REACTION. In psychology, a term used to denote response to sensory stimuli. In experi mentation this response takes the form of physical action, and as the stimulus applied is also physical, the time between the giving of •t he stimulus and the response may lie readily measured by means of electrical appliances and delicately adjusted clocks. This time is called the reaction time, and includes physical, physi ological, and psychical factors in the process measured. Reaction time is found to vary much both with individuals and with practice, and since the physical and physiological processes may he assumed or shown to be fairly constant, it affords a fair test of variation in the times of mental processes.
Simple reaction time may be determined in many ways. Let Its suppose that a clock, record ing accurately to the second, is connected electrically with a hammer and an electric key. The connections are so arranged that the hands of the clock begin to move when the hammer falls upon its block. The sound thus made serves as stimulus to the reactor, who presses the button of the electric• key as soon as he has heard it the pressing of the button stops the clock hands, and the time of reaction (the time elapsing between the giving of the simple stimulus and the execution of the simple move ment made in response to it) is thus registered in thousandths of a second. Very many ob servations have been made, with auditory, visual, and tactual stimuli, so that the norms of re action are now well established. The simple reaction time is, evidently, the time required for the purely physiological process of conduction of excitation from sense-organ to brain; for the psychophysical process of sensation and impulse; and for the purely physiological process of con duction of excitation from brain to ]rand. If we know the 'rate of conduction in sensory and motor nerves, we can. by subtraction. determine the time required for the central or psychophys ical process alone.
The simple reaction time varies largely with direction of the attention. There are three pos sibilities. The reactor may seek to distribute his attention, to attend both to the mining stimulus and to the movement of reaction: or he may attend predominantly (almost exclu sively) to the sensory stimulus, or again to the movement. So we have three norms of reaction time: the central or natural time, the sensorial or complete time, and the muscular or abbre viated time. The natural time of an untrained subject will differ, according as he is disposed by his mental constitution to attend mainly to his own movement or to the stimulus presented to him: but it will. in every ease, lie some where between the two extreme times. The norms for these (in thousandths of a second), as determined with practiced subjects. are: The times differ, that is, with differences of sense-organ appealed to; and the average differ ence between the two times for the same sense department is second.
The simple reaction experiment, presents two aspects, a qualitative and a quantitative. Re garded in the first aspect, the reaction is the exact type of a voluntary action; it is an im pulsive action reduced, by laboratory devices, to its lowest terms; it differs from the impulsive actions of real life only by greater simplicity of motive (stimulus) and greater simplicity of responsive movement. We may therefore make it. the basis of an introspective examination of action at large—in which case the time-values have merely a regulative importance, as an ex ternal check or control upon the validity of in trospection. Regarded in its second aspect, the
reaction measures the duration of a certain com plex mental process. If, then, we vary this process in known ways, the consequent variation of the reaction times gives us a similar measure of other mental processes.
( 1) Under the first heading we note that the course of the simple reaction may be qualita tively changed in various ways. The subject may be required to react to stimuli of different quality (e.g. tones, colors) ; to stimuli of differ ent intensity (e.g. weak and strong pressures): to stimuli of varying intensity or quality (e.g. irregular alternation of loud and weak noises) ; under distraction; and without preadjustment of attention. i.e. without any signal that the ex periment is about to begin. Further, the com plex types of action may be studied. (See ACTION.) The subject may react to one of two or more known stimuli; he is told. perhaps. that either Mack or white will be shown, and that he is not to move until he has 'made sure' of the nature of the impression (discrimination reaction). Or he may be left in the dark, save in a general way, as to the nature of the stimu lus: he is to react to a visual impression, and not to move until he has made sure of it, but the impression may be black, white, gray, or a color (cognition reaction). Again, he may lie told that either black or white is to he shown, and that he is to respond to black by a right-hand and to white by a left-hand movement. Here we have the conflict of impulses characteristic of selective action. Once more, he may be told to respond to black, but not to answer at all if white is shown: here we have the conflict be tween impulse, on the one hand, and a group of ideas which do not prompt to movement, on the other, that is characteristic of volitional action. Selective and volitional actions may also be put together on the basis of the cognition reaction.
(2) Under the second heading we note, first. that an association may be interpolated into the course of the reaction. The reactor is shown a color: he is not to move until he has associated some idea to the color. The time of the associa tion reaction minas the time of the simple re action to the same stimulus gives us a rough measure of the duration of association. It aver ages three-quarters of a second. Secondly, we find that the cognition of intensities takes a longer time than that of qualities. This accords with the fact that quality is the absolute, and in tensity a relative attribute of sensation. Thirdly. the reaction time to tonal fusions lasts longer. the higher the fusion degree: we cognize a minor third more quickly than we cognize a major third. Finally, the reaction method en ables us to trace the course of habituation, fatigue (q.v.), expectation, and practice (q.v.).
BIBLIOGRAPHY. Sanford, American Journal of Bibliography. Sanford, American Journal of Psychology, ii. (Worcester, 1SSS-89) ; Lange, Philosophische .Studien, iv. (Leipzig, 1888) ; Wundt, Physiologische Psychologie (ib., 1893) ; Jastrow, The Time Relations of Mental Phenomena (New York, 1S86) ; Knipe, Out lines (London, 1895) ; Titchener, Outline (New York, 1S99) ; id., Experimental Psychology (ib., 1901) ; Flournoy, Observations sur quelques types de reaction simple (Geneva, 1896) ; Stern, Uebcr Psychologic der indiriduellen Differenzen (Leip zig, 1900). See PSYCHOLOGICAL APPARATvS.