Our use of language depends upon two capacities: a mental lexicon of memorized words, and a mental grammar of rules that underlie the productive sequential and hierarchical composition of lexical forms into complex linguistic representations -i.e., complex words, phrases and sentences. The Declarative/Procedural model posits that the learning and use of lexical knowledge depends upon a well-studied bilateral temporal-lobe "declarative memory" system implicated in the learning and use of conceptual/semantic knowledge (i.e., knowledge about the world), while grammatical computations that underlie the real-time combination of lexical forms into complex representations rely on left frontal/basal-ganglia "procedural" circuits implicated in the acquisition and expression of motor and cognitive skills (e.g., riding a bicycle).

The Declarative/Procedural model predicts double dissociations between lexicon and grammar, with associations among lexical memory, memorized facts, and temporal-lobe structures, and among grammar, motor skills, and frontal/basal-ganglia structures. The model is supported by studies investigating morphology and syntax; using a range of psycholinguistic and neurolinguistic approaches, including behavioral testing of patients with aphasia, neurodegenerative disease or developmental disorders, and neuroimaging investigations of healthy subjects (using fMRI, MEG, and EEG/ERP); with children and adults; examining several languages (English, German, Japanese, and Italian).

Two extensions of the model are discussed. First, sex differences in the neurocognition of lexicon and grammar are examined. Robust evidence indicates that females are better than males at remembering words. This suggests the novel hypothesis that females may tend to memorize previously-encountered complex forms (e.g., played), while males generally compute these forms compositionally (e.g., play + -ed). Both sexes should compute new complex forms compositionally (e.g., proy + -ed). These predictions are confirmed with converging evidence from psycholinguistic, neuropsychological, and neuro-electrophysiological studies examining the processing of complex words and sentences.

Second, neurocognitive differences between first and second language are examined. Evidence suggests a critical (sensitive) period in the acquisition and use of grammar: Older learners have greater difficulty than younger learners. This leads to the hypothesis that older second language learners, being unable to depend upon the procedural/grammatical system, are forced to rely on the declarative/lexical system for the computation of complex linguistic representations. These representations may be either memorized, or constructed by explicit rules learned in declarative memory. This shift to declarative/lexical memory is expected to increase with increasing age of exposure to the language, and with less experience (practice) with the language, which is predicted to improve the procedural/grammatical learning of grammatical rules. Evidence is presented in support of these predictions.