August 29, 1890.] 



SCIENCE. 



I '5 



in varying the fore-knowledge of the subject while still leav- 

 ing it definite enough to call the result a simple re-action, 

 (a) We may leave the precise time of the appearance of the 

 stimulus undetermined. This may be done by experiment- 

 ing with and without a preparatory signal, preceding the 

 stimulus by a regular interval. Wundt re-acted to the sound 

 of a ball falling from a height of 25 centimetres in 76(T with 

 a preparatory signal, but in 253ff if no such signal preceded; 

 to a ball falling five centimetres, in ITSe" in the first case, and 

 266ff in the second. The time between the signal and stimu- 

 lus is here regular, and the most favorable time seems to be 

 about two seconds. Lange found the time with an interval 

 of two seconds less than with one of one or three seconds. If 

 the interval be irregularly varied within two seconds the effect 

 is hardly noticeable, but if irregularly varied within fifteen 

 seconds the time is increased (Cattell). With a normal re-ac- 

 tion to sight of 149(r and to sound of 124(T, the re action to 

 sight with the interval varying within two seconds was 

 1480"; when varying within fifteen seconds, to sight, ITiff, 

 to sound, leSfT (average of two observers). (6) If the time 

 and nature of the stimulus be known, but its intensity be 

 varied, the time is increased. When re-acting to a uniform 

 change between a feeble and a loud sound, the re-action 

 time to the former was 1370', to the latter, 116 ff; but when 

 these changes were made in an irregular and unexpected 

 manner the times were lengthened to 208(7 and 198(7. 



Similarly the attention may be prevented from being ef- 

 fectually directed to the making of the re-action by a variety 

 of circumstances. Some of these we may group under the 

 term (2) distraction. By a constant noise or other means 

 we may be creating a stimulus to which the attention is in- 

 voluntarily drawn, and thus withdrawn from the process of 

 re-action. Wundt re-acted to a sound of mean intensity in 

 189cr, to a strong sound in 158(T, but when a disturbing 

 sound was going on in the room these re-actions required 

 313i7 and 203(T. On the other hand, with Cattell, when in 

 good practice, so that the re-action became almost automatic, 

 the efPect of a disturbing sound both upon siglit and sound 

 re-actions was insignificant, — normal for sight, 149(7, with 

 disturbing noise, 155(7; normal for sound, 124(7, with dis- 

 turbing noise, 124(7. It is quite probable that what acts as a 

 disturbance to one pei-son hardly affects another. In some in- 

 dividuals the re-action time seems to be extremely sensitive to 

 any mental disturbance. One of Obersteiner's subjects, with 

 an average re-action time of about 100(7, requires 142(7 to re- 

 act when music is heard, and another's re-action time is 

 lengthened by 100(7 when talking is going on in the room. 



A more general and thorough form of distraction may be 

 effected by imposing a task requiring distinct mental effort 

 at the same time that the re-action is to take place. Thus 

 Cattell attempted to add 17 consecutively to a series of num- 

 bers, and found that re-actions taken while this was going 

 on were longer by 28(7 (average of two observers). All such 

 effects seem to be much more marked when the re action in 

 question is new than when it has become familiar and partly 

 automatic. The disturbance seems to act by delaying the 

 association between stimulus and movement. 



(3) We have now to notice a distinction which, though 

 but recently brought to light (by N. Lange, 1888), is of fun- 

 damental importance. A re-action may be made in two 

 ways. In the one form of re-action the attention is directed 



to the expected impression: it is identified as the expected 

 impression, and thereupon is initiated the impulse resulting 

 in the re-acting movement. The several processes are per- 

 formed serially, the attention being concentrated upon the 

 sensory part of the process. In the other form of re-action 

 the attention is directed to the movement: the impulse is 

 ready, and is set off by the appearance of the signal almost 

 automatically, the identification of the actual with the ex- 

 pected impression being omitted. The first is spoken of as 

 the "complete" or "sensory " mode of re-action, the second 

 as the " shortened "or " motor " form In the experiments 

 of Lange the simple sensory re-action time to a sound (aver- 

 age of three persons) was 227(7, motor 123(7; to a visual im- 

 pression (average of two persons), sensory 290(7, motor 113(7; 

 to a tactile impression (one person), sensory 213(7, motor 

 lose. These differences, however, seem to be extreme. 

 Mijnsterberg finds for sound, sensory 162(7, motor 120(7. A 

 further characteristic of the motor form of re-action is that 

 its average variation is smaller, i.e., the process is more 

 regular; and that false re-actions occur, either anticipations 

 of stimulus or re-actions to some accidental disturbance. 

 The distinction becomes still more important when the re- 

 action is not simple but complex, and we will return to it 

 later. The distinction is important as aiding in the expla- 

 nation of individual differences, as well as of the path of 

 practice. The somewhat conflicting results obtained before 

 this distinction was taken into account might very well be 

 due to the fact that the one observer re-acted in the one way 

 and the other in the other. Thus the re action times of 

 Kries and Auerbach are motor; foi- they are brief, false re- 

 actions occur, and it is noted that the simple re-actions fol- 

 lowing re-actions involving distinctions were longer by 41(7 

 and 31(7 than before, — a change probably due to a return to 

 a partially sensory mode of re-action. Again, there are 

 doubtless transitional modes between the two, and there are 

 reasons for believing that the path of practice is from the 

 sensory to the motor form of re-action. 



The influences that remain to be discussed may be consid- 

 ered under the heads of " practice," " fatigue," '' individual 

 differences," and " abnormal variations." (4) Practice. As 

 just noticed, the effect of practice is intimately connected 

 with the mode of re-aciion. It is noticed by almost all 

 writers, but the extent to which it influences the time is very 

 various. The observations make it probable that the effect 

 of practice is most marked at first, and that when once the 

 initial stages are over, the effect of continued practice is small. 

 It is greatest in those persons whose time is longest at first, 

 and seems most influential in acts that are complicated and 

 lie somewhat beyond the realm of daily experience. 



When the action is once thoroughly learned, an interval 

 of disuse seems not to affect the time seriously. After not 

 re-acting for three months, Cattell found no essential differ- 

 ence in the time. On the other hand, with some there is a 

 slight newness on beginning each day's work, making the 

 first re-actions of a series rather long (Trautscholdt). 



(5) A similar statement may be made of fatigue: it has 

 greatest effect upon the complicated, less thoroughly learned 

 processes, and varies with the individual and the mode of 

 re action. With an automatic simple process its effect is 

 very slow to appear (Cattell). It may enter at any stage of 

 the process, sensory, motor, or central ; but the last seems to 



