DEAFNESS
Hearing loss differs widely between affected individuals, and is classified as mild, moderate, severe or profound.
Human conversation typically takes place within a frequency range of 250 to 8000 Hz and at an intensity of around 60 decibels. Deaf listeners suffer from a restriction in the frequency range and/or a reduction in the perceived loudness of the signal. Hearing aids can increase loudness but cannot restore gaps in the frequency band. Profoundly deaf listeners appear to compensate by making use of time/ intensity cues. They can perceive changes in the intensity of the signal; the duration of the changes provides a rough indication of voiced/ voiceless contrasts and enables stops and fricatives to be distinguished.
A major issue is the extent to which prelinguistic deafness affects language acquisition. There is evidence of delayed acquisition– infants may reach the 50-word vocabulary threshold around ten months later than their hearing peers. But whether the acquisition route deviates from that of a hearing infant is less clear.
Because of their slower acquisition, it was thought that deaf children might provide evidence as to whether there is a critical period for developing a full mastery of a first language. However, only a very weak association has been found between the age at which a child is diagnosed as deaf and the level of language (speech or sign) which is later achieved. Stronger factors appear to be the degree of hearing impairment and the quality of the input which the child receives. Child directed speech is often impoverished when a child is deaf, with shorter and fewer exchanges and the adult speaker exercising a high degree of control.
The effect of deafness upon phonological development has also been studied. At the babbling stage, the consonant repertoire of deaf infants appears to be smaller and intonation flatter. Visual cues may play a part, favouring the early emergence of labial sounds. However, the speech of deaf infants contains phonological irregularities similar to those in the speech of hearing infants– the major difference being that they occur at a later chronological age. Delay rather than deviance thus appears to be the pattern.
Lexical development differs from that of a hearing infant in that labelling plays a less important part; early nouns are fewer in number. This may reflect the carer’s difficulty in holding and directing the attention of a deaf child. However, the lexical associations which are ultimately built up appear to be similar to those of all language users.
So far as syntax is concerned, deaf infants who achieve speech tend to rely on shorter utterances and to adhere more closely to the standard Subject-Verb-Object word order. Sometimes their speech is telegraphic, with function words and inflections omitted. Even adults sometimes experience problems in making grammaticality judgements and in interpreting passive and negative structures. It would appear that they cope with the unreliability of the signal by relying upon a strategy that is semantic rather than syntactic and that assumes a standard agent-action-patient word order.
Because profoundly deaf children do not have a strong basis of spoken language, they often encounter literacy problems. They are slow in developing reading skills. An obvious explanation is that deaf readers cannot make use of grapheme-phoneme rules to break unfamiliar words into their constituent sounds. However, there is evidence that many deaf readers do achieve a kind of phonological code which enables them to associate words by their rimes and to distinguish homographs.
Typically, the writing of a deaf child uses a limited range of sentence structures and a grammar system which frequently omits inflections, auxiliary verbs and articles. However, spelling is often accurate, suggesting an ability to handle visual vocabulary as whole words.
See also: Blindness, Sign language
Further reading: Gallaway and Woll (1994); Mogford (1993b); Strong (1988)
