In reviewing the preceding literature, a number of methodological procedures suggest themselves for future research on aging and auditory scene analysis. First, and somewhat obvious, it is essential that participants be screened for hearing impairment in both ears. In particular, the integrity of the cochlea should be assessed in a more comprehensive way than just measuring pure tone thresholds in an effort to dissociate nonthreshold-changing peripheral deficits from true central deficits in auditory processing. One test that could provide some information is a distortion-product otoacoustic emission (OAE). Although outer hair cells must be functioning to some degree in order to measure normal thresholds, OAEs are a more sensitive measure of outer hair cell health than pure-tone audiometry and may prove useful in assessing cochlear damage in general. Another potentially useful test would be fine-scale audiometry, which consists of obtaining thresholds with a greater degree of frequency resolution. This method can provide evidence for hearing changes that are not reflected in octave-wide measurements. There are obviously a number of deficits that cannot be easily accounted for by any peripheral problems, but the age-related problems in a wide range of auditory tasks might be accounted (at least partly) by peripheral degradation. Distortion-product OAE assessment might be useful in addressing this issue since it is readily available (and fast), and because it is generally accepted as a sensitive measure of cochlear integrity.
Whenever possible, speech discrimination scores should also be obtained for each ear. Given that the nature of auditory scene analysis research can often rely upon somewhat artificial stimuli, the acquisition of SPIN test scores (or another comparable test) becomes critical. This allows the researcher to establish a correlation between performance in experimental tasks thought to tap into the operation of low-level acoustic mechanisms and speech comprehension in noise, which is an important end product of auditory scene analysis. Also, since musical expertise is likely to influence auditory scene analysis, participants should be asked about the duration of any musical training, and groups should be matched for musical training and listening habits. All of this should be in addition to using standard exclusion screening criteria such as neurological or psychiatric illness and drug or alcohol abuse.
It is well recognized that hearing sensitivity diminishes with age, especially in the high frequency range. Despite screening for hearing impairment, it is not uncommon to find residual differences in hearing thresholds between young and old adults. Hence, several steps should be taken to dissociate the contribution of age-related changes in peripheral processing from those in central auditory processing. First, stimuli should be presented at a suprathreshold level (e.g., 80 dB sound pressure level) rather than at a given intensity increment above hearing threshold. This approach controls for loudness recruitment associated with sensorineural hearing loss (Moore, 1995). Second, stimuli generated using the lower frequency range should be used wherever possible as presbycusis affects primarily higher frequency signals. Third, analyses of covariance should be performed using participants' audiometric levels (pure tone and speech reception thresholds) as covariates to adjust for the contribution of age-related changes in hearing thresholds on behavioral measurements. Whenever possible, three or more age groups should be tested, thereby enabling the examination of when specific maturation processes begin, as well as observing differences in the rate of changes across the various age groups. Such experimental design considerations provide information that goes beyond merely identifying areas where older people perform worse than young people. Taken as a whole, the proposed research strategy provides a means to assess central auditory processing while taking into account differences in hearing thresholds.
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