Because of developments in the literature on ageing, the classic distinction between generalized and process-specific cognitive changes with old age has changed into a distinction between the frontal lobe hypothesis and more differentiated views of neurocognitive ageing.
There is a very heterogeneous pattern of age effects. The cognitive changes that are caused by old age vary in frequency, direction and extent.
The literature on ageing has seen some big changes, caused by a revolution in neuroscience, sophistication of methodology and more different perspective on cognitive ageing. Three of these new distinctions will be discussed now.
This is a distinction between attributing age effects to computational versus control processes. To perform well on a task requires a combination of low-level processes, supportive processes, and higher level control processes. Age effects on low-level computational processes would contribute to the performance on only one task domain. While age effects on executive control processes contribute to performance on almost all domains. As soon as the task becomes more difficult, the deficient control processes become visible. It is then possible that executive control, perhaps together with functional limitations of working memory, are the underlying mechanism that can explain age-related performance decrease. In most of the results it is indeed seen that older adults have a control deficiency instead of a computational deficiency.
For the same task, older brains can use a different balance between control processes and subordinate processes when compared to brains of younger adults. They might do this to compensate for some loss by raising the pressure at the processing level or by stronger recruitment of frontal coordination.
It is becoming less common to think of age effects only as quantitative changes in an invariable cognitive network. But the changing efficacy comes from several qualitative differences. So is speed in older adults less, but accuracy higher. Their episodic memory in information processing is exchanged for semantic memory. And the loss of prospective memory is compensated for by the use of mnemonics. It may seem that the younger adults have brains that can endure more and are more flexible. But the brains of older adults have a lot of experience and work more accurate.
There is a consensus that there is a dominant age effect across processes that is bigger than local effects. Because of this, it is now normal to examine age-related effects only to the extent that they exceed the general effects. To find these age-related effects you have to nullify the a priori differences between age groups. There are a few ways to do this, namely by separating general effects in a covariance model, or by equating task difficulty between subjects, or by transforming data in such a way that multiplicative effects turn into additive effects.
Newer models see frontally mediated executive control as the general mechanism that causes changes across cognitive domains.
This hypothesis states that because of differential decline in old age of neural tissue in the prefrontal cortex, cognitive functions that are being supported by these areas are more susceptible to age effects than functions that rely more on posterior and subcortical areas. However, a direct link between changes at the cell level and changes in performance has not been found. And to really qualify as a mechanism for cognitive ageing, the biological changes have to be reflected in functional decrements. Research so far has not been very promising, often misleading, lacking construct validity and did not come up with more than some weak correlations.
The most important functions of the prefrontal cortex include exerting executive control and adaptively supporting the content of working memory. Many believe that executive control the main role plays in cognitive ageing. However, executive control contains many different skills. These functions are not all completely intertwined and equally sensitive to ageing effects.
Not all cognitive ageing is caused by executive problems, and not all executive functions are responsible for cognitive ageing. Executive control is dependent on the functioning of the subordinate systems. If one of these is not intact, there will be more pressure on coordination and the chance that the coordination fails becomes bigger. Even if control sometimes doesn’t work well for older adults because of difficulties in maintaining a rule, this can still be compatible with the idea that the control mechanism is intact.
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