Normally, our brains do a masterful job of filtering and sorting the information that flows through our ears such that we are able to function in our acoustic world, be it conversing with a particularly charming individual at a social gathering, or immersing ourselves in a joyful piece of music. Unfortunately, as we get older, our sound world becomes impoverished as a result of the normal changes in peripheral auditory structures, which represent a significant challenge for hearing science and medicine because of their prevalence, significant impact on quality of life, and lack of mechanism-based treatments. Moreover, as average life expectancy continues to increase, auditory problems will apply to an ever-greater number of individuals. Older adults experience a general deterioration in auditory processing, the most common form of which is age-related peripheral change in hearing sensitivity or presbycusis, characterized by impaired thresholds particularly for high frequencies, which become increasingly prominent after the fourth decade of life (Morrell, Gordon-Salant, Pearson, Brant, and Fozard, 1996). Presumably, these losses in peripheral sensitivity result in impoverished and noisy signals being delivered to the central nervous system. This initial distortion of the acoustic waveform may lead to further reductions in fidelity as the incoming acoustic signals are submitted to increasingly detailed analyses, resulting in the eventual disruption of higher-order processes such as speech comprehension.

Age-related change in auditory perception varies substantially between individuals and may include: (1) impaired frequency and duration discrimination (Abel, Krever, and Alberti, 1990); (2) impaired sound localization (Abel, Giguere, Consoli, and Papsin 2000); (3) difficulties in determining the sequential order of stimuli (Trainor and Trehub, 1989); (4) difficulties in processing novel (Lynch and Steffens, 1994) or transposed melodies (Halpern, Bartlett, and Dowling, 1995); and (5) difficulties in understanding speech, especially when the competing signal is speech rather than homogeneous background noise (Duquesnoy, 1983), or when speech occurs in a reverberant environment (Gordon-Salant and Fitzgibbons, 1993). Although sensorineural hearing loss is highly correlated with speech perception deficits (Abel, Sass-Kortsak, and Naugler, 2000; Humes and Lisa, 1990) and accounts for some of the difficulties in identifying novel melodies (Lynch and Steffens, 1994), there is increasing evidence that age-related changes in the peripheral auditory system (e.g., cochlea) alone cannot adequately account for the wide range of auditory problems that accompany aging. For instance, some of the auditory deficits experienced by older adults remain even after controlling for differences in audiometric thresholds (Divenyi and Haupt, 1997). In addition, some older individuals with near-normal sensitivity exhibit problems in speech discrimination (Middelweerd, Festen, and Plomp, 1990), whereas others with hearing loss receive little benefit from hearing aids despite the restoration of hearing thresholds to normal levels (Chmiel and Jerger, 1996). Moreover, hearing status does not always interact with age, suggesting that hearing impairment and age are independent sources contributing to many older listeners' difficulties with understanding speech (Gordon-Salant and Fitzgibbons, 2004).

Since reductions in hearing sensitivity fail to account completely for speech perception deficits, it is necessary to consider the additional possibility that the elderly may also suffer from impairment at one or more levels of the central auditory processing pathways. That is, the difficulties commonly observed in older individuals may in fact be related to age-related changes in the neural mechanisms critical for the perceptual separation of the incoming acoustic information to form accurate representations of our acoustic world, rather than simply the reception of a weak and noisy signal from the ear. Auditory scene analysis is one framework that guides our theorizing with respect to the putative mechanisms involved in age-related changes in auditory perception, providing novel insights about how our perception of the auditory world breaks down as a result of normal aging.

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