Aging, Memory, and Information Processing Speed

Measures Sensitive to Aging

The behavioral measurements that are most sensitive to changes in aging brains are memory failures and slowing of the speed with which we can make simple decisions. Slowing seems to reflect a basic change in brain efficiency rather than loss of motivation or greater caution, because the limiting speed at which accurate decisions can be made becomes longer as brain aging progresses.

Until the 1980s, when brain imaging began to provide direct measures of neurophysiological changes in living and functioning brains, speculative models for cognitive aging could be derived only by comparing different groups of older and younger people or by repeatedly testing the same people, as they aged, on the same behavioral tasks. Unfortunately, any behavioral task can give us only two different measures of performance: how fast and how accurately we can perform it. These two measures trade off against each other, as in memory tasks, where, although the immediate measure of performance is the number of errors made, shortening the time that we are allowed to inspect items impairs our ability to remember them. Fergus Craik's excellent metaphor is that we increase the reliability of our memory in proportion to the “depth of processing” that we can achieve by making connections [Page 14]with other material that we already know. Deeper processing takes longer and, because they process information more slowly, older people are at a proportional disadvantage.

For this reason, slowing of information processing speed must, inevitably, appear a sensitive “master” index of age-related change in all tasks. Because people can maintain accuracy by responding more slowly, changes in speed are usually detectable before changes in accuracy are observed. Thus, to be detectable, any observed changes in task performance with age must involve changes in speed. The deeper question is whether, apart from being the critical one of the only two task performance indices that we can measure, speed is also the predominant performance characteristic of our cognitive systems. That is, whether slowing of decision speed is the functional cause of changes in all or most other mental abilities. If it is, we may hope that, by comparing the relative sizes of differences in speed between older and younger people across a variety of tasks, we may be able to use behavioral evidence alone to frame useful hypotheses as to whether aging affects some functional systems earlier and more than others. For example, we might suppose, with the earliest investigators of human behavior such as Wilhelm Wundt and Franciscus Donders, that loss of speed may directly reflect changes in the basic neurophysiological performance characteristics of our brains and central nervous systems. Until the late 20th century, investigators such as Hans Eysenck, Art Jensen, Tim Salthouse, and others suggested that speed on simple tasks directly mirrors elementary biological properties of our nervous systems such as synaptic conduction times, rates of neural transmission, or the degree of degradation of connectivity in neural networks; discussions in cognitive gerontology over the past 50 years have been dominated by this point of view. More recent evidence suggests that declines in maximum information processing speed limit memory efficiency not only by restricting the depth to which we can process information or affecting the maximum rate of rehearsal but also because slowing is a marker for basic neuronal changes in the entire brain that affect all cognitive abilities.

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