Effects of neural aging on speech perception in bilinguals vs. monolinguals.

This study examined the effects of aging and bilingualism on neural speech perception in Spanish-English (SE) bilinguals and American English (AE) monolinguals. As the population of bilingual older adults continues to grow, understanding how bilingual experience influences auditory processing across the lifespan has important theoretical and clinical implications. Electrophysiological measures, including mismatch negativity (MMN) and the sensory obligatory cortical responses (P1-N1-P2 complex), were used to assess early auditory encoding and automatic speech discrimination during passive perception of unfamiliar Japanese vowel contrasts. Participants included younger and older AE monolinguals and SE bilinguals. All participants completed a passive listening EEG task, followed by a phonological memory task, and a second EEG session using the same speech stimuli. Analyses focused on MMN and P1-N1-P2 amplitude, latency, and scalp distribution. Overall, older AE monolinguals demonstrated delayed MMN responses in the left hemisphere and midline compared to both younger monolinguals and bilinguals. In contrast, older SE bilinguals exhibited a delayed MMN response only at a single frontal site when compared to younger bilinguals. While age-related decline in auditory processing was evident, particularly regarding MMN latency, older bilinguals demonstrated relatively preserved MMN responses compared to monolinguals peers. The findings suggest a dissociation between early sensory encoding and automatic speech discrimination processes. Older bilingual adults demonstrated enlarged N1 amplitudes relative to all other groups, potentially reflecting age-related sensory changes, while simultaneously maintaining relatively preserved MMN responses. These findings suggest that lifelong bilingualism may support more adaptable auditory discrimination mechanisms during aging, potentially contributing to greater neural resilience or cognitive reserve. This study contributes to a growing body of evidence suggesting that bilingualism may shape auditory and cognitive processing across the lifespan. The findings also highlight the importance of considering bilingual experience in the assessment and rehabilitation of older adults, particularly in increasingly diverse clinical populations.