Functional Consequences of Brain Deterioration
The major theoretical approach reviewed above focuses on age-related changes in brain function and is typically labeled the frontal lobe theory of aging (Buckner, 2004; Raz, 2000; West, 1996). This theoretical approach suggests that the many age-related declines in cognitive functioning are a function of insults to the frontal lobes, both structurally and neurochemically. Functional neuroimaging examines the neural substrates of cognitive decline. The typical finding that corroborates this approach is reduced activation in older as compared to younger adults in prefrontal and medial-temporal areas supporting cognitive functioning, such as memory (Grady, McIntosh, Horwitz, Maisog, Ungerleider et al., 1995; Logan, Sanders, Snyder, Morris, & Buckner, 2002). Interestingly, some studies also have shown a marked increase in prefrontal activity during cognitive tasks, specifically memory, in older adults as compared to younger adults (Cabeza, Anderson, Locantorre, & McIntosh, 2002; Grady, Bernstein, Siegenthaler, & Beig, 2002; Gutchess et al., 2005). However, we will come back to this point later.
An example of the first finding is a fairly recent study that demonstrated age-related differences in activation of the anterior cingulate cortex during a verbal working memory task (Otsuka, Osaka, Morishita, Kondo, & Osaka,
2006) . As indicated earlier, working memory is linked to executive functioning and involves temporary storage and processing of information which supports higher-order cognitive functioning. In particular, it serves an attentional role by controlling attentional resources for performance on cognitive tasks. The anterior cingulate cortex is affiliated with the prefrontal cortex and is also involved in executive control (Bunge, Klineberg, Jacobsen, & Gabrieli, 2000). The overall finding is that reduced working memory performance is related to decreased activation of the anterior cingulate cortex in older adults as compared to younger adults (Otsuka et al., 2006). Other research further supports this finding, suggesting that reduced brain activation
48 CHAPTER 2 or under-recruitment of the prefrontal cortex occurs during intentional cognitive processing (Buckner, 2004).
Furthermore, Buckner (2004) points out that reduced frontal recruitment in aging is context – dependent. For example, research has demonstrated that older adults show reduced activation or recruitment of the appropriate frontal regions associated with intentional memory. At the same time older adults recruit these regions almost to the same degree as younger adults when support is offered during a memory encoding task (Logan, Sanders, Snyder, Morris, & Buckner, 2002). The researchers have concluded that this may be a matter of decreased flexibility on the part of older adults to produce effective retrieval strategies. However, other studies have shown that younger and older adults display comparable frontal activity or recruitment, for example, when examining memory retrieval (Schacter et al., 1996). As we will see later in this chapter, an increase in frontal recruitment may serve adaptive functioning purposes in older adults.