Non-Linear Dorsolateral Prefrontal Recruitment During Speech-in-Noise Perception in Aging

Age-related speech-in-noise difficulties have been associated with increased activity in prefrontal regions. Whether this upregulation reflects adaptive compensation, neural inefficiency, or changes in task engagement as listening demands increase remains unresolved. Most studies have relied on linear contrasts of task difficulty, limiting our understanding of how the role of prefrontal recruitment evolves with listening demand. The present study examined these competing accounts within a non-linear framework. Using functional near-infrared spectroscopy, we measured activity in the dorsolateral prefrontal cortex (DLPFC) while 36 young and 34 older adults performed a sentence-in-noise task across five signal-to-noise ratio conditions. We tested whether DLPFC recruitment followed a quadratic trajectory across listening demands and whether its relationship with performance changed accordingly. Dorsomedial and ventrolateral prefrontal activity was also recorded to assess regional specificity. Bilateral DLPFC responses followed an inverted U-shaped pattern, increasing from easier to intermediate conditions before declining at the most difficult levels. Greater left DLPFC activation was associated with poorer performance in older adults across the ascending and peak portions of the demand-response function. High-performing older adults showed more youth-like recruitment profiles characterized by lower overall activation. Within the older adult group, higher left DLPFC activation was associated with greater age but not with hearing or global cognition. These results clarify the neural mechanisms underlying age-related speech-in-noise difficulties, suggesting that the shape of DLPFC recruitment reflects a demand-sensitive trajectory broadly preserved across age groups, while the level of activation in older adults may reflect neural processing efficiency, with greater upregulation associated with poorer performance.