Aging is associated with decline in a multitude of cognitive processes and brain functions. A growing body of literature suggests that age-related decline in cognition could sometimes be reduced through fitness training. Several studies found that fitness training was found to have robust but selective benefits for cognition. The magnitude of fitness effects on cognition was also dependent on a number of programmatic and methodological factors, including the length of the fitness-training intervention, the type of the intervention, the duration of training sessions, and the gender of the study participants. This paper provides a discussion of theoretical developments exploring the relationship between fitness training and cognition, a review of studies conducted, and a suggestion for future studies.
The study of the relationship between physical fitness, and more specifically aerobic fitness, and cognition dates back several decades. The questions of whether or not such direct relationship does indeed exist, to what extent, and what variables make this connection the strongest have been the subject of many studies. Research using animal models provides reasonable grounds to expect aerobic exercise to have a positive impact on human cognitive function. According to Kramer et al. (2003), researchers found increases in capillary density in the cerebellum when rats exercised on a running wheel. Other researchers found that increased aerobic fitness, engendered by running, enhanced cortical high-affinity choline uptake and increased dopamine receptor density in the brains of old rats, and increased the number of new cells in the hippocampus of mice.
Despite these promising results with animals, the fitness interventions with humans show more ambiguity. The ambiguity could be the result of methodological factors. For example, some studies have tested widely varying age groups. Other variables include: the nature, intensity, and length of the aerobic fitness manipulations; the type of fitness measures employed; the general health and fitness level of the participants at the beginning of the study; subjects' gender; the tasks used to measure aspects of cognition; and the nature of the control group.
For the purposes of this paper, it becomes important to define exercise and cognition. According to the Webster's online dictionary the definition of exercise is: "the bodily exertion for the sake of developing and maintaining physical fitness" (http://www.webster.com/cgi-bin/dictionary). Many different definitions of exercise do exist though. Another commonly used definition from Webster's dictionary: "something that is performed or practiced in order to develop, improve, or display a specific power." This particular definition does not include physical fitness, but instead leaves it open by using the word something. For the sake of this paper, the first meaning will be the one utilized here. Exercise and fitness training are used interchangeably, along with some other substitutes in this paper. All the descriptions are in line with the above-presented definition of exercise.
A definition of cognition according to Webster dictionary online is: "mechanisms involved in such processes in human beings as perception, attention, learning, memory, thought, concept formation, reading, problem solving, and the development of such behavior in children" (http://www.webster.com/cgi-bin/dictionary). Although different type of cognition tests reviewed in this paper test various parts of the information processing, this definition sufficiently addresses the meaning of cognition used for the purposes of this paper. More specific definition of cognition exist, but are not necessary for understanding of this paper.
Major Theories in Research
A number of experiments have been designed based on a prior predictions derived from theories grounded in the Yerkes-Dodson Law (Yerkes & Dodson, 1908). These studies are characterized by the repeated assessment of an individual's cognitive performance as his or her level of physical arousal increases as a direct function of exercise. Typically, exercise protocols include bouts of aerobic and bouts of anaerobic exercise that are relatively brief, with most protocols lasting less than 20 minutes. Demonstration of an initial improvement in performance followed by a decline in performance as arousal increases from a resting state is taken as support for the Yerkes-Dodson Law.
The results of laboratory studies that attempt to link cognitive performance to specific levels of exercised-produced are ambiguous. There are studies that provide clear evidence for an inverted U-shaped relation between arousal and cognitive performance, studies that provide only partial support for the relation, and still other studies that provide no support for the relation.
Other researchers have, either explicitly or implicitly, assumed that fitness effects would be most likely observed in tasks such as simple reaction time or finger tapping, which presumably tap low-level central nervous system function uncontaminated by subject strategies or high-level cognition (Dustman et al., 1984). This theoretical hypothesis has been termed as the speed hypothesis. Still, other researchers have suggested that fitness effects might be most readily observed for visuospatial tasks (Stones & Kozma, 1989), because visuospatial processes have been demonstrated to be more susceptible to ageing than verbal skills. This belief is termed as a visuospatial hypothesis.
Chodzko-Zajko and his colleagues (Chodzko-Zajko & Moore, 1994) have suggested tasks that require controlled, effortful processing should be more sensitive to fitness differences among older adults than tasks that can be executed via automatic processing. This controlled-processing hypothesis is based on earlier research on a theory of skill acquisition in which task components or skills transition from controlled, effortful processing to automatic processing with consistent practice.
Finally, Kramer et al. (1999) argue that improvements in fitness would be reflected in enhancements in executive-control processes such as coordination, inhibition, scheduling, planning, and working memory. These tasks stand in contrast to controlled tasks in that they do not become automatic over time, but require constant mediation by the central executor. Executive-control processes and the brain areas that support them have shown disproportionate sensitivity to aging. This hypothesis has been coined as the executive-control hypothesis.
Other studies have used a specific model of cognition in order to show at what stage cognition is effected though exercise. Proctor, Reeve, & Weeks, (1990) have presented an information-processing model of cognition. This model assumes that behavior is the result of information that is extracted from the environment and processed through a series of three non-overlapping stages, each operating independently of the others. The three stages include the stimulus-identification stage, which involves a number of discrete processes through which sensory events are transformed and given meaning; the response-selection stage, which is characterized by processes that determine what response, if any, will be made to an environmental event; and the response-programming stage, which prepares the motor system for movement. Considerable experimental research conducted over the past four decades has isolated and examine the function of variety of information-processing system components. The information-processing model has been promoted as a useful framework to assess the impact of exercise on cognition.
Studies Exhibiting No Correlation
Proponents of exercise report that brief bouts of exercise help them think more clearly, improve their mood, and enhance their psychological well-being. The view that bouts of physical activity influence cognitive function is less well substantiated by empirical research, however. Two of the studies reviewed for this paper by Tomporowski, Ellis, and Stephens (1987) and Travlos and Marisi (1995), failed to show a relation between aerobic exercise and cognitive function.
Tomporowski, Ellis, and Stephens (1987) evaluated the effects of aerobic exercise on runners' free-recall memory. In the first of two experiments, 24 college students of average cardiovascular fitness ran on a treadmill at a speed that corresponded to 80% of their VO2 max for 50 minutes. Participants then completed a series of paired-associate memory tests immediately following exercise. Memory performance of the exercisers did not differ from those of a non-exercise group.
In the second experiment, 12 student-athletes with very high levels of cardiovascular fitness completed memory tests immediately following a 50-min treadmill run. The memory performance of highly fit participants did not differ from the performance of individuals with average cardiovascular fitness. The authors concluded from their results that aerobic exercise does not influence the encoding or retrieval information from long-term memory, regardless of subject's level of cardiovascular fitness.
Similarly, Travlos and Marisi (1995) assessed young men classified as high or low fit. Participants performed a 50-min cycling exercise regimen designed to be progressively more effortful every 10 min. A choice-reaction-time test and a test of concentration were administered during and following exercise. The exercise protocol had no effect on either measure and there were no substantial differences between the test performances of high- and low-fit individuals.
Improvement in Cognition
A study by Etnier et al. (1997) assessed fitness in 41 older (age range 60-79 years) and 42 younger (age range 20-29 years) participants. This study was designed to examine differences in performance on fluid and crystallized intelligence tasks as a function of age and fitness. It also examined the influence of age and fitness on the beneficial effects that practice has on both performance and retention on these tasks.
Older participants performed better than younger participants on the crystallized intelligence task; however, younger participants performed better than older participants on the fluid intelligence task. On the fluid intelligence task, older fit participants performed better than older unfit participants. Learning did occur on the fluid task and differed as a function of age and fitness. Learning did not occur on the crystallized task.
Another study by Kramer et al. (2003) in the Journal of Gerontology involved well-educated men and women ages 55 to 79. Their fitness ranged from sedentary to very fit, competitive-ready athletes. Fitness was measured by results of one-mile-walking and treadmill stress test. Three-dimensional scans of the participants' brains were done using MRI equipment. Applying voxel- based morphometry, researchers estimated tissue atrophy in a point-by-point fashion in the targeted regions of the brain. The scans showed differences in three areas of the brain, the frontal, temportal and parietal cortexes, with distinct differences particularly in two types of tissues, the gray matter and whit matter (Fig 1).
Figure 1.
(Colcombe et al 2002)
Gray matter regions preserved with cardiovascular fitness.
Areas of gray matter that shrink with age.
White matter regions most affected by age.
White matter regions as boosted by fitness
A meta-analytic study by Kramer and Colcombe (2001) was conducted to examine the hypothesis that aerobic fitness training enhances the cognitive vitality of healthy but sedentary older adults. Eighteen intervention studies published between 1966 and 2001 involving hundreds of participants ages 55 and older were entered into the analysis. Fitness training was found to have robust but selective benefits for cognition, with the largest fitness-induced benefits occurring for executive-control processes.
A longitudinal study by Barnes, Yaffe, Satariano, and Tager (2003), was designed to determine whether cardiorespiratory fitness at baseline is associated with maintenance of cognitive function over 6 years or with level of cognitive function on tests performed 6 years later on healthy older people. Cardiorespiratory fitness measures were based on a standard treadmill exercise test protocol and included peak oxygen consumption (peak VO2), treadmill exercise duration, and oxygen uptake efficiency slope (OUES). Baseline measures of cardiorespiratory fitness were positively associated with preservation of cognitive function over a 6-year period and with levels of performance on cognitive tests conducted 6 years later in healthy older adults.
Clarkson-Smith and Hartley (1989) investigated relationships between physical exercise and the cognitive abilities of older adults. A series of cognitive tasks was given to 62 older men and women who exercised vigorously and 62 sedentary men and women. Between-group differences persisted when vocabulary, on which the performance of exercisers was superior, was used as a third covariate. Results suggest that the possible contribution of physical exercise to individual differences in cognition among older adults should be further investigated.
Conclusion
The positive effect of exercise on cognition of older adults appears to be more than that of younger adults. This could be due to older adults having more to gain as age-related declines become more prevalent. The meta-analysis indicated that the largest fitness-induced benefit occurred for executive-control processes. The same analysis found that exercise programs involving both aerobic exercise and strength training produced better results on cognitive abilities than either one alone. A study conducted by Etnier et al. (1997) illustrated that maintaining a high fitness level in older adulthood helps with performance on fluid intelligence tasks.
More research is needed to follow up on a promising study by Kramer et al. (2002) that utilized three-dimensional scanning to detect changes in the brain due to exercise. According to Kramer (2002): This, to our knowledge, is the first human data providing a potential anatomical account of the cognitive effects that we and others have found over the years. These findings provide the first empirical confirmation of the relationship between cardiovascular fitness and neural degeneration as predicted in various academic studies on again and cognition in both animal and human populations. Our data also suggest that more research is clearly needed to actually do a thorough examination of brain structure and functioning, and the impact of interventions such as fitness and cognitive training. (p 180.)
Although many theories exist on how to approach the study of correlation between cognition and exercise, a formation of one main theory will facilitate better results in the future. Nevertheless, theoretical accounts of fitness effects on human cognition can only rarely be neatly and precisely fit into the processing components of existing theories. Realization that making sense of a literature that has often produced confusing and seemingly unreliable results will be part of the challenge for future research (Kramer & Colcombe, 2001).
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