Culture, Neuroimaging, and Aging
Before we examine adaptive functioning and changes in the brain, we will look at another important area that is receiving growing attention: the interplay between culture and neurobiological aging as it affects the mind. Denise Park has spearheaded this approach by asking questions such as where we are likely to find neurocultural differences in the aging brain. She became particularly interested in the ventral visual cortex, which is the perceptual region of the brain. This is a highly specialized region that shows little shrinkage with age. Interestingly, some areas of age-related changes in perceptual processing do not show cultural differences, whereas other areas do (Goh, Chee, Tan, Venkatraman, Hebrank et al., 2007). For example, older adults from Western cultures showed significantly greater object-processing adaptation in the lateral occipital complex, which is involved in visual processing, than did older East Asians, who showed almost no adaptation (Goh et al., 2007). This finding provides neuroimaging evidence for
cultural biases in perceptual processing of objects. Overall, however, age-related change seems to have a more profound effect on changes in brain functioning than culture.
Compensation and Prefrontal Bilaterality
One of the most significant findings in the cognitive aging neuroscience literature is the observed discontinuity of neural activation patterns when comparing younger and older adults’ brain activity during the performance of cognitive tasks. As indicated earlier, it is not simply that older adults show reduced activation in regions associated with a particular cognitive task. Initial studies focusing on verbal working memory and long-term memory presented evidence for focal, unilateral activity in the left prefrontal region in younger adults and bilateral activation (i. ein both the left and right prefrontal areas) in older adults when performing the same tasks (Buckner, 2004; Cabeza, 2002; Reuter-Lorenz, 2002). These findings ushered in a flurry of discussion as to what this means for the
aging brain. Is the older brain working harder to compensate for deterioration in these focal regions related to the cognitive task, or is its inefficient operation of inhibitory mechanisms rendering the activation as interference to optimal functioning (Park & Reuter-Lorenz, 2008)?
In a recent review of this literature, Park and Reuter-Lorenz (2008) noted that there is now a growing body of evidence indicating that this bilateral activation in older adults may serve a functional and supportive role in their cognitive functioning. Findings include the association between bilateral activation in older adults and higher performance (not found in younger adults) (Cabeza et al., 2002; Reuter-Lorenz et al., 2001; Rypma & D’Esposito, 2001) in a number of tasks including category learning tasks, visual field tasks, and various memory tasks. Figure 2.2 shows that there is more frontal bilateral activity in older adults during working memory tasks than in younger adults (Park & Reuter-Lorenz, 2008). On the left side of the figure, you can see that there is left lateral – ized frontal engagement in younger adults, whereas older adults further engage the right frontal areas.
The right side demonstrates that younger adults and low-performing older adults show right lateral – ized activation during a long-term memory task. Interestingly, high-performing older adults show bilateral frontal engagement. It may be that high – functioning older adults are more adept at compensating for normative deterioration in the brain by utilizing other areas of the brain. Whether or not this is an accurate conclusion is still being debated in the literature. We will examine this in the Current Controversies section.
More recently, Park and Reuter-Lorenz (2008) have proposed the STAC (scaffolding theory of cognitive aging) model. They maintain that one needs to push the envelope regarding the compensatory role of bilateral activation and over-recruitment. In other words, one must ask what aspects of cognitive function the compensation serves. For example, there is growing evidence that the increase in frontal activity in older adults may be a response to the decreased efficiency of neural processing related to the perceptual areas of the brain (Park & Reuter-Lorenz, 2008). Interesting findings also have emerged when examining the default network of the brain. This refers to regions of the brain that are most active at rest, for example, when an individual lies quietly in the magnetic imaging machine and is not directly engaged in a cognitive task (Raichle et al., 2001). When an individual 50 CHAPTER 2
begins a demanding cognitive task, this default network is suppressed. A number of researchers have found that older adults display less suppression of this default network than younger adults do (Grady et al., 2006) and that this is related to lower cognitive performance (Persson et al., 2007). Thus, this failure to shift from a resting state to a more active state to engage in cognitive processing may be another reason for increased frontal activity in older adults (Reuter-Lorenz & Cappell, 2008; Reuter-Lorenz & Lustig, 2005).
The STAC model (Park & Reuter-Lorenz, 2008) suggests that the reason older adults continue to perform at high levels despite neuronal deterioration is because of compensatory scaffolding. This is the recruitment of additional circuitry to bolster functional decline. A difference between this model and others is that compensation is not simply a neural response to brain insults as we grow older, but the brain’s response to challenge in general. Thus, you may see this occurring when younger adults are learning a new task. Learning moves from effortful processing to overlearning. The neurological shift is from a broader dispersed network (the scaffold) to a more focal and optimal circuit of neural regions. However, what Park and Reuter-Lorenz point out is that the initial scaffolding remains available as a secondary circuitry that can be counted on when performance is challenged. This is what older adults
A number of studies have shown evidence for increased frontal bilaterality and decreased hippocampal activity in older adults across numerous tasks including attention, working memory, and long-term memory (Cabeza et al., 2004), suggesting the global nature of this phenomenon. In sum, additional age-related neural activation (especially in prefrontal areas) may be functional and adaptive for optimal performance. Researchers now suggest that these activation patterns may reflect an adaptive brain that functionally reorganizes (Park & Reuter-Lorenz, 2008).
A number of models have been used to attempt to explain these findings, including the HAROLD model by Cabeza (2002), the CRUNCH model developed by Reuter-Lorenz and her colleagues (Reuter-Lorenz, 2002; Reuter – Lorenz & Mikels, 2006), and most recently the STAC model (Park & Reuter-Lorenz, 2008). Again, these models make the assumption that the primary reason for greater activation in different brain regions as well as bilaterality is the need for the recruitment of additional brain regions in order to successfully execute cognitive functions.
As indicated, numerous studies have documented the fact that younger adults show unilateral brain activation when performing various cognitive tasks. In contrast, older adults’ brains tend to show increased activation in both brain hemispheres.
This finding has led to various theoretical developments including the formation of the HAROLD (hemispheric asymmetry reduction in older adults) model (Cabeza, 2002). The HAROLD model elegantly explains the empirical findings of reduced lateralization in prefrontal lobe activity in older adults. It suggests that the function of the reduced lateralization is compensatory in nature, that is, additional neural units are being recruited to increase attentional resources, processing speed, or inhibitory control.
According to the CRUNCH (compensation-related utilization of neural circuits hypothesis) model developed by Reuter – Lorenz and her colleagues (Reuter-Lorenz, 2002; Reuter – Lorenz & Mikels, 2006), the aging brain adapts to neurological decline by recruiting additional neural circuits (in comparison to younger adults) to perform tasks adequately. This model incorporates bilaterality of activation, but suggests this is not the only form of compensation. Two main mechanisms are suggested that the older brain uses to perform tasks: "more of the same" and "supplementary processes." When task demands are increased, more activation can be found in the same brain region relative to easier tasks. This effect can be found in younger as well as older adults. In older adults, neural efficiency declines, and therefore additional neuronal circuits are recruited earlier than in younger adults. Supplementary processes are taking place when different brain regions are activated to compensate for lacking processing resources. Reduced lateralization is one way of recruiting additional resources. In addition, however, compared to younger adults’ brains, older adult brains also show overactivation in different brain regions, suggesting that compensation can take different forms in the aging brain.
These findings have stimulated scientific debates on the mechanisms and functional adaptiveness of reduced lateralization. Whereas Cabeza and Reuter-Lorenz and colleagues interpret their findings in the light of a compensational framework, other researchers have challenged this interpretation by suggesting that the over-recruitment of neurons is not necessarily specific to the areas that control the identified cognitive processes (see Kramer et al., 2006; Logan, Sanders, Snyder, Morris, & Buckner, 2002). If researchers cannot link the over-recruitment to specific processes to improve cognitive functioning, then a compensation interpretation is in question. This highlights the fact that to date theoretical work and empirical data in the field of neurocognitive science are still in the early stages of development. Therefore, definitive conclusions about the adaptiveness and functionality of the observed patterns of increased brain activation cannot be drawn at this point in time.
Neuroscience as a Basis for Adult Development and Aging 51
may be doing. However, scaffolded networks are less efficient than the honed, focal ones. For older adults, the focal networks become less efficient, and therefore they engage in the scaffolded circuits which may be less efficient and associated with poorer performance. The trade-off is that without the scaffolding, performance would be even worse because older adults would have to rely on the more focal areas. The elegance of this model is that older adults’ performance can be understood in terms of factors that impact decline and those that impact compensation. As Park and Reuter-Lorenz argue, this integrative approach embraces a lifelong potential for plasticity and the ability to adapt to age-related changes.
In sum, neuroscientific methods have demonstrated that cognitive functioning can be understood at new levels. Advances in methods allow us to adequately test conditions under which age-related structural change is associated with decline, compensation, or even improvement in functioning. Rather than using general biological deterioration as the default explanation for cognitive changes, we can now identify specific biological mechanisms reflected in different structures of and activation patterns in the brain. In addition to identifying regions of the brain that decline, these techniques have also allowed us to differentiate preserved areas of the brain, such as the amygdala (which is involved in emotional processing) from areas that are more prone to decay, such as specific areas in the prefrontal cortex. These respective areas relate to preserved emotional processing on the one hand, and decline in other more higher-order executive cognitive processes on the other. These areas will be explored in a later section on social-emotional neuroscience and aging.
1. What age-related changes occur in the structure of the brain? What cognitive functions are they related to?
2. How are age-related changes in the dopaminergic system related to cognitive aging?
3. What support is there for the argument that bilateral activation serves an adaptive role for older adults in their cognitive functioning?
4. How does the STAC model improve upon the CRUNCH and HAROLD models of brain activation change and aging?
5. What are the cultural differences in brain activation?