REACTIONS AND REACTION TIMES IN PSYCHOLOGY. Owing to the large place occupied by movement in man's life and the practical importance of knowing the time required in movements in making records in physics and astronomy, much labor has been spent by the psychologist and physiologist in measuring the time used in making different reactions. The first interest in the topic was aroused when it was found that different men recorded the time at which a star crossed a wire in the telescope differently. Experiment showed that this was in part due to the varia tion in the time needed to make the recording movement. Later experiments were devoted to measuring the times of other mental processes that were involved in more complex mental and physical operations, with the object of explain ing movement in general. All measurements involve determining the time that elapses be tween some stimulus and a prescribed move ment. The times are recorded electrically. The apparatus most frequently employed is a Hipp chronoscope with instruments for giving stimuli and keys to make or break circuits as the move ment is completed. The Hipp chronoscope is a clock controlled by the vibration of a fork or metallic tongue which vibrates 1,000 times per second. The hands are so adjusted that an electric magnet will articulate them with the clock work. As the stimulus is shown a cir cuit is made through the clock which engages the hands in the gears, and they continue to revolve until the movement breaks the circuit. One reads the time elapsed in thousandths of a second (ordinarily designated a). One distin guishes simple and compound reactions. The simple are made as quickly as possible after the stimulus, the compound are delayed until some mental operation is introduced. The simple reaction varies in length with the individual and with the mental attitude. If one attends to the movement alone the response is quicker than if one attends to the stimulus and makes sure that one knows what it is before one acts. The latter or sensory times for auditory stimuli range from 180a to 220a; for visual stimuli from 200a to 290a, and for tactual stimuli are about 210a. The muscular reactions require about 100a less. It was at first asserted that they were always less by this amount, but in most of the more recent experiments the differ-: ence has been smaller and has varied for indi viduals. Sound stimuli involve times of 110 125a; light, 170-180a, and tactual, 105-110a. The mean variation for the muscular reaction is smaller—from 6-9a, as compared with 24-28a for the sensory, and premature reactions are more frequent. The time required for any movement may be analyzed into four parts: the time required to stimulate the sense organ, the time of transmission along the nerve fibre, the time involved in passing from one nerve element to another and the time needed to ex cite the contraction in the muscle. The more important variables are the time required in the sense organ and the time for passing from one neurone to another. The greater length of the visual reaction is due to the longer time required to excite the retina, a photochemical process. The fact that reactions are shortened by training and by the attitude or preparation is a result of reducing the time needed to pass from one nerve element to another. The very short time of the reflex winking on stimulation of cornea (as little as 35a) is probably explained by its having a path in the subcortical tracts with relatively few elements to pass over. The simplest of the compound reactions is the dis crimination reaction. In this the reactor is asked to discriminate between two or more stimuli before he responds. The time required
increases with the number of objects or quail ties between which discrimination must be made. Distinguishing between two qualities adds 20 30 to the sensory time, and when one must distinguish between 10, the time may be in creased by 500a or more. In the choice re action, the reactor must make a different move ment for each of the qualities. He may, e.g., react with the right hand to red and with the left to white, or he may be shown one of 10 colors and react with a different finer or thumb to each. The times for the choice re action vary greatly with the individual and the amount of practice as well as with the corn plexity of the response demanded. They add to the discrimination time from 60-80a when one of two movements may be chosen to 400a or more when 10 movements are possible. In addition to these motor processes and processes essential to movement, measurements have been made of association times and the times in volved in other more complicated mental proc esses. Association times are usually investi gated by showing the observer a word and asking him to give some other word as quickly as possible. To think this word takes from 400a to two seconds or more longer than it does to read a word. This time varies greatly with the individual and also with the conditions pre scribed for the association. When one is left free to give any word whatever, one uses more time than when one is restricted in some way. Thus, when two numbers over each other are shown and one has been told in advance that one may either add, subtract or multiply, the response is more delayed than when one is told to add. In general, the smaller the number of possible associates the shorter the time. To name a member of the class when a class is given takes longer than to name the class when one member is shown. It might be noted that the association time is increased when the word arouses an emotion. This fact has been used by Jung as an aid in the detection of emotional complexes that may be responsible for hysteria. In explaining all responses and attempting to allot portions of the time to different acts, we must recognize that preparation of the act be gins with the instructions. We may divide the process into two parts, the fore period from the time of receiving the instructions until the stimulus is given and the time between the stimulus and responses. During the fore period there is apparently a process of partially arous ing the possible paths of discharge, and all that is necessary after the stimulus presents itself is to select the appropriate response from those that are already prepared. This
tendency( acts with the stimulus in deciding both the form of association and the nature of the response. This fact, that instruction and stimulus unite in determining the act, makes it impossible to divide the compound reaction into a number of part processes each distinct from the other. One does not see first, then dis criminate, then choose and finally react, but the various processes overlap. Much of the choice is made before the stimulus is Shown and the way for the association begins to be prepared when the instructions are heard. Studies of these conditions are valuable, for the explana tion of action in general as well as for under standing the nature of reaction times. Consult Titchener, 'Text-book of Psychology) (pp. 428 447) ; Ladd-Woodworth, (Elements of Physi okeical
(pp 476499) ; Myers,
of Experimental Psychology' (ch. XI) ; Ach,