Abstract
Sentence comprehension remains largely intact across the lifespan, except for when domain-general cognitive resources are taxed. Specifically, as successful sentence comprehension requires the encoding of rich and detailed verbal information, difficulties may arise from a decline in verbal working memory capacity with age. In the literature outside of sentence comprehension, memory encoding success has been associated with oscillatory power increases within the theta band and decreases within the alpha and beta band in young adults; in older adults, these power changes are attenuated. However, it remains an open question whether age- related oscillatory attenuation associates with difficulties in verbal working memory- intensive sentence comprehension. To address this question, we here assessed sentence-encoding success in 18 younger (mean age: 24 years), 16 middle-aged (mean age: 43 years) and 13 older adults (mean age: 64) via a verbal working memory-intensive comprehension task; in parallel, the electroencephalogram (EEG) was recorded. Comprehension accuracy was quantified as the ratio of later- remembered (LR) and later-not-remembered (LNR) sentences—indirectly indicating encoding success. From the EEG, oscillatory power within the theta, alpha, and beta band was derived, separately for LR and LNR sentences. First, we then assessed the subsequent memory effect (SME) by comparing LR and LNR power within each age group and frequency band, using cluster-based permutation t-tests. Then, the band-wise oscillatory power differences (i.e., LR – LNR) were compared between age groups, using a cluster-based permutation analysis of variance. As expected, the behavioral results showed better encoding performance for younger than middle- aged than older adults. The EEG results showed lower oscillatory power within alpha band, most pronounced over the fronto-parietal midline, to predict better sentence encoding, in young adults only. This negative SME was less pronounced in middle- aged adults, turning into a positive SME in older adults. Potentially, this neural desynchronization-to-synchronization shift across the lifespan reflects a cognitive shift in encoding strategies: At young age, bottom-up encoding may dominate, achieved through cortical disinhibition—allowing enriched information routing throughout the language network. At old age, resource limitations may necessitate an increased reliance on top-down encoding, mirrored in cortical inhibition to avoid information overload. We suggest that declining language comprehension across the lifespan is characterized by changes to the underlying electrophysiological processing networks that are, in turn, associated with changes in the functional dynamics within these networks during task performance.
