On the Inseparability of Grammar and the Lexicon: Evidence from Acquisition, Aphasia and Real-time Processing

Within linguistic theory, many phenomena that were previously handled by a separate grammatical component have been moved into the lexicon; in some theories, the contrast between grammar and the lexicon has disappeared altogether. In a review of findings from language development, language breakdown and real-time processing, we conclude that the case for a modular distinction between grammar and the lexicon has been overstated, and that the evidence to date is compatible with a unified lexicalist account. Studies of normal children show that the emergence of grammar is highly dependent upon vocabulary size, a finding confirmed and extended in atypical populations. Studies of language breakdown in older children and adults provide no evidence for a modular dissociation between grammar and the lexicon; some structures are especially vulnerable to brain damage (e.g. function words, non-canonical word orders), but this vulnerability is also observed in neurologically intact individuals under perceptual degradation or cognitive overload. Finally, on-line studies provide evidence for early and intricate interactions between lexical and grammatical information in normal adults. A possible characterization of this unified lexical processor is offered, based on distributed representations in recurrent neural networks.