Our auditory environment is inherently complex. Hence, it should not come as a surprise that successful identification of simultaneous and sequentially occurring acoustic events depends on listeners' ability to adequately parse sound elements emanating from different physical sounds, and at the same time, combine those arising from a particular source of interest across time. Solving this problem involves low-level mechanisms that take advantage of physical properties in the acoustic environment as well as high-level mechanisms that take advantage of our knowledge about the auditory world. The evidence reviewed in this chapter indicates that aging impairs listeners' ability to parse concurrent sounds based on both spectral and directional cues. Further research is nevertheless needed to clarify the link between age-related decline in low-level sound segregation and higher-level processes such as the perception of speech in noise. It is also unclear whether extended training could be used to alleviate some of the age effects on simultaneous sound and speech separation.

The robust and reliable age-related decline in concurrent sound segregation contrasts with the more subtle age effects observed in sequential sound segregation. An important difference between concurrent and sequential sound segregation is that the former relies on a more precise temporal and frequency representation. The studies reviewed in this chapter suggest that the likelihood of deficits in auditory stream segregation increases with hearing loss, in that the greater the hearing loss the less likely the individual perceptually segregates the incoming stimuli into two distinct streams of sounds. This suggests that sequential sound segregation involves a relatively coarse temporal and frequency representation, which may be more resilient to the aging process than concurrent sound segregation.

We propose that speech and music perception can be viewed as complex auditory scene analysis problems that engage both low-level and schema-driven processes. It is likely to be the interaction between low-level and high-level mechanisms that lead to successful speech identification in adverse listening situations since primitive grouping mechanisms alone are not sufficient to fully account for perceptual organization of speech (Remez, Rubin, Berns, Pardo, and Lang, l994). In solving the scene analysis problem, older adults appear to rely more on schema-driven than on low-level processes. Although knowing what to listen for helps both young and older adults, older adults appear to benefit the most from prior learning in understanding their acoustic environment. Once again, these results leave many questions unanswered, and therefore further research is needed to understand the extent to which schema-driven processes are affected by age. Nevertheless, by using the scene analysis framework scientists can bridge various areas of aging and hearing research that encompass signal detection and the formation of more abstract representations of our auditory environment. Only by considering both bottom-up and top-down influences, as the current framework allows us to do, may we arrive at a complete picture as to how the auditory scene changes as a function of aging.

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