The time between the onset of a stimulus and the start of an overt response.Reaction time, or RT, is a measurement of the speed of response. As the definition states, it is the interval between the onset of the stimulus and the start of the response rather than its conclusion. These distinctions are important in such activities as driving a car. It takes about .55 seconds to put on the brakes when the light changes to red, and in this time a car traveling at sixty miles per hour covers forty- eight feet. Then it takes, on the average, a full two seconds to bring the car to a stop, and in that time it travels an additional two hundred feet. The reaction time is the first figure, since it measures the behavior of the driver and not the behavior of the car.The first RT experiments were performed by the German physiologist Hermann Helmholtz in 1850 during his investigation of the speed of the nerve impulse. By applying an electric shock to the skin at various points, he discovered that the interval between stimulus and response varied with the distance from the brain. He carefully measured both the distance and the reaction time, and discovered that the nerve impulse travels at approximately 90 meters per second. He also found that the reaction time was longer for complex reactions involving choice than for simple reflex activities. This suggested—and subsequent experiments have proven—that most of the time taken by complex responses is consumed by brain activity.Since these early experiments, a vast number of reaction time experiments have been performed. The research literature can best be summarized under separate headings:Sense modalities. The reaction time of any individual varies somewhat from moment to moment since it is affected by changes in attention, emotion, receptor sensitivity, and readiness for muscular response. Nevertheless, fairly stable averages for each sense have been obtained by subjecting large numbers of subjects to repeated trials. Simple withdrawal of the finger was used in response to all the following stimuli; electric shock, .143 sec.; sound, .140; touch, .140; light, .180; odor, .210; taste (salt), .308; rotation.400; pain, .888; warmth and cold, .300 to 1.60. The relatively long reaction time for pain may be surprising since it does not seem to be helpful for survival, but it is due to the fact that the sensation depends on injury to tissue, and it takes longer for tissue to break down than merely to respond to warmth or pressure. The superiority of sound over light may also be unexpected, but it is demonstrated by the fact that we react more quickly to an automobile horn than to a red light. The reason for the slower reaction to light is that it involves a chemical change in the retina, while sound only requires a mechanical change in the ear.The reaction times just given have all been classed as simple responses to stimuli. Many of our reactions, however, involve decision—that is, we have to make different responses to different stimuli. The ball player reacts one way to one signal from his coach and a different way to another signal; and in a tight situation the driver has to decide quickly whether to step on the accelerator or the brake. In an early series of experiments, a subject was required to react with individual fingers of the right hand when the Arabic numerals 1 to 5 were presented, and with the left hand when the Roman numerals I to V were presented. The reaction times for these “disjunctive reactions,” as they are called, were about three times as long as for simple reactions. Also it has been found that the more alike the stimuli, the longer the reaction time—for instance, it takes more time to discriminate between red and green than between black and white. Even the factor of preference makes a difference. Subjects were found to react more quickly when one color was greatly preferred to the other than when they were liked equally (Shipley et al., 1945, 1946). These findings are useful in constructing highway signs and signals.Strength of stimulus. Intense stimuli produce quicker reactions than faint stimuli. In the case of sound, there is a continuous drop in speed of reaction from loud sound (.110 sec.) to faint sounds (.400 sec.), according to Choch- olle, (1945). This is why we shout or blow a loud blast on the car horn in warning people of danger. Similarly, we are careful to touch a strange dog lightly and move our hands slowly, since more intense stimulation would quickly evoke defensive responses.Motivation. The speed of response is usually increased when motivation is intensified. If a team is behind, its members are likely to be more alert and react more quickly than when they have a comfortable lead. Since employees rarely work close to their physiological limits, incentives such as bonuses are frequently used to increase their working speed.Group differences. Men, on the average, react about 10 per cent faster than women, although there are wide individual differences. Age is also a factor: the RT decreases steadily until we reach the age of twenty-five, remains relatively unchanged until about sixty, and then slowly increases. Here, too, there are large individual differences, probably because people age at different rates. (Miles, 1942).Specific influences. Moderate amounts of alcohol have been found to lengthen the reaction time only slightly, but large amounts slow us down considerably. Small quantities of caffeine have little effect, but large doses quicken our responses, especially when a choice is involved (Hollingworth, 1912). Nutritional deficiencies and insufficient oxygen tend to slow down reactions. Increased gravitational forces (the G factor) also decrease the speed of response, particularly in airplanes and satellites, since these forces reduce the amount of blood that feeds the brain (Canfield, Comrey, and Wilson, 1949). Tight-fitting pressure suits are now worn by pilots and astronauts to keep the blood from rushing to or from the brain, for either increased or decreased reaction time can be disastrous in flying situations. See air hunger.Set. An individual’s set, or readiness for response, has a considerable effect on reaction time. The standard practice in experiments is to signal the subject during a “foreperiod” that a stimulus will soon be presented, just as we say to a runner, “On your mark, get set.” The foreperiod is varied so that the subject will not react to a constant interval. Experiments indicate that the optimal foreperiod is from two to four seconds in most modalities, although it appears to be a little shorter (1.5 seconds) for a foot race, probably because the contestants are keyed up (Kobayashi and Matsui, 1938). Other studies have shown that in starting a race and in other simple reactions, we respond more quickly if we concentrate on the reaction we are to make (motor set), but in choice or “disjunctive” reactions, it is best to attend to the stimulus (sensory set). See set.Applications. The determination of reaction time has been put to use in a number of psychological fields. Safety studies have revealed that unusually rapid responses are as dangerous as slow responses, since accidents are frequently caused by impulsive, thoughtless behavior. In driving, the fast reactor tends to get hit in the rear, while the slow reactor collides with the car ahead. The time it takes to respond on word association tests is often measured, and it has been found that quick responses indicate freedom from conflict,while hesitation (among other responses) indicates that a stimulus word has touched upon an area of emotional difficulty. In learning tests, such as naming state capitals, rapid responses are taken as an indication of a higher learning level than slower responses. A long RT is also used as an indicator of possible neurological defect, especially on the pupil dilation test. See WORD ASSOCIATION TEST.