In Gulliver´s Travels (1726) Jonathan Swift described the darker side of very advanced age. The immortal Struldbrugs of Luggnugg did not enjoy the everlasting wisdom, knowledge and happiness that Gulliver longed for himself. Rather, they all lost their memory, forgot names and suffered personality and emotional changes. After seeing this, Gulliver´s “Appetite for Perpetuity of Life” declined. While the reality may not be as gloomy as described by Gulliver it is a fact that a large percentage of the very old (> 85) either have dementia or some decline in cognitive function. But a small minority of individuals retain their cognitive abilities into very advanced age. That these individuals exist shows that cognitive impairment among the very elderly is not unavoidable, although it may be the exception.
Age- related changes in cognition
It is generally accepted that episodic memory, working memory and cognitive speed decline with advancing age. Other parts of cognition are well preserved or change relatively little until very late in life. It has been a matter of debate when cognitive decline starts. Some maintain that it starts quite early in middle age (i.e. age 45-49) and even in the 20s and 30s, others claim that there is little evidence for decline until after age 60. Large studies, with thousands of participants, have been devoted to this issue. But is this interesting research? Is it useful to know that on average people do worse on cognitive tests when they are 70 than when they are 40? I am not sure and I think it is time to move beyond such studies.
Individual variability on many cognitive tests increases with age. In general, younger people are healthier and it is mostly differences in innate abilities that contribute to observed differences in memory among them. Other factors can also contribute, such as education, anxiety and fatigue at time of testing as well as certain diseases. More people in their seventies are burdened with diseases that can lower test scores. They may have vitamin deficiencies, thyroid disease, Parkinson´s disease, sleep apnea, depression, cerebrovascular disease and hypertension. The list goes on and on! They might also have early Alzheimer´s disease which can go unnoticed in large sample studies. This leads to more variability in their cognitive test scores; some do well but a larger percentage does worse and this lowers the mean score. Therefore, one might argue that trying to find the age at which memory, or other cognitive factors, starts to decline is an uninteresting and even futile resarch question.
What should we do?
Age per se does not explain changes in cognition, not any more than the passage of time explains why iron rusts. Research has shown that many neurobiological indices (e.g., neurotransmitters, synaptic density, brain volume) change with age, and this might be the basis for changes in cognition as we age. These changes take place at a different rate among different people and why that is needs to be investigated. Furthermore, these changes do not correlate perfectly with cognitive performance. Our brain can resist some damage before it is reflected in our behaviour (i.e. memory, attention, cognitive speed). So, instead of trying to find the average age when cognitive decline starts we should try to find out why some individuals can maintain cognitive health into ripe old age, even in the presence of pathological changes in the brain. And why others with similar changes in the brain do much worse cognitively. We need to focus on the variability among the elders instead of looking at differences between the young and old. Studies like this might eventually increase the number of individuals who maintain brain health and cognitive health into very old age.
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