Neural Substrates of Component Lexical Semantic Processes

Extracting meaning from words (lexical semantics) is critical to normal language functioning. Lexical semantic impairment can result from a variety of causes including developmental abnormalities, stroke and degenerative diseases but may reflect damage to one or more component processes. Current language models have identified two qualitatively different components of the lexical semantic system. (1) An automatic component enables rapid access to semantics within milliseconds of lexical input; (2) a controlled component evolves more slowly and guides strategic retrieval of semantic information not recovered automatically, In addition, some processes associated with these components are thought to depend upon the modality of incoming stimuli. The objective of the project is to identify the neural correlates of these behaviorally separable automatic and controlled components of the lexical semantic system, within and across visual and auditory modalities. In a preliminary study, we identified dissociable neural correlates of visually cued automatic and controlled lexical semantic processing in healthy volunteers using event-related fMRI and a primed lexical decision task. A series of similar studies are proposed to characterize more precisely the roles of several frontal and temporal lobe brain regions in components of the lexical semantic system. The design is novel in that automatic and controlled semantic priming conditions will be compared directly, using randomized designs identical to those used in behavioral research. There are three goals. The first goal is to characterize more precisely the role of a left frontal region in visually cued controlled retrieval of lexical semantic information. The second goal is to characterize more precisely the role of a left temporal region in visually cued automatic lexical semantic processing. The third goal is to identify neural regions involved in auditorily cued lexical semantics that overlap, and are dissociable from, those involved in visually cued lexical semantics. Identifying neural systems associated with component processes involved in extracting meaning from words is of central importance to understanding the structure and organization of the lexical semantic system and should aid diagnosis and rehabilitation of language disorders.