Timothy A. Salthouse

THE PROCESSING-SPEED THEORY OF ADULT AGE DIFFERENCES IN COGNITION

A theory is proposed to account for some of the age-related differences reported in measures of Type A or fluid cognition. The central hypothesis in the theory is that increased age in adulthood is associated with a decrease in the speed with which many processing operations can be executed and that this reduction in speed leads to impairments in cognitive functioning because of what are termed the limited time mechanism and the simultaneity mechanism. That is, cognitive performance is degraded when processing is slow because relevant operations cannot be successfully executed (limited time) and because the products of early processing may no longer be available when later processing is complete (simultaneity). Several types of evidence, such as the discovery of considerable shared age-related variance across various measures of speed and large attenuation of the age-related influences on cognitive measures after statistical control of measures of speed, are consistent with this theory.

Decomposing Adult Age Differences in Working Memory

Two studies, involving a total of 460 adults between 18 and 87 years of age, were conducted to determine which of several hypothesized processing components was most responsible for age-related declines in working memory functioning. Significant negative correlations between age and measures of working memory (i.e., from -.39 to -.52) were found in both studies, and these relations were substantially attenuated by partialing measures hypothesized to reflect storage capacity, processing efficiency, coordination effectiveness, and simple comparison speed. Because the greatest attenuation of the age relations occurred with measures of simple processing speed, it was suggested that many of the age differences in working memory may be mediated by age-related reductions in the speed of executing elementary operations. Working memory is generally defined as the preservation of information while simultaneously processing the same or other information. It is distinguished from other forms of memory because the assumption that it reflects both processing and storage implies that it plays an important role in many cognitive tasks (e.g., Baddeley, 1986; Carpenter & Just, 1989; Salthouse, 1990). An illustration of the hypothesized functioning of working memory in one cognitive task, mental arithmetic, is presented in Figure 1. The left column in this figure indicates the operations to be performed, and the right column represents the intermediate products that must be temporarily stored while carrying out those operations. This figure is useful because it graphically illustrates both the importance and the complexity of working memory. That is, it is clear from this example that effective storage of information is essential in order for the successful performance of certain cognitive tasks. Figure 1 also suggests that it may be fruitful to think of working memory not as a single discrete structure, but rather as a dynamic interchange among three conceptually distinct aspects or components-processing efficiency, storage capacity, and coordination effectiveness. Processing is represented by the series of operations in the left column, storage is represented by the entries in the right column, and coordination can be assumed to correspond both to the sequencing of operations and to the arrows portraying the exchange of information between processing and storage. A primary purpose of this article was to investigate the contribution of these three hypothesized components to age-related differences in measures of working memory. Each of the components has been hypothesized to be an important source of adult age differences by one or more researchers, but few definitive conclusions have been possible because the currently available evidence is both weak and inconsistent. To illustrate,

Theoretical perspectives on cognitive aging

Contents: The Need for, and Requirements of, Theories of Cognitive Aging. What Needs to Be Explained? Environmental Change. Disuse Interpretations. Qualitative Differences: Structures, Strategies, and the Relation Between Competence and Performance. Analytical Approaches to Localization: I. Memory Abilities. Analytical Approaches to Localization: II. Reasoning and Spatial Abilities. Reduced Processing Resources. Final Words and Future Directions.

When does age-related cognitive decline begin?

Cross-sectional comparisons have consistently revealed that increased age is associated with lower levels of cognitive performance, even in the range from 18 to 60 years of age. However, the validity of cross-sectional comparisons of cognitive functioning in young and middle-aged adults has been questioned because of the discrepant age trends found in longitudinal and cross-sectional analyses. The results of the current project suggest that a major factor contributing to the discrepancy is the masking of age-related declines in longitudinal comparisons by large positive effects associated with prior test experience. Results from three methods of estimating retest effects in this project, together with results from studies comparing non-human animals raised in constant environments and from studies examining neurobiological variables not susceptible to retest effects, converge on a conclusion that some aspects of age-related cognitive decline begin in healthy educated adults when they are in their 20s and 30s.

Executive Functioning as a Potential Mediator of Age-Related Cognitive Decline in Normal Adults

Critical requirements for the hypothesis that executive functioning is a potential mediator of age-related effects on cognitive functioning are that variables assumed to reflect executive functioning represent a distinct construct and that age-related effects on other types of cognitive functioning are reduced when measures of executive functioning are statistically controlled. These issues were investigated in a study involving 261 adults between 18 and 84 years of age. Although age-related effects on various cognitive abilities were substantially reduced after statistical control of the variance in measures hypothesized to represent executive functioning, there was only weak evidence for the existence of distinct constructs corresponding to executive functioning or to aspects of executive control concerned with inhibition, updating, or time sharing.

Aging and measures of processing speed

Many variables have been assumed to reflect speed of processing, and most are strongly related to age in the period of adulthood. One of the primary theoretical questions with respect to aging and speed concerns the relative roles of specific and general age-related effects on particular speed variables. Distinguishing between specific (or unique) and general (or shared) age-related influences on measures of speed has been complicated, in part because the issues are sometimes framed in terms of extreme all-or-none positions, and because few researchers have employed analytical procedures suitable for estimating the relative contributions of each type of influence. However, recent methods focusing on partitioning age-related variance have indicated that large proportions of the age-related effects on individual speed variables are shared with age-related effects on other variables. Although these theoretical ideas and analytical procedures are fairly new, they may be relevant to a variety of psychophysiological or neurobiological variables.

Processing speed as a mental capacity

Throughout the lifespan, there are pronounced age differences in speed of processing, differences that are consistently related to performance on measures of higher-order cognition. In this article, we examine domain-specific and global explanations of these age differences in processing speed; we conclude that although experience can play a role in age differences in speed, there is also evidence that a general mechanism limits speeded performance. We also review research that shows the influence of processing speed on the quality of performance on nonspeeded tasks such as reasoning and memory. We suggest that speed of processing should be viewed as a fundamental part of the architecture of the cognitive system as it develops across the entire lifespan.

Mediation of Adult Age Differences in Cognition by Reductions in Working Memory and Speed of Processing

Three studies, involving a total of 672 adults between 20 and 84 years of age, were conducted to evaluate the relative importance of working memory and perceptual comparison speed in the age-related differences in selected measures of cognitive functioning. The same measures of working memory and comparison speed were used in each study, but the studies differed in the specific cognitive measures examined. A common finding across all studies was that the magnitude of the age-related cognitive differences was greatly reduced by statistically controlling measures of working memory and perceptual comparison speed. Many of the age differences in process or fluid aspects of cognition therefore appear to be mediated by age-related reductions in working memory, which may in turn be largely mediated by age-related reductions in the speed of executing simple processing operations.

Effects of age and skill in typing.

What are the factors responsible for skilled typing performance, and do they change with the age of the typist? These questions were addressed in two studies by examining time and accuracy of keystrokes in a variety of typinglike activities among typists ranging in speed from 17 to 104 net words per minute and ranging in age from 19 to 72 years old. Typing skill was related to the temporal consistency of making the same keystroke, the efficiency of overlapping successive keystrokes, the speed of alternate-hand tapping, and the number of characters of to-be-typed text required to maintain a normal rate of typing. Older typists were slower in tapping rate and in choice reaction time but were not slower in speed of typing, apparently because they were more sensitive to characters farther in advance of the currently typed character than young typists.

What and When of Cognitive Aging

Adult age differences have been documented on a wide variety of cognitive variables, but the reasons for these differences are still poorly understood. In this article, I describe several findings that will need to be incorporated into eventual explanations of the phenomenon of cognitive aging. Despite common assumptions to the contrary, age-related declines in measures of cognitive functioning (a) are relatively large, (b) begin in early adulthood, (c) are evident in several different types of cognitive abilities, and (d) are not always accompanied by increases in between-person variability.