Cognition through the lifespan: mechanisms of change

Cognitive abilities rise steeply from infancy to young adulthood and then are either maintained or decline to old age, depending on the specific ability. This pattern suggests corresponding continuities of mechanism and process, but it is striking that the fields of cognitive development and cognitive aging make little contact with each other's methods and theories. In this review we examine reasons for this cultural separation, and show how recent findings from both areas fit a framework couched in terms of cognitive representation and control. These two broad factors have very different lifespan trajectories; consideration of their relative growth and decline makes it clear that cognitive aging is not simply ‘development in reverse’. This framework is offered in light of recent interest in finding greater continuity throughout the lifespan and creating a more comprehensive explanation of cognitive function and cognitive change.

Introduction

There is symmetry to our physical lives: we are independent and robust in youth and middle age, but dependent and frail in infancy and old age. On the surface, cognition appears to follow the same general pattern of building up and wearing down. In the brain, too, the consolidation of networks in infancy and early childhood is mirrored by the reduction of connectivity and structural atrophy in older age (Box 1). In all these cases, there is a vulnerability in youth and old age that is not present in the middle of life. However, the conclusion that cognitive aging is ‘development in reverse’ is an oversimplification of a dynamic that unfolds over the lifespan, fuelling the changes that are reflected in distinct types of cognitive ability at different times of life. In the brain, for example, similar behaviors in older and younger adults are often mediated by different neural circuits 1, 2. Our purpose in this article is to propose a framework for examining changes in cognition over the lifespan and consider the implications of that framework for conceptions of cognition and the factors responsible for its change.

There are remarkably few integrated accounts of lifelong changes in cognitive ability, making the exceptions particularly noteworthy. For example, in a dynamic view of lifespan development, Baltes and his colleagues 3, 4 have stressed that change can occur at any time, that development depends on interactions among genetic, environmental and social factors, that all processes of development entail both gains and losses, and that the relative mixture of biological and social-cultural factors change with age. From a different perspective, Salthouse 5, 6 has shown that processing speed increases from infancy to young adulthood and then declines from the twenties to old age; he has argued that this general slowing is the primary cause of age-related declines in cognitive performance. Aside from these exceptions, the fields of cognitive development and cognitive aging have shown little contact.

An integration of the processes of cognitive change in development and aging is essential to the construction of a comprehensive account of the structure of cognition and the factors that influence cognitive performance. The details of cognitive change at each end of the lifespan are now sufficiently known that a broader perspective can be applied. However, a lifespan description of cognitive change requires more than a simple blending of the two fields. Taking language as an example, vocabulary and grammar develop through childhood with only small age-related losses from age 70 on [7]. But aging brings problems of access to stored information, even if there is no decrease in knowledge. The most common memory complaint of older adults is their difficulty in recalling names and words that are specific labels [8]. This information has not been lost from memory, as it can be retrieved later either spontaneously or with better cues. Therefore, although knowledge deficiencies are associated with both development and aging, the limitation in children is due to incomplete acquisition whereas the limitation in old age is associated with difficulties of access. Such asymmetries need to be explained.