senility can be defined as ‘showing the feebleness of old age’ and in this sense it is commonly used to imply feebleness of the mind. As Samuel Johnson has stated in his life of Boswell (1783), there is a wicked inclination in most people to suppose older people decayed in their intellects. He goes on to suggest that if younger or middle-aged men do not recollect where they have left their hats when leaving company it is nothing, whereas if the same happens with an old man then people will shrug up their shoulders and say ‘His memory is going.’ But are there such mental changes associated with normal ageing or is the term ‘senility’ a generalization from the increasingly frequent dementias which occur in old age?

Examining initially the gross changes which occur in the brain with ageing, it can be observed that the normal volume and weight of the adult brain begins to decrease from about 50 years of age. This is due to a reduction in the number of cells in a wide area of the brain: the cerebral cortex, the hippocampus, the substantia nigra, and the cerebellum. Depletion however does not necessarily result in impaired function because there is a generous reserve of cells. There is also a reduction in nerve cell size from the age of 85 years onwards in some areas of the brain.

A high resolution scanning technique, magnetic resonance imaging (MRI), has demonstrated increased white matter hyperintensities, and radioisotope scanning techniques such as single photon emission tomography (SPET) and photon emission tomography (PET) have shown a decrease in cerebral blood flow and metabolism, in normal people with advancing age.

At the microscopic level, we also know that old age is accompanied by the accumulation of certain substances in and around the brain cells. Abnormal structures known as ‘neurofibrillary tangles’ appear in some nerve cells of the cerebral cortex and ‘senile plaques’ containing the protein beta amyloid may be widespread. In a third of older people's brains amyloid deposits are also found in the thin membranous covering (leptomeninges) and in the walls of the blood vessels in the cerebral cortex. Some nerve cells show evidence of shortening or loss of dendrites (their multitudinous fine extensions through which incoming messages are received), whilst others have excessive branching dendrites. In addition to changes in the nerve cells, the intermingling glial cells, which normally form more than half of the brain's substance, spread themselves diffusely in some areas in a manner which resembles the way in which they would respond at any age to local damage.

But do all these changes result in an alteration in ‘normal’ brain function? Certainly there are neuropsychological changes associated with normal ageing and these can best be described as not inevitable and affecting different people at different ages to a variable extent. When population samples of different age groups are compared (cross-sectional studies), intellectual function appears to be at its peak at around 25 years of age, to remain static until the middle 50s and then to decline gradually. But consecutive assessments of large samples of people as they become older (longitudinal studies) demonstrate considerable individual differences, with 60–80% remaining stable or actually improving in some specific abilities. It appears that tests emphasizing the need for speed are particularly prone to deterioration with age. Stored knowledge and its access is less susceptible to change than the more ‘fluid’ aspects of intelligence such as working memory. Thus older people can be viewed as demonstrating significantly faster forgetting of newly acquired information and having a smaller pool of processing resource available. This is evident for example in their difficulty in dealing with divided attention tasks. But at the same time age brings with it an enriched database and intellectual skills which have been accumulated during a lifetime.

The concept of senility may also be a generalization from the increasing number of older people who develop dementia. Dementia is a syndrome due to disease of the brain, usually of a progressive nature, where there is impairment of multiple higher cortical functions and deterioration in emotional control, social behaviour, and motivation, all occurring in clear consciousness. The commonest causes of dementia are Alzheimer's disease and those associated with arteriosclerotic changes in brain blood vessels. The prevalence of dementia in the population is 5% at 65 years and 20% at 80 years of age. The cognitive changes in dementia are widespread, affecting intellectual, language, and memory functioning and as with normal ageing, some functions may change more than others, indicating that there may be subgroups of patients with different cognitive impairments.

Martin Blanchard

senility (sin-il-iti) n. the state of physical and mental deterioration that is associated with the ageing process.
—senile adj.