Abstract
Older adults have more language production difficulties than younger adults but display largely comparable language comprehension abilities. The Transmission Deficit Hypothesis suggests that production difficulties stem from an age-related increase in phonological signal transmission failures, while the semantic system, being more redundant than the phonological system, allows comprehension to be relatively preserved despite signal failures. Though the neural instantiation of the Transmission Deficit Hypothesis remains an open question, white matter represents one important factor to investigate. Metrics indicative of white matter connectivity across the brain, namely, Radial Diffusivity (RD) and Fractional Anisotropy (FA) have also been linked to age-related cognitive differences including naming difficulties. Using a Picture-Word Interference (PWI) task with 18 younger and 19 older healthy adults, we found that, across ages, better picture naming in the presence of phonological distractors was associated with lower RD across dorsal (r = −.35, p = .03), ventral (r = −.34, p = .04), and fronto-striatal (r = −.33, p = .04) tracts, and higher FA along dorsal tracts (r = .43, p = .008). The pattern of lower RD and higher FA, which is thought to reflect better white matter structure, points to the dorsal stream tracts as critical for performance on the PWI task. Moreover, the effects of RD and FA on performance were attenuated by the effect of age, reflecting the shared variance between age and white matter as it relates to language production ability.
Highlights
Despite relative stability in language comprehension abilities across the lifespan, older adults, typically considered to be individuals over 60 years old, have more language production difficulties compared to younger adults (Burke and Shafto 2008)
Post-hoc Fisher R to Z tests comparing the strength of the relationship between phonological accuracy and Radial Diffusivity (RD) along each tract indicated that the RD-accuracy relationship was comparable across tracts (p > .05)
Fractional Anisotropy (FA) along dorsal tracts was positively correlated with phonological accuracy (r = .43, p < .01) but FA along ventral (r = .22, p > .05) and fronto-striatal (r = .16, p > .05) tracts was not correlated with phonological accuracy
Summary
Despite relative stability in language comprehension abilities across the lifespan, older adults, typically considered to be individuals over 60 years old, have more language production difficulties compared to younger adults (Burke and Shafto 2008). Cross-sectional work has revealed that older adults experience more tip-of-the-tongue states (Burke et al 1991), produce more filler-words and pauses in speech (Kemper et al 1992), and make more naming errors (Feyereisen 1997). As such, explaining the underlying mechanisms of age-related language production deficits has critical downstream consequences for the overall well-being of individuals as they age. Theoretical models and behavioral evidence suggest that agerelated language production difficulties arise from phonological deficits, such as a slow or incomplete retrieval of a word’s sound form (Burke et al 1991). The neural mechanisms underpinning such deficits remain an open question
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