On the surface, the two systems are so distinct that one would think, left to their own devices, they would seek out
quite different realizations in the brain. Yet they do not. Sign does make use of certain special opportunities of the
visual modality, for instance it often expresses verbal modification by modulating the form of the verb's sign rather
thanbyadding an affix. However,aside fro mthese differences we provetobedealingwiththevery sa me syste m. This
suggests that children mobilize the same resources in acquiring sign—that as soon as something in the world can be
categorized as symbolic communication, the language machine is engaged.
This case is a dissociation between grammatical capacity and modality of expression. The other cases are dissociations
between aspects of language and general intelligence. There are three.
- Smithand Tsimpli(1995) discuss a“linguisticsavant,”an individual whois deeplyretarded onmostmeasures
but exhibits a remarkable talent at learning languages. His competence, however, extends only so far as the
grammaticalsystem: histranslationsamong languages are grammaticallyimpeccable, buttendtobeword-for-
word in a way that neglects overall sense. - Williams Syndrome (Bellugi et al. 1994) was originally described as a genetically based syndrome that results
in retardation, particularly in spatial cognition, but preserves language-learning ability. Children with Williams
Syndrome are indeed highly verbal, often atfirst giving an impression of impressive intelligence. However,
further study has revealed selective deficits in Williams Syndrome language: for instance, syntactic tasks and
regular inflection are unimpaired, but irregular inflection is disrupted (Clahsen and Almazan 1998). - Specific Language Impairment (SLI) wasfirst brought to the attention of the linguistics community by
Gopnik and Crago (1990), who studied a familysome of whosemembers suffered from thisimpairmentand
others did not, in a pattern familiar from studies of genetic inheritance of other characteristics. The
impairment was said to affect only language, specifically morphology, without affecting general intelligence.
Vargha-Khadem and Passingham's (1990) study of this family, however, found a parallel pattern of more
general auditory and articulatory problems, afinding that has been widely taken to discredit Gopnik and
Crago's argument for a genetically based language specialization. On the other hand, more extensive studies
of SLI by Gopnik (1999) and others (e.g. van der Lely 1999); van der Lely and Christian 2000; Clahsen and
Almazan 1998, with data from a variety of languages, overall confir mthe character of the i mpair ment. What
is interesting is that this impairment seems roughly the con-verse of (one aspect of) the impairment in
Williams Syndrome: irregular inflection seems to be acquired, but regular inflection is impaired.