Effects of Aging on Cortical Representations of Continuous Speech and Directional Connectivity during Difficult Listening

Jonathan Simon, University of Maryland, College Park 

Abstract

Compared to young adults, older adults often have increased difficulty comprehending speech, especially in challenging acoustic environments. However, previous research has surprisingly found that their cortical responses to speech demonstrate more robust tracking of the acoustic speech envelope than those of younger adults, even though the opposite result holds for subcortical responses to speech. Here we analyze magnetoencephalography responses to continuous narrative speech in older and younger listeners with clinically normal hearing, while listening to speech in quiet and in the presence of competing speakers. We show multiple lines of evidence that older adults show exaggerated cortical responses compared to younger, and at several distinct cortical processing stages. Exaggerated responses at early latencies are consistent with excitation/inhibition imbalance seen in animal models, whereas exaggerated responses at greater latencies (strongly dependent on selective attention) are consistent with the recruitment of additional neural resources in order to aid speech comprehension. Exaggerated responses are only seen for cortical processing of slow speech features (≲ 10 Hz), however, and not at faster rates associated with pitch tracking (≳ 80 Hz). Additionally, we analyze directional functional connectivity between different brain areas using Granger Causality. In the theta band, for younger adults, as background noise increases there is a transition from predominantly temporal-to-frontal (bottom-up) connections to predominantly frontal-to-temporal (top-down). In contrast, older adults showed bidirectional information flow between frontal and temporal cortices even for speech in quiet, not changing substantially with increased noise.  Additionally, younger listeners did not show changes in the nature of their cortical links for different listening conditions, whereas older listeners exhibited a switch from predominantly facilitative (excitatory) links to predominantly sharpening (serial excitatory+inhibitory), when noise increased.