Reaction Time

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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 participants 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 occurswe 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 ones 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 words 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 targetsprovides 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).


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


Allen, P. A.; Kaufman, M.; Smith, A. F.; and Propper, R. (1998). A Molar Entropy Model of Age Differences in Spatial Memory. Psychology and Aging 13 (1998): 501518.

Allen, P. A.; Madden, D. J.; and Slane, S. Visual Word Encoding and the Effect of Adult Age and Word Frequency. In Age Differences in Word and Language Processing. Edited by P. A. Allen and T. R. Bashore. New York: North-Holland., 1995.

Allen, P. A.; Madden, D. J.; Weber, T. A.; and Groth, K. E. Influence of Age and Processing Stage on Visual Word Recognition. Psychology and Aging 8 (1993): 274282.

Allen, P. A.; Smith, A. F.; Jerge, K. A.; and Vires-Collins, H. Age Differences in Mental Multiplication: Evidence for Peripheral But Not Central Decrements. Journal of Gerontology: Psychological Sciences 52B (1997): P81P90.

Balota, D. A., and Ferraro, F. R. A Dissociation of Frequency and Regularity Effects in Pronunciation Performance Across Young Adults, Older Adults, and Individuals with Senile Dementia of the Alzheimers Type. Journal of Memory and Language 32 (1993): 573592.

Birren, J. E. Age Changes in the Speed of Behavior: Its Central Nature and Physiological Correlates. In Behavior, Aging, and the Nervous System. Edited by A. T. Welford and J. E. Birren. Springfield, Ill.: Charles C. Thomas, 1965.

Burke, D. M., and Light, L. L. Memory and Aging: The Role of Retrieval Processes. Psychological Bulletin 90 (1981): 513546.

Cerella, J. Information Processing Rates in the Elderly. Psychological Bulletin 98 (1985): 6783.

Light, L. L. Memory and Aging: Four Hypotheses in Search of Data. Annual Review of Psychology 42 (1991): 333376.

Lima, S. D.; Hale, S.; and Myerson, J. How General Is General Slowing? Evidence from the Lexical Domain. Psychology and Aging 6 (1991): 416425.

Luce, R. D. Response Times. New York: Oxford University Press, 1991.

Madden, D. J.; Pierce, T. W.; and Allen, P. A. Age-Related Slowing and the Time Course of Semantic Priming in Visual Word Identification. Psychology and Aging 8 (1993): 490507.

Pachella, R. The Use of Reaction Time Measures in Information Processing Research. In Human Information Processing. Edited by B. H. Kantowicz. Hillsdale, N.J.: Erlbaum, 1974.

Salthouse, T. A. The Processing-Speed Theory of Adult Age Differences in Cognition. Psychological Review 103 (1996): 403428.

Sternberg, S. Two Operations in Character Recognition: Some Evidence from Reaction Time Measurements. Perception & Psychophysics 2 (1967): 4553.

Wickens, C. D. Engineering Psychology and Human Performance. New York: Harper Collins, 1992.

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Reaction Time

Strength variables

Minimum RT and comparison of senses

Complexity conditions

Explanations of RT

Relation to other aspects of behavior


Special conditions


When a human subject follows instructions to make a specific response as soon as he can after the presentation of a specific signal, the latency of the response is called reaction time (RT). Average values of between 150 and 250 milliseconds (msec.) are typically found, for example, where the subject must press a telegraph key when a light is flashed; however, under some conditions, RTs even shorter than one hundred msec. and even longer than one second may occur. The term “RT,” is also applied to the continuous lag of responses to the stream of ongoing events.

Beginnings of the RT experiment are found in two sources. One is Helmholtz’ study of the speed of neural transmission. In an unsuccessful attempt, reported in 1850, to adapt the nerve muscle preparation technique to the intact human organism, he measured the latency of voluntary hand responses to light electric shocks applied to different areas of the skin. It may be supposed that Helmholtz did not appreciate what could be learned through this approach as he did not proceed with it. However, it was not long before many others were actively experimenting and theorizing on the problem.

The other source is the astronomer Friedrich Bessel’s analysis in 1822 of differences among observers in their estimates of the instant, within a series of clock ticks, in which a star passed a cross wire, the “eye and ear” method. With the interest thus aroused in the variability of human behavior, and with the concurrent development of accurate timing apparatus designed to eliminate this factor from astronomical observations, there eventually resulted the measurement of true RTs by two astronomers. One was O. M. Mitchel, an American, who used light and sound signals in 1858; the other was A. Hirsch, a Swiss, who also used electric shock in 1861–1864.

Three early programs of research on RT are of special importance. The first is that of the Dutch physiologist Frans Donders, who in 1868 introduced the experiment in which response is made dependent upon a choice between signals, i.e., the disjunctive RT. By subtracting the RT to a simpler situation from that to a more complex situation, he hoped to find the time required for the additional mental act. The second is that of Siegmund Exner, who first used the term “reaction time” in 1873. His emphasis was on the importance of preparation and on the subject’s feeling of involun-tariness of the response. The third is that of Wundt, started in 1880, which was notable for the attempt to measure the RT for the identification of a stimulus. At least the germ of much of the later experimentation on RT and of the accompanying conceptionaUzations is found in the work of these pioneers. (See Boring [1929] 1950, chapter 8; and Woodworth 1938, chapter 14 for a more complete historical coverage.)

Two characteristics of RT have especially attracted the attention of investigators. First, RT is shortened by “strength” conditions, but not below an irreducible minimum. Second, RT is increased by “complexity” conditions. Much of the research in this field has aimed at developing functional relations between RT and variables concerned with strength or complexity.

Strength variables

Considered here are variations which, in some respect, might be expected differentially to activate, facilitate, or energize some part of the chain of processes between signal and response.

Intensity of signal

It regularly has been observed that more intense signals have a shorter RT. Moreover, two signals of different frequency but of the same subjective intensity will produce extremely similar distributions of RT. If the signal is the cessation of a stimulus, a shorter RT is again found for a strong stimulus. RT to a sudden change of stimulus intensity is shorter as the amount of change is greater.


Discriminability may also be shown to affect RT, and with a wider variety of examples, by the disjunctive, or choice, procedure. The more different the signals, the shorter the RT. This has been demonstrated for frequency of tones, length of lines, gradation of colors, number of objects, and distance between points of tactile stimulation.


Motivation may, in several ways, be shown to have an effect. For example, informing the subject of his RT after each trial decreases RT. A more marked reduction is found by applying an electric shock for high RTs.


In most studies of RT a warning signal is given, perhaps a second or two before the signal for response. If the foreperiod (or warning interval) is increased to seven or eight seconds, RT will be lengthened. Too short a warning interval will increase RT, but a greater increase will result from highly variable or unexpected fore-periods (Klemmer 1956). Degree of muscular tension was found to vary inversely with RT under such manipulations.


Practice has been found effective in reducing RT, especially for complex conditions, where considerable reduction may take place over many daily sessions. Warm-up within a session has also been noted, lowest values not ordinarily being attained before the tenth or fifteenth trial.

Minimum RT and comparison of senses

A typical subject has a mean auditory RT of about 140 msec, under usual testing conditions and mean visual RT of about 180 msec. The difference is similar to that required to set the respective sensory nerves into action by appropriate stimuli to the sense organs, and the argument has been made that the difference in RT for the two modalities lies in this differential for receptor stimulation. The argument would be more convincing if their irreducible minimums of RT were composed of known processes. However, in the case of auditory stimulation, only about 55 msec, is accounted for by measurable “peripheral” events and in visual stimulation about 85 msec. (Woodworth [1938] 1960, p. 19). It is risky to assume that, except for the matter of stimulation, the same mechanisms are involved in auditory and visual RT. As a matter of fact, in the study where the lowest RT values were found for the two modalities, the two mean RTs were both close to 110 msec. (Hovland & Brad-shaw 1935). In this study the view was presented that the typical difference might be explained by the poorer figure-ground segregation under usual visual testing conditions (or, in other terms, a lower signal-noise ratio), a factor that was eliminated by making the visual signal a bright light in completely dark surroundings. In view of the difference in time for the stimulus process apparently demonstrated for the two modalities, this then would imply, if the subtractive is accepted, that the unknown processes require less time for visual than for auditory signals as the irreducible minimums are approached.

RT for touch is sometimes as short as that for sound, sometimes perhaps 25 msec, longer. There are special difficulties in finding RT for other types of stimulation: taste, smell, pain, etc. One problem is the isolation of specific sense qualities: a sharp point will be felt as pressure as well as pain. Another is to ascertain the instant when the stimulus acts on the receptor, as in the case of olfaction. The RTs reported for these modalities generally range between 300 msec, and one second.

Complexity conditions

Complexity is introduced by any circumstance that changes the situation from that of the simple RT, where the same signal appears on each trial, requiring the same response, and in which there is no problem of the relation between signal and response.

Multiple options

The RT where the choice of reactions depends upon which of a set of signals appears has been approached from the point of view of information theory. The same increase in RT has been found between two and four options —one and two bits—as between four and eight options—two and three bits (Hyman 1953). However, the effect of number of options is much reduced by extensive practice and by elimination of coding requirements, for example, using a tactual signal to the finger. Also, if options permitting simultaneous response of several fingers are allowed, RT does not increase greatly with an enormous increase in the number of possible patterns.


Any introduction of indirectness or incompatibility between signal and response will increase RT. If the lights on a display no longer directly indicate the required response, RT will rise (Garvey & Knowles 1954).

Prior involvement

If there is an unpredictable succession of two signals, each requiring a response, RT to the second signal is elevated over normal. The shorter the interval between signals, the greater will be the elevation. The explanation of a “psychological refractory period” holds that the processing of a new response cannot start until the end of the processing of the first response (Welford 1959). Alternatively, it has been suggested that the preparation for the second response is interfered with by the first event (Poulton 1950).

Explanations of RT

It may be well to start with the major problem raised by the relation between intensity of signal and RT. Since speed of neural transmission is independent of intensity of stimulus, the relation between intensity and RT cannot be explained in terms of delays in transmission along neurons. Four modes of explanation have been proposed (Chocholle 1963).

Two of the modes specify kinds of neurological mechanisms; the other two imply the properties they must have. The first explanation is that there is actually a faster over-all neural transmission for stronger signals despite the constant transmission speed along single neurons. One possibility cited is that of more rapid traversal of synapses. This appears inadequate to account for the extent of differences found. Another possibility, that a stronger stimulus obtains access to more direct pathways in the central nervous system (CNS), has not yet been directly tested. The second mode of explanation is that there are sensing mechanisms in the CNS which are activated more quickly by a stronger stimulus because of a higher rate of impulses along ascending neurons or because of the involvement of more fibers. This, too, has yet to be verified.

The third mode of explanation is that RT is short to the extent to which part processes between signal and response can be prepared (Poulton 1950). Perhaps full preparation of the effectors (motor set) cannot be attained or maintained if involvement is required of the mechanisms of detection (sensory set).

The final mode of explanation is in terms of a stochastic decision process which distinguishes between a noisy signal and noise alone (McGill 1963). A strong signal might reach a “criterion count” sooner than a weaker one.

The last two modes of explanation lend themselves to aspects of RT other than its relation to intensity. With fewer alternatives, there can be greater preparation for those that do occur—especially for those most likely—as well as a “clearing of the ways” for the response. Also relevant for this expectancy formulation is the relation between RT and variability of the length of the foreperiod.

The kind of decision model described explains the relation between RT and number of alternatives by the time necessary to distinguish among the “noisy” alternatives. The information-theory approach appears to give a fixed amount of time to each successive binary decision, although the means by which this is accomplished have not been stipulated [seeInformation theory; see also Bricker 1955].

Relation to other aspects of behavior

Reaction time is related to several other areas of investigation.

Association time

In the association experiment, the subject makes a verbal response to a stimulus. Unlike in the case of RT, he employs some already available association: he reads a word, names an object, gives the opposite of a word, or simply replies to one word with another (Woodworth [1938] 1960, chapter 3).

Operant conditioning

In operant conditioning, reinforcement of an operant is first made contingent upon the occurrence of a signal. Next, reinforcement is made to depend upon the rapid appearance of the operant after the signal. Latencies may become very short, in the range of values found for RT (Skinner 1946). Thus, experiments with animal subjects can be made to parallel those with human subjects. The relation of latency to signal intensity has already been studied. [SeeLearning, articles oninstrumental learningandreinforcement.]


In tracking procedures the subject keeps a pointer on a moving target or keeps a target from being moved off center. If the position of the target varies continuously it is difficult—although possible in theory—to determine the RT to the appearance of an error. However, the step-tracking task amounts to no more than a variant of the RT procedure.


Several procedural aspects are relevant to a discussion of RT.


The greatest problem in instrumentation is to make the signal appear substantially instantaneously. Neon lamp and tachistoscopic techniques are superior in this respect to incandescent lamps. Although tones may be introduced instantaneously, a click will be heard at their onset, often contrary to the aims of the experiment.


Manipulation of the telegraph key and the light-pressure snap switch are both satisfactory responses. The response may be either a press or a release. Finger responses are usual, but any movement of the hand, arm, leg, etc. may be employed. A microphonic switch is the preferred method of recording a vocal response. In the step-tracking procedure, a pointer or lever is generally moved laterally.


An immediately accessible RT after each trial may be obtained by means of a continuously running synchronous clock. The direct-current clutch is engaged with the signal and disengaged with the response. Although such clocks are sufficiently accurate for most purposes and are widely employed, electronic counters are superior in avoiding mechanical problems and in not producing clicks. Moreover, permanent registration may be made by means of print-out devices. In the step-tracking procedure, graphic techniques, often utilizing signals produced by transducers, are used to find an index of the start of the response. It is also possible to use such instruments in conjunction with key pressing and other ungraded responses that provide an early indication of the onset of the response. Muscle action potentials provide an even earlier indication (Bartlett 1963). The delay arising from the mass of the recording element may be avoided by use of a camera technique in conjunction with a cathode-ray tube or by use of magnetic tape recording.

Special conditions

For a given condition, where the RT for a three-year-old is 500 msec., an exponential decrease to 150 msec, might be expected to about age thirty, with a shallow increase starting thereafter. For as yet unexplained reasons, slightly lower RTs are found for males than for females.

Predictive use has been made in a number of ways. In testing or training automobile drivers a measure is made of time taken to apply the brake on the appearance of a danger signal. Some negative correlations have been revealed between RT and measures of athletic performance.

A variety of special conditions have been tested; following are a few examples. Prolonged vigilance has been found to increase RT, but no consistent effect has been found to result from simple loss of sleep. Benzedrine can overcome the effect of prolonged vigilance but otherwise has no discernible effect. Alcohol has been found to increase both the mean and the variability of RT.

Robert M. Gottsdanker

[Other relevant material may be found inAttention; Hearing; Nervous system; Senses; Vision.]


Bartlett, Neil R. 1963 A Comparison of Manual Reaction Times as Measured by Three Sensitive Indices. Psychological Record 13:51–56.

Boring, Edwin G. (1929) 1950 A History of Experimental Psychology. 2d ed. New York: Appleton.

Bricker, Peter D. 1955 Information Measurement and Reaction Time: A Review. Pages 350–359 in Henry Quastler (editor), Information Theory in Psychology. Glencoe, III.: Free Press.

Chocholle, RenÉ 1945 Variations des temps de réaction auditifs en fonction de l’intensité à diverses fréquences. Année psychologique 41/42:65–124.

Chocholle, RenÉ 1963 Les temps de réaction. Volume 2, pages 63–112 in Paul Fraisse and Jean Piaget (editors), Traiteé de psychologie expérimental. Paris: Presses Universitaires de France.

Garvey, W. D.; and Knowles, W. B. 1954 Response Time Patterns Associated With Various Display-Control Relationships. Journal of Experimental Psychology 47:315–322.

Hovland, Carl I.; and Bradshaw, Dorothy A. 1935 Visual Reaction Time as a Function of Stimulus Background Contrast. Psychologische Forschung 21: 50–55.

Hyman, Ray 1953 Stimulus Information as a Determinant of Reaction Time. Journal of Experimental Psychology 45:188–196.

Klemmer, Edmund T. 1956 Time Uncertainty in Simple Reaction Time. Journal of Experimental Psychology 51:179–184.

McGill, William J. 1963 Stochastic Latency Mechanisms. Volume 1, pages 309–360 in R. Duncan Luce, Robert R. Bush, and Eugene Galanter (editors), Handbook of Mathematical Psychology. New York: Wiley.

Poulton, E. C. 1950 Perceptual Anticipation and Reaction Time. Quarterly Journal of Experimental Psychology 2:99–118.

Skinner, B. F. 1946 Differential Reinforcement With Respect to Time. American Psychologist 1:274–275.

Teichner, Warren H. 1954 Recent Studies of Simple Reaction Time. Psychological Bulletin 51:128–149.

Welford, A. T. 1959 Evidence of a Single-channel Decision Mechanism Limiting Performance in a Serial Reaction Task. Quarterly Journal of Experimental Psychology 11:193–210.

Woodworth, Robert S. (1938) 1960 Experimental Psychology. Rev. ed. by Robert S. Woodworth and Harold Schlosberg. New York: Holt.

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reaction time Your hand accidentally touches the hot plate of an oven and is withdrawn immediately. A young child runs out in front of your car and you hammer on the brakes. A lottery ball falls into its position upside down and you have to shout out the correct number as fast as you can to a colleague who is checking off the numbers for your syndicate. All three examples of reaction time are the time it takes to make a movement in response to a sensory stimulus. However, even if we try to respond as fast as possible in each situation, the reaction time is quite different.

In this context, time is measured in milliseconds (ms) — thousandths of a second. It may take only 100 ms to withdraw our hand from the stove, 200 ms to stamp on the brakes, and 500 ms to read out the number on the ball. The difference occurs because of the different amount of time it takes for the central nervous system (CNS) to process the sensory signals and to choose the appropriate course of action.

The quickest reaction times have the simplest neuronal circuitry. Tap the knee and the leg moves. This is the tendon jerk beloved of clinical neurologists. The tap excites receptors in the quadriceps muscle at the front of the thigh and these send signals back to the lumbar part of the spinal cord. There, a direct connection is made to the motor neurons that innervate the quadriceps muscle and cause it to contract, making the leg kick forwards. It takes a total of about 30 ms for this to happen. The receptors take 1–2 ms to respond, and another 1–2 ms is needed for the connections to operate in the spinal cord. The remaining 27 ms or so is taken up with the time it takes nerve impulses to travel from muscle to spine and back again. There is of course a price to pay for such a fast circuit. The circuit is so simple that the same thing happens every time the tendon is tapped; it is impossible to control what happens no matter how hard we try. Because of this we refer to this type of reaction as a reflex, and the time it takes as the reflex response time. In electronic jargon we can imagine that it is a hard-wired input–output circuit.

There are rather few examples where the circuit is so simple. The corneal reflex, which causes an eye blink when a speck of dust hits the cornea, is one of the few other familiar examples. Most other very rapid reactions turn out to be more complex. Withdrawal of a hand from a hot cooker is certainly automatic, but can, with great effort of will, be controlled. The neural circuit is more complex than that for the tendon jerk, and this gives it more adaptability at the expense of a longer response time. However, like the tendon jerk, this is a circuit that is innate, and ready for action from the moment we are born.

More complex reactions, like hitting the brakes to stop a car in an emergency, are neither innate nor hard-wired. After all, a person who had never been in a car before would have no idea how to stop the vehicle. They are learned responses that can be selected with remarkable speed in the correct conditions. In the simplest situation we may be asked to press a button as soon as possible after a light is illuminated. There is no ready-made circuit to do the job. Instead, the motor system prepares in advance the instructions for the response (move the arm), and all our attention is concentrated on the light. As soon as a change in illumination is detected, the instructions for movement are released and the button is pressed. In this situation the CNS narrows down the total possible number of movement options and sensory events to just one of each, and links them together with high security. Of the millions of possible connections between sensation and movement, one is highlighted by the preparation to respond in a particular manner. In the case of driving a car, there may be several circuits that have a particularly high probability of being called into action. One of them may link the operation to press the brakes hard with the unexpected arrival of an object in the path of the vehicle. Such very fast responses are sometimes referred to as voluntary, to indicate the necessary involvement of volition in preparing to respond in a particular way to what may well be an arbitrary event. The term ‘voluntary’, however, does not mean that we need consciously identify the sensory signal before issuing the instructions to move. Drivers will often volunteer that they pressed the brakes before knowing what it was that was in front of the car. They may well say that it was a ‘reflex’ response, presumably indicating that conscious appreciation of the action occurred only after the event.

There are some responses that require much more careful evaluation of the sensory input before an appropriate movement can be selected. These have longer reaction times, since the circuits cannot be prepared in advance with any certainty. Calling out upside-down numbers on lottery balls is probably in this category. First of all the visual field must be rotated mentally by 180 degrees, and even then, fifty possible responses are available, perhaps narrowed down to 10 if the colour of the ball is known. All of this takes the CNS a good deal of processing, and by the time the response (vocalization of the number) is selected, the sensory impression has probably reached consciousness.

In summary, reaction times span a spectrum of response types. At one end are very fast, predefined neural circuits such as the tendon jerk and the withdrawal reflex that always operate, but which can be modulated, depending on how complex their connections are, by volitional control. At the other end, sensations must first be evaluated and then assigned the correct motor response, which prolongs the response time by a factor of ten or more. In the middle, situations occur in which the CNS can accurately predict what to do when a certain simple sensation is received. In these circumstances, sensory and motor circuits are selected in advance and joined with high probability so that processing time is reduced to an absolute minimum.

J. C. Rothwell

See also reflexes.
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Reaction time

Generally, in psychological measurement, the interval of time between the presentation of a stimulus to a subject and the beginning of the subject's response to that stimulus.

Several categories of reaction time, such as simple reaction time, have been established and studied in experimental psychology . In a simple reaction time experiment, the subject is presented with one simple stimulus, such as a light, and instructed to perform one simple response, such as pressing a button. In a discrimination reaction time experiment, the subject is presented with one of two or more different stimuli, such as a red light and a green light, and instructed to perform a response to only one of the stimuli, such as pressing a button when the red light is presented but not when the green light is presented. In a choice reaction time experiment, the subject is presented with one of two or more different stimuli, such as a red light and a green light, and instructed to perform different responses depending upon which stimulus is presented, such as pressing a red button when the red light is presented and pressing a green button when the green light is presented. There are other types, and many variations of reaction time experiments.

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reaction time
1. In geomorphology, the time taken for a system to react to a sustained change in external conditions. Representative reaction times are difficult to define, because of variations both in the resistance of systems to change and in the magnitude of the external change. For example, a sand-bed river channel reacts more readily to change than does a rock-floored channel. See also RELAXATION TIME.

2. See CORONA.

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reaction time In geomorphology, the time taken for a system to react to a sustained change in external conditions. Representative reaction times are difficult to define, because of variations in the resistance of systems to change and in the magnitude of the external change (e.g. a sand-bed river channel reacts more readily to change than does a rock-floored channel). See also relaxation time.

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reaction time (latent period) The period of time between the detection of a stimulus at a sensory receptor and the performance of the appropriate response by the effector organ. This delay is caused by the time taken for the impulse to travel across the synapses of adjacent neurons. The reaction time for a reflex response, involving only a single linking synapse, is very short.