Reaction Time

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, Figure 1 Attentional Resources SOURCE: Author 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).