Human language depends upon a dictionary of words (the mental lexicon) and rules which specify how those words are put together into larger words, phrases, and sentences (the mental grammar). The computational and brain bases of these two fundamental language capacities are still not well understood. This project investigates the novel view that the learning and use of the two language capacities depends upon two more general-purpose memory systems whose neural foundations are relatively well studied. Specifically, it is hypothesized that the acquisition and use of words depends upon "declarative memory" temporal-lobe brain circuits previously shown to underlie the learning and use of fact knowledge, such as what a zebra is and where it lives. It is also hypothesized that the acquisition and use of grammatical rules depends upon "procedural memory" left frontal/basal-ganglia brain circuits previously shown to underlie the learning and expression of motor and cognitive "skills," such as riding a bicycle. On this "declarative/procedural" view of language, lexicon and grammar are linked to distinct general-purpose systems, each responsible for a particular set of language and non-language functions. The declarative/procedural view contrasts with the two main classes of previously proposed theories regarding the computational and neural mechanisms of language. It shares the perspective of traditional "dual-system" theories that lexicon and grammar depend upon distinct systems, but it diverges from their claim that these systems are responsible for language alone. Conversely, while the declarative/procedural view agrees with "single-system" theories that the two language capacities depend upon general-purpose brain circuitry, it diverges from their claim that both capacities are linked to a single system with a broad distribution in the brain.
To distinguish the three views, the project will focus on a simple distinction in language: between regular and irregular transformations in past tense and in other aspects of language. In this distinction lexicon and grammar can be contrasted, while other psychologically relevant factors are held constant. Irregular transformations are at least partially arbitrary (e.g., sing-sang, bring-brought), and therefore must depend upon memory, whereas regular transformations are predictable (e.g., verb + -ed), and so could be computed by a grammatical rule. Thus links are predicted among irregulars (lexicon), facts, and temporal-lobe circuits, and among regulars (grammar), skills, and frontal/basal-ganglia circuits. Patients with brain damage limited to particular temporal-lobe areas (e.g., "posterior aphasics") should have more trouble with irregulars than regulars, and with facts than skills. Patients with damage limited to particular left frontal/basal-ganglia structures (e.g., "anterior aphasics") should show the opposite pattern.
If indeed the declarative and procedural memory systems play a role in lexicon and grammar, then data from numerous investigations of the neural, computational, and developmental underpinnings of the two brain memory systems in humans and in animals will contribute to our understanding of language. Conversely, new approaches to the study of the two memory systems will become possible on the basis of our knowledge of language structure, processing, and acquisition. Tests of lexicon and grammar, such as those used in the proposed project, may become useful probes of the two brain systems. In addition to the advancement of fundamental knowledge in cognitive science, the contributions of animal and human studies of the two brain systems to our understanding of language should lead to new clinical approaches for the diagnosis and treatment of acquired and developmental language disorders.