Temporoparietal brain structures support sentence processing across the adult lifespan

Although older adults maintain most language abilities, they can experience declines in complex sentence comprehension. How brain structure and function mediates age-related changes in sentence processing remains underspecified. Here, using a novel neuroimaging approach to perform region-based mediation statistics, we investigated the neural correlates of memory capacity and sentence processing in 187 neurologically intact adults aged 20-80. Working and auditory-verbal short-term memory was measured using two National Institutes of Health Cognitive Toolbox tasks, and reading sentence processing was measured with a self-paced reading (SPR) task consisting of closely matched noncanonical (object-relative; OR) and canonical (subject-relative; SR) conditions. Behaviorally, we found that memory capacity mediated the relationship between age and overall sentence comprehension. Age, and not memory capacity, was the most important factor for sentence processing speed. Neuroanatomically, we found that bilateral grey matter volume (GMV; structure) in posterior temporal/inferior parietal areas and partial amplitude of low-frequency fluctuations (pALFF; function) in the left SMG mediated age-related processing speed for noncanonical compared to canonical conditions after controlling for memory capacity. These findings suggest that bilateral temporoparietal brain areas support the processing of complex syntactic structures across the lifespan in ways that are partially independent of memory abilities.