Jonathan Z. Simon1,2,3
1Department of Electrical & Computer Engineering, University of Maryland, College Park, U.S.A.
2Department of Biology, University of Maryland, College Park, U.S.A.
3Institute for Systems Research, University of Maryland, College Park, U.S.A.
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 have analyzed magnetoencephalography responses to continuous narrative speech in older and younger listeners with clinically normal hearing, acquired in two separate experiments. Responses to clean speech and to speech from two simultaneous talkers were used to distinguish between bottom-up and task-related brain activity. We show multiple lines of evidence that older adults show exaggerated cortical responses compared to younger, 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 (which are also 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). Additional insight into the cortical processing of continuous speech is gained from the analysis of sustained pupillometric measures and non-phase-locked alpha-band neural activity, simultaneously obtained during the continuous speech listening.
Acknowledgements: Funding provided by NIH (P01-AG055365 & R01-DC014085), and NSF (SMA-1734892).