PROJECT SUMMARY/ABSTRACT The goal of this research is to address the critical barriers to communication that older and hearing-impaired listeners face in complex acoustic scenes, in which multiple competing sounds impede speech understanding. Our approach includes simultaneous behavioral and electrophysiological measures of auditory stream segregation, and importantly, we also determine the costs and benefits of hearing instruments on segregating multiple speech sources. Our research has uncovered neural mechanisms associated with age-related declines in auditory temporal processing that play a critical role in binaural and spatial hearing. Our current tools, however, lack the precision necessary to gain deeper understanding of this age-related deficit. In the present study, we propose a series of innovative approaches that aim to determine the functional boundaries of spatial hearing in listeners with age-related hearing loss (ARHL). In Aim 1, we use a well-established paradigm of auditory scene analysis to define functional, ecologically relevant boundaries of perception as they relate to individual binaural cues, and measure how these boundaries are affected by ARHL at the neural level. Aim 2 introduces novel behavioral and electrophysiological tasks that precisely measure functional boundaries between segregation and integration of competing speech. Aim 3 evaluates the costs and benefits of directional microphone technology to hearing-impaired listeners as a function of their individual spatial segregation boundaries. The present study draws from a long line of research on auditory scene analysis and introduces a novel approach to cortical tracking of speech envelope in complex scenes. This project pushes the field forward in how we measure mechanisms of stream segregation in older and hearing-impaired listeners. The results of these experiments will: (1) precisely characterize individual binaural and spatial segregation boundaries; (2) provide evidence of the neural circuitry and patterns of activity underlying the influence of spatial cues on stream segregation; (3) discover evidence for the relationship between binaural segregation boundaries, individual differences, and the effects of hearing loss and aging; and (4) investigate a relationship between individual suprathreshold hearing abilities and specific hearing aid processing technology. Whereas hearing devices currently have remarkable benefits to specific users, the present research should inspire a new push for innovative ways to address hearing challenges of an aging population. Our team is well- positioned to execute behavioral and electrophysiological measures sensitive to auditory function and intervention outcomes, and critically, to bridge these to shift the field toward individualized hearing health care.