Introduction
Aging is one of the most evident biological processes whose molecular and cellular mechanisms are still poorly understood. Age-related changes involve almost all aspects of human beings such as the brain, bones, joints, eyes, ears, digestive, metabolic, urogenital, dental system, skin and functional abilities.
Regarding the bones and joints, the weight-bearing bones and the movable joints take much wear and tear as the body ages. Consequently, osteoporosis and arthritis occur commonly among older adults. Likewise, there are age-related changes in the brain; however, still much is not known about age-related changes in the brain.
Among brain functions, cognitive function and speech perception abilities seem to be affected by aging. There is still no consensus about the mechanisms underlying cognition and speech perception problems in the aged. Changes in hearing thresholds, memory, attention, and speed of processing all could be causes of the difficulties older adults encounter in everyday life.1-3)
The objective of this article is to review age-related changes in cognition and speech perception and to investigate their interrelationship. In addition, this study will provide a current understanding of the mechanism of age-related decline in cognition and speech perception.
Cognition: Association with Aging
Cognitive function is broadly defined as an intellectual process by which one becomes aware of, perceives, or comprehends ideas. It involves all aspects of perception, thinking, reasoning, and remembering.
Studies of cognitive aging suggest that age is associated with cognitive decline. The average course of cognitive abilities over the last several decades of life is accelerating decline.4) This phenomenon is shown in cross-sectional, longitudinal studies, and studies on neural stem cells.5)
Growing evidence indicates that a few types of resident neural stem cells (NSCs) that are located in specific brain regions, most notably the subgranular zone of the hippocampus, seem to play a major role in cognitive functions such as learning, memory, and emotional behavior.6,7) A possible mechanism of age-related changes in neurogenesis is introduced (Fig. 1).5)
Aging constitutes one of the major factors reducing the proliferation of NSCs,8,9) while cognitive decline, including reduced learning and memory performance, is commonly observed in aged individuals.9,10)
Manipulations that decrease neurogenesis typically worsen cognitive performance in senescent animals while, conversely, an increase in neurogenesis tends to improve learning and memory.11,12)
However, some studies indicate that cognitive decline is not intrinsic to aging, but there is some heterogeneity.13,14) According to a recent study, cognitive decline is not a normal part of aging and is associated with some form of neuropathology.15) In the study, individuals were classified into 3 groups by three different rates of decline: slow, moderate, and fast. Individuals in the fast decline groups had high levels of amyloid plaques and neurofibrillary tangles. On the other hand, individuals in the slow decline class had the least amount of neuropathology.
Therefore, there is still controversy as to whether aging and cognitive decline are associated, and there may be individual differences such that not all older adults will experience cognitive decline.
Speech Perception: Association with Aging
Speech perception is the process by which the sounds of language are heard, interpreted and understood. In general, older adults complain that talkers seem to mumble or talk too fast and that it is hard to hear when there is background noise. Many researchers have shown that speech recognition declines with increasing age.16,17) According to one report, as many as one-third of seniors find it difficult to understand conversations in everyday listening situations.18)
Age-related difficulties in speech understanding may be attributed to changes in higher-level cognitive processes such as language comprehension, memory, attention, and cognitive slowing, or to lower-level sensory and perceptual processes.19) That is, some of the age-related decline in speech perception can be accounted for by peripheral sensory problems, but cognitive aging can also be a contributing factor.20)
Many studies of speech recognition in older adults report age-related differences in performance. However, the interpretation of decreased speech perception in older adults is complicated by the change in pure-tone thresholds that occurs with increasing age.
Age-related peripheral hearing loss, called presbycusis, is highly prevalent and hearing loss in older adults is associated with a number of pathologies. Among them, strial degeneration is assumed to be the dominant cause of presbycusis.21)
The effects of age-related decline in hearing thresholds on speech perception have been extensively studied. The finding is that high-frequency threshold hearing loss accounts for much of the loss of phonetic information and word identification in quiet listening conditions.22,23)
In addition to the decline in hearing threshold, older adults show a greater reduction in speech recognition than younger adults, with increasing speech rate.24) The potential sources of reduced recognition for rapid speech are reduction in processing time and reduction of the acoustic information in the signal.
However, other many studies also indicated that speech perception does not decline with age, after correcting for the effects of different hearing thresholds.25) In some cross-sectional studies, pure-tone thresholds that change with increasing age and rates of change that vary among individuals are factors that complicate the interpretation of the differing levels of speech perception in older adults. In one study. when thresholds across age groups were equated, speech perception did not differ from age 50 to age 90, except for the Synthetic Sentence Identification.26) In other study, no significant differences were found in speech recognition for individuals in three age groups: 55-64, 65-74, and 75-84 years who were selected so that the average pure-tone thresholds for the three groups were within 5 dB of each other.27)
According to a longitudinal study, age-related decline in word recognition was more consistent with underlying changes in auditory function, rather than decline in cognitive function, as a result of peripheral, rather than central, system pathology.28)
Analysis of Characteristics of Auditory Aging
Fast speech rate and the presence of background noise disrupt word identification more for older than younger listeners even when their pure-tone thresholds are normal.16,27,29,30)
Some changes in auditory processing are assumed to give rise to the particular vulnerabilities of older listeners when speech is heard with background noise or at fast rates. Those changes include declines in monoaural auditory temporal processing,31) binaural processing,32) and behavioral and physiological factors in auditory temporal processing.31,33)
Gap detection thresholds are higher for older adults, potentially reducing temporal segregation of events. That is, older adults do not detect a gap in the signal until the size of the gap is about twice as large as the smallest gap detectable by younger adults (approximately 6 ms vs. 3 ms).34) Gap detection thresholds are not predictable from pure-tone hearing thresholds in listeners with normal audiograms.35,36)
The Relationship between Cognition and Speech Perception
Cognitive abilities are inherently involved in speech perception and they are highly correlated. Some studies suggested the importance of cognitive function, in addition to hearing factors, that limit speech perception among older adults.37) On the other hand, some studies reported that the effect of cognitive decline on the speech perception performance of older adults is not significant.22) Cognitive function is not composed of a single component but is composed of many distinct components such as attention, memory, and language comprehension.
Considering the cognitive components specific to speech perception, two cognitive factors that decline with age may influence speech perception performance. Those two factors are working memory capacity and the rate of information processing that is defined generally as the speed at which an individual can extract content and construct meaning from a rapid signal.3,38)
Working memory refers to a temporary short-term store where information is held and related to later-occurring events. Cohen39) (1987) noted that working memory capacity could affect speech understanding because parts of the spoken message must be retained in the memory and related to later parts of the message for the comprehension of logical relationships between words and concepts. Working memory is known to decline minimally with aging.40)
According to one study, age-related decline in auditory processing, rather than changes in the capacity of the working memory, primarily influences speech perception,2) and reduction in the available processing time causes reduced recognition for older adults.25) Therefore, older adults tend to have difficulties in speech perception with rapid speech.
Brain Systems: Integrating Auditory and Cognitive Information
Davis and Silverman41) (1970) recognized long ago how brain systems integrate auditory and cognitive information would be crucial to our understanding of speech communication.
Acoustic analysis of the information occurs in the middorsal superior temporal region then proceeds either anteriorly along the superior temporal gyrus (STG) to the inferior frontal cortices in mapping speech to meaning or posteriorly along the STG to the inferior parietal lobe and then to the premotor cortices (Fig. 2).42) Left dorsolateral prefrontal cortical areas are thought to be involved in semantic processing and working memory. PFC activation is particularly important in the aged43) as it is supposed to be a reflection of recruitment of more working memory resources.40)
Decline-Compensation Hypothesis
Cognitive aging involves a combination of decline in sensory processing and a following increase in the recruitment of more general cognitive areas such as the PFC, as a means of compensation.20)
For example, when younger and older adults perform equivalently on various perceptual and cognitive tasks, there is more widespread activation in older brains than in younger brains, with one interpretation being that this reflects compensatory processing in older adults.43)
These findings that correlate adaptive cognitive processes with age are consistent with the concept that the brain itself has potentially a life-long capacity for neural plasticity and adaptive reorganization.20)
Value of Cognitive Function Testing for the Evaluation of Speech Perception in the Aged
Aging affects both the peripheral and central auditory system. Simply evaluating the peripheral auditory system is not enough to fully understand the pathophysiology of impaired speech perception in older adults. Therefore, diagnostic evaluation for speech perception difficulty in older adults should not only include standard audiometric testing, but also include measures of central auditory function and related cognitive functions. Tests of cognitive function could also be used as sensitive outcome measures of hearing rehabilitation.44) Likewise, regarding the management of speech perception problems in older adults, treatment aimed at compensating only for the peripheral auditory impairment is inadequate, but it might still be better to include this strategy to compensate for central auditory impairment and cognitive function deficits.
Conclusion
The effect of age on speech perception and cognitive processing is not well understood. Although there is still controversy as to whether cognitive function and speech perception decline with age, in general, older adults have difficulty in everyday communication whether the cause is in the peripheral hearing system or in the central cognitive system. Hearing becomes more difficult with age but brain plasticity and cognitive function might continue to compensate for the peripheral impairment. Therefore, cognitive function testing could also be considered in the evaluation of speech perception especially for the aged and there is significance for adopting central auditory rehabilitation training for the aged. Further studies should be performed to delineate which cognitive skills are more sensitive in the perception of speech.
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