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Over 100 years of observations have established that slowness of behavior is a characteristic of becoming old, although it is now recognized that health, use of medications, and physical activity may modify the extent of the slowing. Early research indicated that there is a limited contribution to slowing by peripheral sensory-motor factors. Substantial evidence has pointed to the central nervous system as the locus of the slowing. Recent investigators have expressed divided opinions about whether there is a pervasive general slowing of behavior by the central nervous system or whether there are specific localized mechanisms. This is not unlike early disputed views of the brain as having localized or global behavioral functions: Both principles appear to be simultaneously true. Sufficient research has been conducted to indicate that there are specific factors as well as a general process associated with the slowing of behavior with advancing age. Whether such slowing is a primary or secondary cause of age differences in cognitive processes is a significant scientific issue. A marked broadening of research on aging has been accompanied by an interest in identifying both the neurophysiological correlates of slowing as well as its role in specific cognitive processes. Yet another aspect of the changing research picture is the trend to move beyond the mere use of chronological age as the sole basis for comparing performance differences. Measurement of more independent variables is suggested as part of clusters or causal complexes that will indicate sources of the changes in speed and other aspects of behavior. These causal complexes include biological indicators such as disease, physiological capacity for work, and length of life, as well as causal complexes of social factors involving such variables as education, occupation, and ethnicity. There has been considerable discussion of markers of aging. In this approach, factors found to be closely associated with advancing age are used as measures of the effectiveness of attempts to modify the course of aging, e.g. by diet, exercise, new learning, and drugs. Along with other biomarkers of aging, speed of behavior may prove to be a criterion for assessing the impact of interventions on the rate and processes of aging. As a marker of aging, speed needs further exploration that will compare the slowness observed in different subgroups of adults with a wide range of outcomes in their productivity, capacity for adaptation to life's demands, and health. The present status of information about slowness of behavior with advancing age indicates that it is one of the most reliable features of human life.(ABSTRACT TRUNCATED AT 400 WORDS)
Birren et al. (Sun,) studied this question.