sumption, diet, intake of antioxidants, and levels of physical activity (see Waldstein, Appendix E). Further investigation of the effect of the mechanisms that link experiential factors on cognitive aging promises to increase our understanding of the mechanisms involved in age-related cognitive changes.
Considerable evidence exists that peripheral sensory and motor systems decline with age. The sensory input to an older nervous system is attenuated, spatially blurred, and temporally smeared compared with that of a healthy young sensory system. The motor output of the aged individual is also often slower, less agile, and more variable than that of a younger person. Evidence is mounting that there is a correlation between sensory-motor decline and cognitive decline—that much of the interindividual variation in cognitive decline is associated with sensory-motor decline.
Much of the decline in sensory function with age is due to ''wear and tear" on these systems (see Kline and Scialfa, 1997; Schneider and Pichora-Fuller, in press, for reviews). For instance, the hair cells of the inner ear, which transduce sound vibrations into neural impulses, tend to wear out over repeated use, leading to loss of hearing sensitivity and poor ability to resolve frequency differences, which are the major hearing problems associated with aging (presbycusis). Once mammalian hair cells are damaged or lost, they cannot be replaced (see Tsue et al., 1994). Hair cells similar to those in the auditory system are sensory transducers for the vestibular system that controls balance and helps in locomotion. These also deteriorate over time. In vision, structural changes in the tissues of the cornea, iris, lens, and their fluids occur over time, one result being that less light reaches the retina for neural transduction. Similar forms of structural decline have been documented for taste, smell, the sense of balance, and proprioception. For these reasons, the information most older people receive through their senses is degraded relative to that received by younger people. Even when corrections are made (e.g., hearing aids, eyeglasses) for some forms of sensory loss, older people still tend to perform poorer on perceptual and attentional tasks than do younger people, especially in terms of temporal processing.
Strong correlations have been reported among declines in sensory, motor, and cognitive function. Mayer and Baltes (1996, see also Baltes, 1997; Baltes and Lindenberger, 1997) developed a measure of general intelligence as a part of the Berlin Aging Study. This measure of intelligence correlates strongly with age (from 70 to 105 years) with more than 32 percent of the variance accounted for. When simple measures of hearing, vision, and motor balance are treated as covariates, the correlation drops so that almost none