LEARNING OBJECTIVES
• How do animal models contribute to our understanding of neural plasticity and aging?
• What evidence is there for neural plasticity in aging humans?
• How does aerobic exercise influence cognitive aging?
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arisa has been playing incessantly with her latest Nintendo Wii video game. Her grandmother, Leticia, became captivated by her granddaughter’s gaming and asked her granddaughter to teach her how to do it. Marisa was delighted and helped her grandmother learn the game. After months of practice, Leticia noted that she seemed stronger in her normal physical activities and her perceptual skills seemed to improve. In addition, she and her granddaughter Marisa had more in common than ever before.
As discussed previously, models explaining age — related changes in brain functions as a form of compensation embrace the notion that there is plasticity in both behavior and at the level of changes in the brain across the adult life span. Plasticity involves the interaction between the brain and the environment and is mostly used to describe the effects of experience on the structure and functions of the neural system. This is certainly evidenced by Leticia’s observations of the positive changes that took place after she started playing the Wii video game.
Plasticity is a multifaceted concept that applies across the life span. For example, as illustrated in the previous section, it can refer to the ability to compensate for declining performance from a behavioral perspective or to the reorganization of neural circuitry as a form of compensation from a
neuroscience perspective. Many attempts have been made to assess the potential for plasticity in cognitive functioning by focusing on the potential to improve cognitive performance following training (a good example would be Leticia’s practice on the video game).
One of the more prominent behavioral findings in the literature can be found in P. Baltes and col — legues’ attempts to examine the range of plasticity in older adult cognitive performance (e. g., Baltes & Kliegl, 1992; Willis, Bliezner, & Baltes, 1982). They found that whereas older adults are able to improve cognitive ability in memory tasks through tailored strategy training beyond the level of untrained younger adults, this is highly task-specific, and the ability-level gains are very narrow in focus.
More recently from a behavioral perspective, research has suggested that basic cognitive processes affected by aging can be improved through training and transfer to multiple levels of functioning as long as the basic functions are shared across tasks (e. g., Dahlin, Neely, Larsson, Backman, & Nyberg, 2008). From a neural plasticity perspective, more recent work on animal models of plasticity have revealed compelling evidence that demonstrates the effects of experience on various aspects of brain functioning in adulthood and aging (see Jessberger & Gage, 2008). Interesting
research shows that neural stem cells (which give rise to new neurons) persist in the adult brain and can generate new neurons throughout the life span (Gage, 2000; Jessberger & Gage, 2008). This has shed doubt on the long-standing belief that neurogenesis (i. e., the development of new neurons) dwindles away at the end of embryonic development, and gives new life to debunking the old myth that you cannot teach an old dog new tricks.
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Moreover, this more recent research has added a whole new level of understanding to what happens to individuals as they grow older. For example, even though aging is associated with a striking decrease in the number of new neurons, this may be altered even at advanced ages. When older mice were exposed to enriched environments, they performed much better on spatial memory tasks than mice in more deprived environments (Kempermann et al., 2002). This was demonstrated by changes in the structure of the brain. Figure 2.3 illustrates that younger mice showed a high number of neural stem cells (depicted in the sprouting black illustrations on the left of panel A). For older mice, there was a drop in the amount of hippocampal neurogenesis, as seen in the middle of panel A. However, with enriched experience, this was somewhat reversed (see the third part
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Playground area for Hamster and mouse. Made out of paper tubes, gift box package and cartons. |
of panel A). Panel B shows that the number of dendrites in young (on the left) and older (on the right) mice does not differ. This suggests that the speed of neuronal maturation is not substantially affected in older rodents.
Similarly, physical exercise in elderly mice enhanced the number of newly generated neurons about fivefold (van Praag et al., 2005), and this was also associated with improved performance. The most compelling finding from this study is that the exercise started very late in the life of the mice. Thus, even in advanced stages of aging, the brain retains its capacity to react to external stimuli that are related to improved cognitive performance. Furthermore, environmental enrichment periods were short and yet sufficient to enhance both the generation of new neurons and behavioral performance in aged mice (Jessberger & Gage, 2008). Of course, these leaps and bounds found in aging mice may not be comparable to the leaps and bounds of younger mice, as indicated by the Baltes et al. research discussed above, but this new research certainly sheds a more optimistic note to the concept of aging.
Evidence of these types of effects in human brains is growing. For example, new learning has been linked to structural changes in the brain.
Draganski and colleagues (2004) demonstrated positive structural changes in the brain in individuals who were novices at juggling to begin with in comparison to learning the skill after a period of three months. Similarly, evidence indicates that hippocampal volume increased in medical students after extensive studying for an exam (Draganski, Gaser, Kempermann, Kuhn, Winkler, Buchel et al., 2006).
These types of findings extend to older adults as well, although limited in nature. For example, positive biochemical changes, such as increased creatine and choline signals in the hippocampus, in healthy older adults were observed after five weeks of training using method of loci mnemonic strategies to improve memory (Valenzuela, Jones, Wen, Rae, Graham, Shnier, & Sachdev, 2003). Mnemonics strategies involve using personally relevant cues to help one remember information. In a study using positron emission tomography (PET) scans, Nyberg and collegues (2003) further investigated brain activation, comparing younger and older adults, after the method of loci training. They found that younger adult brains’ showed increased metabolic activity in the left dorsolateral prefrontal cortex when applying the method of loci, but this activity was not observed in older age groups. However, both younger and older adults who showed a benefit from the method displayed increased left occipito-parietal activity. Another group of unimproved older adults showed no increase in activity of either brain areas. Similar to what we discussed earlier, they concluded that, at a neural level, although neural plasticity is present across the adult life span, there are age-related reductions in this plasticity and the potential for functional improvement.
Certainly, studies on neural plasticity in humans are limited. However, with advancements in histological and molecular techniques for examining human postmortem samples, along with in vivo noninvasive methods such as MRI, fMRI, PET, and others, future research may be able to close the apparent gap between animal and human research in the context of adulthood and aging neural plasticity. Some of the most compelling work so far has moved
54 CHAPTER 2
HOW DO
WE KNOW?