Neural processing during older adults' comprehension of spoken sentences: age differences in resource allocation and connectivity.

Speech comprehension remains largely preserved in older adults despite significant age-related neurophysiological change. However, older adults' performance declines more rapidly than that of young adults when listening conditions are challenging. We investigated the cortical network underlying speech comprehension in healthy aging using short sentences differing in syntactic complexity, with processing demands further manipulated through speech rate. Neural activity was monitored using blood oxygen level-dependent functional magnetic resonance imaging. Comprehension of syntactically complex sentences activated components of a core sentence-processing network in both young and older adults, including the left inferior and middle frontal gyri, left inferior parietal cortex, and left middle temporal gyrus. However, older adults showed reduced recruitment of inferior frontal regions relative to young adults; the individual degree of recruitment predicted accuracy at the more difficult fast speech rate. Older adults also showed increased activity in frontal regions outside the core sentence-processing network, which may have played a compensatory role. Finally, a functional connectivity analysis demonstrated reduced coherence between activated regions in older adults. We conclude that decreased activation of specialized processing regions, and limited ability to coordinate activity between regions, contribute to older adults' difficulty with sentence comprehension under difficult listening conditions.