Changes in memory processing with age

Abstract

Over the years, a large body of literature has shown that humans display losses in memory with age, but that not all types of memory are affected equally. Similarly, recent evidence from functional neuroimaging experiments has revealed that, depending on the task, older adults can display greater or lesser activity in task-relevant brain areas compared with younger adults. Recent behavioral and neurophysiological experiments are furthering our understanding of the effects of aging on cognition. It appears that some brain changes seen with age may be compensatory.

Introduction

There is general agreement in the literature on cognitive aging that memory performance declines from early to late adulthood, and that such age-related losses in performance are much greater in relation to some tasks than in others. Decrements are typically slight in implicit memory tasks (including priming) in which a stimulus that has been presented previously affects current behavior when presented again, often without the person realizing that the stimulus was encountered beforehand. Age-related declines are also slight both in short-term memory span tasks, in which subjects repeat back a short string of words, letters or numbers, and in many recognition memory tasks, in which previously encountered events (e.g. words, sentences, pictures, faces) are re-presented along with new distractor items of a similar type. In contrast, age-related losses are substantial in tasks involving free or cued recall and those involving recollection of the original context in which an event occurred, as well as in ‘prospective memory’ tasks (i.e. remembering to carry out an action at a future time) and in ‘working memory’ tasks. These last-named tasks usually necessitate the manipulation of information held in mind or the maintenance of some information while dealing concurrently with further incoming stimuli. Evidence in support of these findings is summarized in a number of recent reviews 1•, 2•, 3•, 4.

The fundamental biological changes underlying this pattern of relatively spared versus reduced memory abilities in the elderly are still under active debate, although there is emerging agreement from recent neuroimaging studies that certain cortical areas (such as the prefrontal cortex) are less active in older adults under some conditions, but more active under other conditions. Much of the recent neuroimaging work has been in the area of verbal memory, and a common finding in young adults is that they have increased activity in the left prefrontal cortex during encoding and in the right prefrontal cortex during retrieval (for reviews, see 5, 6). In contrast, older adults often have less activation of left frontal areas during encoding [7], but bilateral prefrontal activation during retrieval 7, 8. Older adults have also been found to have greater activity in left prefrontal cortex during performance of some nonverbal tests of retrieval [9], but not all [10]. These age-related differences in prefrontal activity are probably attributable to some extent to the different task demands involved in the various experiments, but increased left prefrontal activity in older adults may somehow be necessary for them to perform adequately on the tasks. In some cases, the additional recruitment of left prefrontal cortex during retrieval was seen during memory tasks when performance in the older group was comparable to that of younger adults, leading to the suggestion that this recruitment may be a compensatory mechanism [7]. A fundamental issue in this respect is whether the recruitment of additional areas in older adults actually aids their ability to perform the task.

With regard to the theoretical mechanisms underlying age-related memory deficits, current suggestions at the cognitive level include the notions of a general slowing of cognitive operations [11] and an age-related decline in attentional resources [12]. Other researchers have postulated that aging is associated with a reduction in the efficiency of inhibitory processes [13]. It has also been postulated that older adults are impaired in their ability to consciously recollect events and and the contexts in which they occurred, but are relatively unimpaired in their experience of the familiarity of a recognized object or person. Thus, when an older individual encounters a slightly known acquaintance, especially in an unusual setting, the acquaintance may seem familiar, but the older person may be unable to recollect where or when they had originally met [14]. Similarly, the influence of learned habits on behavior is unchanged with age, although the older person may be less aware of the original source of the learned information. This last combination of impaired and unimpaired processes means that the cognitive performance of older adults is dominated to a considerable extent by habitual patterns of thought and action 4, 15••. Another popular notion is that older adults have difficulty in associating aspects of an event [16] or in ‘binding’ such aspects into a coherent representation [17]. The present brief review focuses largely on recent studies of the effects of aging on episodic memory for personally experienced events, their neural correlates, and their behavioral manifestations.