Differential age effects on lexical ambiguity resolution mechanisms

Abstract

Multiple neurocognitive subsystems are involved in resolving lexical ambiguity under different circumstances. We examined how processing in these subsystems changes with normal aging by comparing ERP responses to homographs and unambiguous words completing congruent sentences (with both semantic and syntactic contextual information) or syntactic prose (syntactic information only). Like young adults in prior work, older adults elicited more negative N400s to homographs in congruent sentences, suggesting mismatch between the context and residual activation of the contextually-irrelevant sense. However, the frontal negativity seen in young adults to homographs in syntactically well-defined but semantically neutral contexts was absent in older adults as a group, suggesting decline in recruiting additional neural resources to aid difficult semantic selection. A subset of older adults with high verbal fluency maintained a young-like effect pattern.Human language is difficult in part because it consists of inherently arbitrary pairings between forms and meanings. Moreover, a single form is oftentimes associated with multiple interpretations. For example, 'watch' can refer to a timing device or an action of observing. One of the challenges of human language processing is therefore to solve these ubiquitous one-to-many mapping problems in the face of time pressure, using (when possible) a variety of contextual constraints such as lexical associations, syntactic structure, and message-level semantic information.Aging is usually associated with cognitive as well as biological decline. However, it has been shown that different aspects of cerebral and cognitive aging take place along multiple trajectories. For example, frontal lobe regions undergo more age-related deterioration than other brain areas in terms of volume (Raz et al., 2005), amount of grey and white matter (Moseley, 2002;Resnick, Pham, Kraut, Zonderman, & Davatzikos, 2003), and white matter integrity (Head et al., 2004;Sullivan & Pfefferbaum, 2006). Similar disproportional agerelated changes are also found across cognitive domains: in particular, executive/controlled processes (which are usually considered to be frontally mediated), as opposed to stimulusdriven ones, tend to be more affected by age (DiGirolamo et al., 2001;Foster, Black, Buck, & Bronskill, 1997; Hasher, Zacks, & May, 1999;Jonides et al., 2000;Vanderaspoilden, Adam, Van Der Linden, & Morais, 2007).Differential age effects have also been observed within the specific domain of language processing. Overall, language has been described as a facet of cognitive processing that is (Burke & Peters, 1986;Howard, 1980;Park et al., 2002;Stern, Prather, Swinney, & Zurif, 1991). However, in fact, some aspects of language abilities are better preserved than others. On the one hand, there is ample evidence that word-related knowledge and the ability to link perceptual word forms to meaning remains relatively intact across the lifespan. For example, it has been shown that, across adulthood, vocabulary ...