Event-related potential measures of lexical activation in Alzheimer’s disease

Although the hallmark deficit in Alzheimer’s disease (AD) is an impairment in episodic memory, language abilities are also impaired very early in the disease course (for a review, see Caramelli, Mansur, & Nitrini, 1998). Declines in semantic processing are well-documented, but less explored are the lexical abilities of AD individuals. However, there exists one study suggesting, on the basis of error patterns, that these patients exhibit a deficit in lexical activation (Paganelli, Vigliocco, Vinson, Siri, & Cappa, 2003). The present study aims to explore this possibility further, using an event-related brain potential (ERP) methodology to examine response to items that vary in terms of neighborhood density (ND). ND refers to the number of words existing in the lexicon that differ from the target word by only one letter (e.g., carol has one neighbor, carob, while take has many neighbors, e.g., tale, make, toke, etc.). On the basis of behavioral studies, it has been claimed that high ND lexical items elicit higher levels of global lexical activity than low ND items due to partial activation of neighbors (Grainger & Jacobs, 1996). In a study examining ND effects on ERP response, Holcomb, Grainger, and O’Rourke (2002) found a larger N400 response to high ND than to low ND items in healthy young adults. The authors argue that this reflects higher global levels of lexical activation when a high ND item is seen, as postulated by Grainger and Jacobs (1996). We tested the hypothesis that deficits in lexical activation in AD will result in a diminished N400 effect to high ND items relative to healthy older adults.