4.6 Consequences of neural ageing
While we are beginning to understand the underlying molecular and cellular changes that take place in the ageing brain, the consequences of these changes are all too familiar. As people age, their mental competence may change and their ability to cope with the demands of everyday life may alter. A decline in the speed of acquisition of new information is typical of people in middle and old age.
To what extent are observed declines in memory a function of ageing as such, and to what extent are they attributable to other factors, such as educational or job experience, differences in motivation, or differences in health?
Is there an age-related decline in every type of memory during ageing?
To address these questions, cognitive psychologists studying neural ageing have developed specific memory assessment procedures to enable them to understand age differences in memory in older adults. Using memory assessment procedures and personal interviewing approaches, psychologists study memory changes in the population of older adults by tracking the same people year after year as they age. Advancing age is the greatest risk factor for development of the two main human neurodegenerative disorders: Alzheimer's disease (AD) and Parkinson's disease (PD). These two diseases, and some others, are characterised by the occurrence of distinct neuronal changes – in particular, neuronal death and the increased presence of ‘senile plaques’ and ‘neurofibrillary tangles’, which you will read about in the Selected Readings that follow. The relationship of these two ‘pathological’ features to disease onset and progression is still a matter of debate, as you will find out, not least because they are also found in ‘normal’ ageing brains. Exactly why ageing is the major risk factor for AD and PD is largely a mystery; but it is clear that if we wish to understand the mechanisms underlying age-related brain diseases, as well as brain physiology during senescence, we need to study the brain during healthy, normal ageing. However, until recently, changes in the properties (such as gene expression) of brain cells during healthy ageing received less attention than studies of cells in specific brain diseases. A tragic consequence of these neurodegenerative disorders is dementia. However, this term encompasses a range of conditions, and dementias can have other causes. Recently, the World Health Organization has agreed a classification of different types of dementia. Over the last 40 years or so, the average human lifespan has increased by more than 20 years – an extra 20 years spent in an environment that has become more chemically polluted and much more complex, both socially and physically. Perhaps it is not surprising then, that in recent years there has been a progressive increase in the incidence of not only brain disorders that are stress- and environment-related, but also age-related neurodegenerative disorders.