Ageing and the human brain

There is little doubt that the brain changes with time, and all research in psychiatry is predicated on holding age constant in comparing groups of patients or estimating the effect sizes of causal factors. Nevertheless, relatively little is known about the mechanisms that are responsible for translating time into ageing. This chapter tries, after an overview of the principal mechanisms involved in biological ageing, to summarize the age-related changes observable in brains in vivo and to demonstrate the types of investigations that may cast light on such mechanisms in the future. A useful heuristic device to order the multiple potential causes of ageing is the chronic stress–allostatic load model, widely employed in epidemiology, public health medicine, and health psychology. In vivo imaging provides a method to test the translation of intermediate stress markers, such as vascular risk, metabolic syndrome, or allostatic load, into predictors of age-related brain changes.