Abstract
The structure of the brain and the nature of evolution suggest that, despite its uniqueness, language likely depends on brain systems that also subserve other functions. The declarative/procedural (DP) model claims that the mental lexicon of memorized word-specific knowledge depends on the largely temporal-lobe substrates of declarative memory, which underlies the storage and use of knowledge of facts and events. The mental grammar, which subserves the rule-governed combination of lexical items into complex representations, depends on a distinct neural system. This system, which is composed of a network of specific frontal, basal-ganglia, parietal and cerebellar structures, underlies procedural memory, which supports the learning and execution of motor and cognitive skills, especially those involving sequences. The functions of the two brain systems, together with their anatomical, physiological and biochemical substrates, lead to specific claims and predictions regarding their roles in language. These predictions are compared with those of other neurocognitive models of language. Empirical evidence is presented from neuroimaging studies of normal language processing, and from developmental and adult-onset disorders. It is argued that this evidence supports the DP model. It is additionally proposed that “language” disorders, such as specific language impairment and non-fluent and fluent aphasia, may be profitably viewed as impairments primarily affecting one or the other brain system. Overall, the data suggest a new neurocognitive framework for the study of lexicon and grammar.
Highlights
Specific Language Impairment (SLI) is associated with deficits of a number of other functions which depend upon the brain structures underlying procedural memory, including working memory (Gathercole & Baddeley, 1993), processing rapidly-presented sequences (Merzenich et al, 1993; Tallal, Stark, & Mellits, 1985), and mental rotation (Johnston & Weismer, 1983) and other “dynamic” mental imagery tasks involving the mental manipulation of images (Leonard, 1998)
I have argued that certain developmental and adult-onset “language” disorders may be best viewed as disorders that affect brain structures underlying the memory systems
The DP model has a number of implications in addition to those discussed above
Summary
The study of language has focused largely on language itself. That is, in order to understand the representation, processing, development, neural correlates and other aspects of language, most theories and investigations have directed their attention to language. Other cognitive domains are much better understood than language in a number of respects, including their neuroanatomy, physiology, biochemistry, evolution, development, and neural computation. Theories that are motivated by non-language domains as well as language have a much wider potential predictive range for language, and are likely to lead to important advances in our understanding of this faculty. This should be likely for areas of research that have been given greater attention in non-language than language domains, such as functional neuroanatomy, physiology, biochemistry, neuroendocrinology, and pharmacology. Multiple characteristics of these two systems, including their computational, neuroanatomical, physiological and biochemical substrates, have been quite well studied, and should lead to important predictions about language
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