When right becomes less right: Neural dedifferentiation during suprasegmental speech processing in the aging brain.

This study combines event-related sparse-temporal acquisition fMRI with structural MRI to investigate older participants (n = 26, mean age = 70.64) with age-typical peripheral hearing. While participants were presented with locally time-reversed sentences of varying temporal integrity, they performed an auditory pattern-matching task. The major aim of the study was to find out whether functional lateralization for speech perception according to the 'Asymmetric Sampling in Time' (AST) hypothesis shows a similar pattern in elderly individuals as has been previously observed in younger adults. Our findings indicate that, unlike results previously obtained from younger adults, older individuals did not demonstrate the same pattern of rightward lateralization in response to suprasegmental speech cues in the three auditory regions of interest (ROI), namely Heschl's gyrus (HG), planum temporale (PT) and posterior lateral superior temporal gyrus (pSTG). A frequentist statistical approach did not yield evidence for functional lateralization in the aging brain, and this was corroborated by Bayesian evidence which supported the absence of lateralization in older adults in response to the suprasegmental manipulation. However, a relationship between structural measurements and functional responses demonstrated that individuals with thicker right PT showed less variance in lateralization. Hence, this study extends the division of labour between the left and the right auditory cortex during the processing of continuous spoken language as proposed by the 'AST'-hypothesis in younger adults to a lifespan context.