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by Kelly L. Tremblay and Bernhard Ross
Aging can be defined as the biological process of growing old, regardless of chronological age (Timiris, 2003). Intrinsic (nature) and extrinsic (nurture) factors, as well as their interactions, influence the degree and rate at which we age. For this reason, the aging process affects individuals differently, resulting in a heterogeneous group of older people with varying degrees of auditory impairment and different communication needs. Despite this heterogeneity, a consistent complaint heard by clinicians from older adults is, "I can hear you, but I can't understand you."
The speech understanding difficulties expressed by older adults likely arise from multiple sources. From the ear to the brain, numerous structural and chemical changes coincide with advancing age (for a review, see Willott, 1991; 1999). Not only can these changes negatively impact the audibility of sound, physiological changes throughout the auditory system also affect the way frequency and timing information (in the incoming signal) is encoded and perceived (for a review see Chisolm, Willott, & Lister, 2003). Some higher-level cognitive functions, such as attention and memory, also may decline with age.
Because perception depends upon lower-level sensory as well as higher-level cognitive processes, it is likely that decreased audibility, slowed neural conduction time, and a struggle to selectively attend to a voice in the presence of competing noise all occur when an older listener tries to understand what is being said (for a review see Pichora-Fuller, Schneider, Benson, Hamstra, & Storzer, 2006). It therefore follows that rehabilitation of the older patient involves more than improving sound audibility, because for many people detecting, discriminating, and attending to the signal of interest might be compromised.
Measures of Brain Function
To better understand the effects of aging and hearing loss on the brain, non-invasive tools are used to examine the physiological correlates of sound detection and discrimination, as well as higher-level processing [for reviews specific to clinicians, see Tremblay & Burkard (2006); Tremblay & Ross (2007); and Alain & Tremblay (in press)]. When these measures are combined with behavioral/perceptual measures, researchers are able to explore brain-behavior relationships in the same individual.
Electro- and magneto-encephalography (EEG/MEG) are examples of physiological tools that are familiar to most audiologists and speech-language pathologists. One subcategory of recorded brain activity is the auditory evoked potential (AEP). Testing for AEPs is a passive procedure that does not require the participants to attend to the stimuli or execute a task while brain activity is measured; EEG and MEG provide an opportunity to examine how the brain automatically (passively) encodes sound without the assistance or interference of cognitive processing. An equally important but different approach is to record brain activity while a person executes a behavioral task, which allows researchers to examine some of the active physiological processes engaged during perception and stimulus processing.
Passive and active recording paradigms have been used to study different types of automatic sound processing in aging adults. A recent focus of interest is the effect of aging on the physiological capacity to process time-varying (temporal) acoustic cues. Temporal cues convey a tremendous amount of information to a listener (Rosen, 1992; McAlpine, 2005) and there is converging evidence from animal and human studies that older adults have more difficulty processing these types of cues (Frisina, 2001; Fitzgibbons & Gordon-Salant, 1996; Ginzel et al., 1982; Scheider, 1997).
We have been using the P1-N1-P2 complex—a particular type of AEP that reflects synchronous neural activity of structures in the thalamic-cortical segment of the central auditory system—to examine the effects of age and age-related hearing loss on the physiological processing of temporal cues. Most recently we used this tool to examine the effects of aging on binaural hearing because the ability to localize sound in space, as well as understand speech in noise, is partially dependent on hearing with two ears. Sound arriving at both ears is characterized by interaural time delays (ITDs) as well as interaural level differences (ILDs). One example of an ITD is the detection of changes in interaural phase differences (IPDs). In this example, the phase of the acoustical signal at one ear is compared with the phase of the signal at the opposite ear in the central auditory system, and a physiological response is evoked by a change in the stimulus phase.
Using MEG equipment and P1-N1-P2 auditory evoked responses, we examined the effects of aging on the physiological capacity for detecting IPD cues (Ross, Fujioka, Tremblay, & Picton, 2007). In short, we found that with increasing age, the upper frequency limit at which IPDs are physiologically detected and perceived by the listener is reduced. Most surprising, this decline begins earlier in life than one might think. Physiological and behavioral responses to IPD cues were already reduced in a group of middle-aged people (ages 46–53). These physiological results provide evidence of impaired sound processing during normal aging that is likely unrelated to hearing status or cognitive ability, as audible stimuli were used to measure brain activity passively.
Rehabilitation
So, what do these findings mean to the clinician? They reinforce the observations made by many clinicians that improving audibility through hearing aid amplification alone does not automatically improve a person's ability to understand what is being said. Even when sounds are made audible for older adults, the way they are processed in the brain is different from the process in younger adults.
The degree to which impaired physiological detection translates to impaired communication likely differs from one individual to another. One person might be able to compensate for abnormal timing cues while another person might not. The questions then arise: Is it possible to improve temporal processing in older adults? And how should clinicians approach rehabilitating older adults?
As clinicians, we are well-trained to adopt a "bottom-up" or "low-level" peripheral approach to remediation by improving the audibility of sounds, and this is obviously an important first step. A recent example of a bottom-up approach is to modify and examine the effects of spectral shaping on the perception of speech sounds (Shrivastav et al., 2006). A spectrally enhanced stimulus, where the second formant of stop consonants is enhanced, has proved to significantly increase the perception and physiological representation of the stimulus (Harkrider, Plyler, & Hedrick, 2006).
But there is also increasing interest on the contribution of "top-down" processes, such as attention and memory. Audiologists are not typically trained in these areas as they are usually examined by cognitive psychologists and in some cases by speech-language pathologists, and less so by hearing scientists. However, clinicians and scientists are beginning to work together to find integrated approaches for improving speech communication in older adults (Pichora-Fuller & Sing, 2006; Kricos, 2006).
Some of these approaches include modifying relevant cues of the incoming speech signal as well as using training exercises in focused listening to help improve the perceptual detection of important acoustic cues (Burke & Humes, 2007; Sweetow, 2007; Fu & Galvin, 2007; Tremblay, 2007).
When working with older adults, it is important to recognize that effective rehabilitation will likely require more than improving signal detection. Beyond the aging ear is a central auditory system that is biologically different from that of younger people.
Kelly L. Tremblay is an associate professor in the Department of Speech and Hearing Sciences at the University of Washington (UW), Seattle. Contact her at tremblay@u.washington.edu.
Bernhard Ross is a senior scientist at the Rotman Research Institute of Baycrest Centre for Geriatric Care in Toronto, Ontario. Contact him at bross@rotman-baycrest.on.ca.
Support and funding from the National Institutes of Health (R01DC007705-01 AUD) and Virginia Merrill Bloedel Hearing Research Scientists Traveling Scholar Award are acknowledged and appreciated.
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