Developmental milestones: sign language acquisition and motor development.
Bonvillian JD, Orlansky MD, Novack LL
Child Dev 1983 Dec 54:6 1435-45

Abstract
The sign language and motor development of 11 young children of deaf
parents were studied across a 16-month period. The subjects showed accelerated early language development producing, on the average,
their first recognizable sign at 8.5 months, their tenth sign at 13.2
months, and their first sign combination at 17.0 months. In contrast, children learning to speak typically do not attain the equivalent spoken language milestones until 2-3 months later. The structure and content of the subjects' 50-sign vocabularies closely resembled those for children at the same stage in previously published studies of spoken language acquisition. The pattern of synchrony between motor and language development previously reported by investigators of children learning to speak was found not to apply to the population of the present study: most of the subjects learning to sign did not slow down in their rate of language acquisition after achieving a new motor milestone, but rather continued to show a gradual increase in the size of their sign language vocabularies.

First-language acquisition after childhood differs from second-language
acquisition: the case of American Sign Language.
Mayberry RI
J Speech Hear Res 1993 Dec 36:6 1258-70

Abstract
This study determined whether the long-range outcome of first-language
acquisition, when the learning begins after early childhood, is similar to that of second-language acquisition. Subjects were 36 deaf adults who had contrasting histories of spoken and sign language acquisition. Twenty-seven subjects were born deaf and began to acquire American Sign Language (ASL) as a first language at ages ranging from infancy to late childhood. Nine other subjects were born with normal hearing, which they lost in late childhood; they subsequently acquired ASL as a second language (because they had acquired spoken English as a first language in early childhood). ASL sentence processing was measured by recall of long and complex sentences and short-term memory for signed digits. Subjects who acquired ASL as a second language after childhood outperformed those who acquired it as a first language at exactly the same age. In addition, the performance of the subjects who acquired ASL as a first language declined in association with increasing age of acquisition. Effects were most apparent for sentence processing skills related to lexical identification, grammatical acceptability, and memory for sentence meaning. No effects were found for skills related to
fine-motor production and pattern segmentation.

Abstract
Analysis of the patterns of breakdown of a visuospatial language in deaf
signers thus allows new perspectives on the nature and determinants of cerebral specialization for language. First, these data show that hearing and speech are not necessary for the development of hemispheric specialization--sound is not crucial. Second, the data show that in these deaf signers, it is the left hemisphere that is dominant for sign language. The patients with damage to the left hemisphere showed marked sign language deficits but relatively intact capacity for processing nonlanguage visuospatial relations. The patients with damage to the right hemisphere showed much the reverse pattern. Thus, not only is there left hemisphere specialization for language functioning, there is a complementary right hemisphere specialization for visuospatial functioning. The fact that much of the grammatical information is conveyed via spatial manipulation appears not to alter this complementary specialization. Furthermore, the finding that components of sign language (e.g., lexicon and grammar) can be selectively impaired suggests that the functional organization of the brain for sign language may turn out to be modular.

Finally, patients with left and right hemisphere damage showed
dissociations between two uses of space in the language--one to represent
spatial relations and the other to represent syntactic relations. Right hemisphere damage disrupts the former but spares the latter; left
hemisphere damage disrupts the use of space for syntactic relations but
spares its use for spatial relations. Taken together with studies of the processing of sign language ''on line'' by neurologically intact deaf signers, these data suggest that the left cerebral hemisphere in humans may
have an innate predisposition for language, independent of language modality. Studies of the effects of brain damage on signing make it
clear that accounts of hemispheric specialization are oversimplified if
stated only in terms of a dichotomy between language and visuospatial functioning. Such studies may also permit us to come closer to the real
principles underlying the specializations of the two cerebral hemispheres,
since in sign language there is interplay between visuospatial and linguistic relations within the same system.

Abstract
The speech and language training for deaf children at our clinic is performed using a multisensory method, which consists of reception and expression training for sign language and fingerspelling as well as auditory training, lip reading, and written language training (the Kanazawa Method). We have already reported that acquisition of written language is not dependent on oral language, and that written language is easier to learn than oral language for deaf children. In the present investigation, we analyzed the acquisition of comprehensible and expressive vocabulary in sign language and fingerspelling.

The subjects were two children congenitally deaf at levels higher than 105dB. Recorded language samples by the age of 48 months were analyzed. Acquisition of sign language was found to be significantly
easier than acquisition of oral language. The development of expressive noun words, function words, and Wh-question words in sign language at the early period was almost equivalent to that of hearing peers, and then the sign language appeared transfer to the oral language. These results suggest that early presentation of sign language with written and oral language is effective in the acquisition of communicative attitudes, function words and interrogative sentences which are most difficult for the hearing-impaired. It was shown that early presentation of sign language with written and oral language serves to promote acquisition of oral language.