The Role of Age-Related Neural Timing Variability in Speech Processing

Abstract

Age-related difficulties in speech processing remain a concern, especially as technology continues to depend heavily on successful speech comprehension on the part of users. Event-related potentials (ERPs) have frequently been used to assess age-related changes in the processing of language. Specifically, the amplitude of the ERP is often compared between conditions or groups of interest. In constructing ERPs, many neurophysiologic responses are averaged together to reduce the contribution of uncorrelated background activity in the electroencephalogram (EEG). However, if variability in the timing of each potential on a trial-by-trial basis (i.e., “latency jitter”) is confounded with a variable of interest, then the presence of amplitude differences observed in the average ERP might not be solely the result of genuine amplitude differences, but also timing variability. We examined the role latency jitter may play in the well-established observation of age-related changes in the processing of natural speech as indexed by the N400. Older and younger adults were presented with sentences that ended in either expected or unexpected final words. In agreement with previous findings, a reduction in N400 amplitude was observed in the older adults compared with the younger adults in response to the unexpected final words. However, analyzing ERPs on a single trial basis reveals the older adults to have significantly greater intra-subject variability in N400 latency for incongruent speech stimuli, in comparison to younger adults. This increased intra-individual latency variability may contribute to the smaller N400 amplitudes observed in each subject’s average ERP. Age-related reductions in N400 amplitudes may indicate less precise timing of neurological processes. Conversely, they may indicate that the older brain exhibits greater specificity in the processing of individual sentence stimuli.