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REACTION TIME

In cognitive psychology, reaction time (RT) is used to measure the amount of time that it takes an individual to process information (Luce). It is the duration of the interval between presentation of a stimulus (e.g., a word on a computer monitor) and the participant’s response to the stimulus. RT is considered to be a dependent variable because it ‘‘depends’’ on the manipulation of an independent variable (such as the exposure duration of a stimulus). RT is related to response accuracy (the other primary dependent variable in cognitive psychology), because participants can often trade off speed for increased accuracy, or conversely, trade off accuracy for increased speed (Pachella). It is important to note, though, that accuracy and RT are often used for different purposes. Accuracy tells us whether a series of perceptual and mental processes is completed correctly. RT is used to infer process duration.

Stages of information processing

Overall task RT data can certainly be interesting; older adults have consistently been shown to be slower than younger adults, for example. But it is the decomposition of RT into times for individual stages in mental processing that is of most scientific interest. Figure 1 illustrates attentional resources and the basic stages of human information processing: perceptual encoding, memory activation, decision-making, response selection, and response execution (Wickens). Attentional resources provide the processing ‘‘energy’’ to the information processing system. Encoding involves the initial processing of sensory and perceptual information. For example, while driving we must convert the physical energy of the light waves hitting our eyes into neural impulses that the rest of the cognitive system can understand before we can begin to identify a circular yellow approaching object. After encoding has occurred, we compare the perceived stimulus to information stored in long-term memory. This comparison process is likely based upon the similarity of the input stimulus code to codes stored in long-term memory. Pattern recognition has occurred when the system identifies the yellow stimulus as a ‘‘yellow traffic signal.’’ The decision[M1]-making stage of processing then begins. Based on vehicle speed and distance from the intersection, we must decide whether to slow down or to continue to accelerate. Response selection then occurs—we decide to press either the brake or the accelerator pedal. And finally, response execution involves carrying out the decision made during response selection (actually moving one’s foot to the brake pedal).

It is seldom possible to get exact processing times for each stage of mental processing. As a result, psychologists frequently study peripheral or sensorimotor processing by combining input (encoding) and output (response execution) times. The central processing stages of memory retrieval, decision-making, and response selection are also combined. Processing times for peripheral and central processing can be empirically separated with experimental manipulations that affect one stage (say, central), but not the other (peripheral). Consider an experiment using a lexical decision task (does a letter string form a real word or not) with three levels of word frequency. Since a word’s frequency, how common it is, should affect neither initial registration of the light waves nor speed of response execution, we can reasonably assume that differences in RT that are dependent on word frequency must be due to central processes. In Figure 2, separate functions are plotted for younger and older adults across word frequency. Older adults have a higher y-intercept than younger adults, but both age groups show the same slope. Our logic, supported by past research, suggests that the level of the function is primarily a measure of peripheral processing, but that the slope of the function is a measure of central processing (Allen, Smith, Jerge, and Vires-Collins; Sternberg). Since slopes are the same, there is no evidence of age-related slowing of the central processes affected by word frequency. In this case, overall age differences in RT are due to peripheral processes and possibly some central processes that are not affected by word frequency (Allen, Madden, Weber, and Groth).

Age differences in reaction time

More generally, how does adult age affect RT? Information processing takes longer (Cerella; Salthouse) and its duration becomes more variable (Allen, Kaufman, Smith, and Propper) with increasing age. This has led many people to believe that aging is invariably associated with slowing and decline. However, increased adult age does not affect all processing stages equivalently.

For example, the lexical decision data in Figure 2 show that while older adults show a peripheral-process decrement, they show no drop in speed compared to younger adults in lexical access speed (a central process involving memory retrieval). Using a word-naming task, similar results were observed by Balota and Ferraro (1993). Reviews of the literature on lexical processing conclude that there are no appreciable age differences in central processes, but that older adults do show longer overall RTs due to slower peripheral processing (Allen, Madden, and Slane; Lima, Hale, and Myerson; Madden, Pierce, and Allen). Lexical tasks involve semantic memory or knowledge, including vocabulary (Tulving, E., 1985). Semantic memory tasks all tend to show a similar pattern of age differences: peripheral-, but no central-process decrements.

Other types of information processing, though, do show both central- and peripheral-process age differences. Episodic memory tasks ask individuals to remember personally experienced events and their temporal relations (e.g., what you had for breakfast this morning; see Tulving, E., 1985). Large age differences are found in episodic memory (Burke and Light, 1981; Light, 1991), and as can be observed in Figure 3 (from Allen et al., 1998, Experiment 1), these appear in both slope and intercept. The steeper slope shown by older adults across transposition distance—i.e., how far probe items are shifted relative to where they occurred as targets—provides specific evidence for slowing of central processes in this episodic task (smaller distances require more central processing). Central slowing is a hallmark of episodic memory tasks, as well as many other information-processing tasks (Cerella).

Conclusion

While it is true that older adults do show longer overall processing time than younger adults (Birren), this RT slowing is not constant across all processing stages and tasks. For semantic memory tasks such as a lexical decision (Allen et al., 1993) or a naming (Balota and Ferraro), older adults show slower peripheral processing (encoding and response execution), but there are no appreciable age differences in central processing (particularly for memory retrieval). However, for many episodic memory tasks, there are actually larger central-process than peripheral-process age differences (Cerella). Research using RT, especially when it can be decomposed to shed light on specific stages of mental processing, will ultimately move us toward a deeper understanding of the changes in thinking that accompany aging.

PHIL ALLEN

BIBLIOGRAPHY

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