2. Aging on different levels
3. Li's critique on resource-reduction theories
4. Li's theory on aging cognition
Aging is a process usually associated with less flexibility, cognitive and physical decline. Especially the cognitive abilities like fluid intelligence which is related to working memory, attention and processing speed seems to be influenced largely by aging processes. However, aging is not only a natural phenomenon that concerns everyone personally. In an industrialized society there is an increasing number of aged individuals. Thus, the pressure to deal with an aging society and its consequences of reduced cognitive flexibility and its practical impact back on the community pushes research on aging into the center of attention. But the question remains: What exactly is cognitive aging? It is a phenomenon whose effects appear on different levels such as on neuromodulation, cognition and behavior. There have been certain theories on what processes underlie cognitive aging such as the resource reduction theory that tries to link cognition to behavior. However, Professor Shu-Chen Li from the Max Planck Institute for Human Development and her colleagues criticize this simplistic view of the resource reduction theory as it does not adequately explain the interaction between the different levels. Therefore they come up with a new cross-level theory of what mechanisms could underlie behavioral manifestations of cognitive aging deficits introducing the neurotransmitter dopamine as a significant correlate for aging. Thus, it needs to be shown first why the traditional view of aging from the resource reduction theory's point of view seems to be insufficient to explain the aging effects. Secondly, it is to be demonstrated what Li's own theory can contribute to this complex domain. Thirdly, one will return to the question of what cognitive aging might be and what other factors influence it. Finally it will be discussed to what extent Li's approach is satisfactory concerning the understanding of the processes of aging on different levels.
2. AGING ON DIFFERENT LEVELS
Concerning the cognitive side of aging there seem to be three main effects: On the one hand, the ability to activate, represent and keep information is declining. The capability to process information efficiently is declining as well. On the other hand, as consequence it seems harder for older brains to filter relevant and ignore irrelevant information (cf. Li et al., 2001: 479). The decline of information processing also demonstrates neurobiologically as with a decreased efficiency of neuromodulation. Hence, neurotransmitters like monoamines especially serotonin, dopamine or noradrenalin are less produced with aging. This could have serious effects on working memory and processing speed (cf. Li et al., 2001: 479). Effects on working memory in turn are related to the ability to process information because the working memory activates and holds information simultaneously in the immediate memory while processing the same or other information. But there is also evidence that working memory subserves attentional and mnemonic control functions (cf. Li et al., 2001: 479).
3. LI'S CRITIQUE ON RESOURCE-REDUCTION THEORIES
Regarding the aging effects on neuromodulation, cognition and behavior the next question occurs: how are these different levels related to each other and how do they interact? On explaining this process traditional reduction theories give the following explanation: Behavioral changes in aged individuals are caused by impairment of cognitive resources such as working memory, processing speed and attention regulation, which decline with aging (cf. Li et al., 2001: 480). There is supporting evidence that resource reduction is linked to cognitive aging. Gray matter changes have been observed with increasing age especially in the hippocampus and in the frontal cortex. Both entities are related to working memory just as declarative memory functions (cf. Nyberg, Bäckman; 2006: 243). There exist also studies that tried to correlate reduced frontal lobe volume in older age to errors on tests (Nyberg, Bäckman; 2006: 244).
However, Li criticizes this potential theory as being too simplistic with logical flaws. She sees especially two problems with the conventional explanation of the resource reduction theory:
1) On the one hand the declining cognitive recourses like working memory, processing speed and attention regulation cannot be regarded independently from each other as they are interlaced empirically just as conceptually. This means that for example attentional control could be an important component of working memory.
2) On the other hand the resource reduction explanation seems to hold a circular argument: It is claimed that the processing resources like working memory are declining and therefore cause cognitive impairment. But concurrently the impaired behavior of aged persons is being explained by the declined cognitive resources.
Because of the existing difficulties that the resource reduction theory holds by merely linking cognition to behavior Li et al. suggest to impose a link between the processing resources and its neurobiological basis. Besides the fact, that she regards neuroanatomical degeneration as a too drastic way to explain normal cognitive aging1 Li refers light cognitive deficits rather to "neurochemical shifts in still relatively intact neuronal circuits" (Li et al., 2001: 480).
4. LI'S THEORY ON AGING COGNITION
She sees the promising neurochemical correlate for aging in the neurotransmitter dopamine as important link between neuromodulation and cognition. There has been certain empirical evidence that dopamine is related to aging processes as its quantity declines with aging. Since dopamine receptors are active in the extrastratial regions, in the striatum and in the frontal cortex, dopamine D1 receptor loss could also be witnessed in the striatum and the frontal cortex. Besides, there is empirical evidence for a parallel between working memory, processing speed and the level of dopamine (cf. Li etal., 2001:481).
1 Neuroanatomical degeneration she attributes rather to pathological aging like Alzheimer's disease.