "pronounce" ASL words properly.
Despite baby's limited expression, a study at the University of California, San Diego, shows that babies can understand what adults are saying.
At this two-word stage, toddlers are able to utter words more identifiable as adult words.
The toddler can express over 50-200 words and make two-word combinations (e.g. "Where ball?", "More pull", "daddy car", "where dog", "water on").
For example in this video, the toddler "Juli" produced fire hot and yummy burger.
Linguistic pointing (apart from gestural pointing) for the first person ("me") emerges at 17-20 months.
The pronoun "you" in ASL emarges at 22-24 months. And, the third-party pointing (she/he) at 24 months.
Like hearing toddlers at the same stage, reversal errors in pronouns do appear in toddlers who are native in ASL or other signed languages. Despite the nature of iconicity in pronominal pointing, language is amodal
The inflection of verbs and prepositions emerge at the stage of 20-36 months. For example in English, the present progressive (e.g. --ing), prepositions (in, on), and plural.
In ASL, for example: inflecting verbs (GIVE-ME/YOU/HER, GIFT-ME/YOU/HER, etc), classifier verbs (CL3-drive, CL1-person), etc.
At this stage, the toddler has a word for almost everything and can utter two or three words long. She/he also uses pronouns (e.g. I, you, he, she, they, me) and prepositions (e.g. on, in).
E.g. "mummy make cake", "daddy kiss mummy", etc.
From about 34 months, the preschooler can combine between four to six words in any one utterance.
There is greater use of contrast between prepositions such as "in", "on", and "under", and adjectives such as big and little. E.g. "mummy on little bike", "mummy playing with the big spoon".
They can talk about what have happened away from home. Most of the time, other people can understand what the child is saying.
In speech and signlan, a child speaks clearly and fluently.
At this stage, the child can produce longer and detailed utterances of over six words in length. She/he can speak/sign clearly and fluently.
The concept of past and future time develops.