FATIGUE STUDIED IN REACTION TIME EXPERIMENTS. By. furnish results of a general kind, and does not enable one to arrive at a

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1 FATIGUE STUDIED IN REACTION TIME EXPERIMENTS. By T. H. MILROY. (From the Physiology Laboratory, Queen's University, Belfast.) (Received for publication 5th April 1909.) MANY methods have been employed to test the degree of fatigue which is produced after various kinds of mental work. Most investigators have used the ergograph in order to show the effect of mental fatigue upon a regular series of voluntary muscular acts; but such a method can only furnish results of a general kind, and does not enable one to arrive at a knowledge of the nature of the mental fatigue. In carrying out a large number of experiments on reaction time I was struck by the various manifestations of fatigue which were evident on the analysis of the various records, and so I varied the experimental conditions in different ways with the view of discovering the nature of the fatigue which sets in after a prolonged series of such experiments. As one judges of the degree of fatigue by the lengthening of the reaction time, it is essential to gain some idea of the conditions apart from fatigue which affect the time of the response. In the first place, as Exner I pointed out, persons of a more or less phlegmatic disposition respond as a rule more uniformly, and on an average more rapidly, than those of a nervous disposition. This is undoubtedly due to the fact that the former can more readily keep the attention directed towards the one psycho-motor act than the latter. Thus, as can be readily understood, the main factor in reaction time responses is the attention one. In the second place, practice gradually shortens the time, apparently in most cases by the person gradually becoming more capable of sharply focussing his attention on the act. This appears to be accomplished, not so much by what might be referred to as a linkage of the sense impression change with the willing of the motor act, but rather by directing the attention sharply to the sensory impression change, the subsequent motor act following almost involuntarily. Thus when a person responds to a particular auditory or visual stimulus, he places himself in such a mental attitude, prior to the stimulus, as to be able to appreciate with as little delay as possible the time occurrence of the impressional change. The sensory impression with practice forces to a certain extent the motor act, which at the outset was of a distinctly willed character. This is to be frequently observed towards the close of a long series of reaction time experiments, when, on the occurrence of a sensory stimulus at an unexpected moment, the person involuntarily carries out the usual motor act of re- 1 Hermann's Handbuch der Physiologie, Bd. ii. Abt. 2, S. 268.

2 278 Milroy sponse. Thus if it be true that attention is the main factor which influences the duration of reaction time responses, it is more than probable that fatigue produces its effects by gradually weakening the power of attention. If this be correct, a change in the nature of the sensory stimulus after a prolonged series of reaction time responses should not act as a recuperative period, if the same degree of attention be directed towards the second psycho-motor act as towards the first one. For example, after fatigue has been produced by a series of reaction times for sight, the person should show no sign of recuperation setting in on the interpolation of a series of reaction times for hearing between the first and second series of sight responses. If, on the other hand, the fatigue mainly affected the sensory paths and centres the change from one sensory path to another should have a beneficial effect, and the reaction time responses should show an improvement. Again, if the fatigue phenomena shown in a prolongation of reaction time be due to an overstrain in the attention process, then the stronger the degree of attention directed to each act the more rapidly should fatigue be produced. It was in order to arrive at some conclusions based on experimental evidence that the following investigation was carried out. METHODS. The reaction times were recorded graphically, using a pendulum myograph. In the case of reaction time for sight the pendulum plate opened a galvanic circuit in which there were two electromagnets, one carrying a small plate which on descent opened at once a narrow brightly illuminated slit, the other electromagnet acting as signal for the opening of the circuit and carrying a writing point which marked on the pendulum plate the time of opening. The response was by the middle finger of the right hand closing a Morse key which was connected with the recording signal through a short-circuiting arrangement bridging the interval between the two bars of the pendulum key and thus bringing again the electromagnet into action which had been thrown out by the opening of the key. There were thus recorded in the usual way the time of opening (the light stimulus) and the time of response, while a tuning-fork tracing (100 per sec.) was recorded at the same time. In the case of reaction time for hearing the pendulum contact operated on a sharp, stiff metal spring which emitted a well-defined note. Everything was done to prevent any other slight sound from interfering with the experiment; the person remaining in every case thoroughly well isolated, so that he could not see what was going on in the neighbourhood. Care was also taken that the experiments were carried out under conditions which prevented distraction of the attention. In order to produce distinct signs of a lengthening in reaction time, it was found necessary to carry out prolonged series, and therefore every response was not recorded. As a rule, ten per minute were taken, but only one of these was recorded, in order that the plate might take all the records which were necessary in one series. The experiments referred to in this paper were all carried out

3 Fatigue studied in Reaction Time Experiments 279 on the same person, because the reaction times were, although by no means short, extremely uniform. At the beginning of the investigation the reaction times were rather longer, but without any difficulty an average could be obtained based on extremely uniform series. A. Prolonged Reaction Time Experiments. (1) Hearing.-Reaction time responses were taken over a period of 40 minutes. There was no evident lengthening of the time during the first half of the experiment, but during the latter half the responses began to be markedly irregular, some being missed and the average time being distinctly longer than at the outset. During the first 10 minutes the average reaction time was *220 sec., during the second similar interval *208 sec., while during the last 20 minutes the average was *240 sec. The person was unable at the close to keep the attention firmly fixed, a peculiar feeling of drowsiness becoming so strong as to require strong will-power to overcome. During the 40 minutes approximately 350 responses were given. After some three months more or less regular practice in reaction time work the intervals became shorter, but fatigue always occurred after the first 20 minutes (about responses). The results of a series taken at this later period were as follows: First 20 minutes reaction time, *167 sec.; second 15 minutes, '210 sec. The average lengthening in reaction time based on a large number of experiments was found to be -033 sec. during the second half of a 40 minutes' series of responses. (2) Sight.-Fatigue phenomena were not so evident in these experiments as in the case of hearing, and a longer time elapsed before fatigue set in. Thus, in the earlier series which were taken, the reaction time began to lengthen only after about 30 minutes, and even then not to the same extent as in the case of hearing. E.g. the average of a series of about 250 responses taken during 25 minutes was *222 sec.; in the succeeding 10 minutes, *247 sec. In the later series, after some months' practice, the average time during the first 20 minutes was *180 sec., during the next 20 minutes, *201 sec. Thus a lengthening of about *02 sec. was the average observed in the later responses of a 40 minutes' series. B. Effect of a Resting Period. Reaction times for sight were recorded for 45 minutes, then a rest was taken for 10 minutes, and again reaction times for sight for 10 minutes. First series:-(a) first, 20 minutes, *185 sec.; (b) second, 25 minutes, -203 sec. Rest, 10 minutes. Second series, *208 sec. Another example may be given, taken from a period when the person showed the effects of prolonged practice: Reaction time for sight at outset, *175 sec.; after 30 minutes, *197 sec. Rest for 10 minutes.

4 280 Milroy Reaction times for sight again taken for 6 minutes (60 responses); average, -201 sec. The slight degree of fatigue which was produced in no case disappeared after a 10 minutes' rest, although the person felt the better of the resting interval, and one expected to find an improvement in the reaction time. C. Effect of Prolonged Stimulation of the Afferent Tracts on Reaction Time. In order to test the effect of a prolonged stimulation of the auditory path the following experiments were carried out. The reaction times for hearing were taken for a period of 20 minutes, the person responding approximately 10 times per minute. The average reaction time was *206 sec. Following this the person was asked to count the number of ticks sounded by toothed wheels driven at such a rate as to produce about three distinct sounds per sec., a record of these ticks being taken by means of an electromagnet circuit to serve as a control over the accuracy of the count. It was found that this act of counting, even when prolonged from half to one hour, produced extremely little sign of fatigue, the attention not requiring to be directed sharply to the sounds owing to the regular rate of their production. In the case mentioned above the person counted 2864 ticks in 20 minutes, the actualnumber being The reaction time for hearing was taken immediately afterwards for a period of 15 minutes, and the average time was now found to be *210 sec. Experiments of a similar nature were carried out, using the metronome in the intermediate period, the metronome beats being 3 per sec. The reaction time for hearing taken for 20 minutes averaged *208 per sec. The metronome beats were then counted for 50 minutes (actual number 11,000, counted 10,914). Reaction time for hearing was taken immediately afterwards for 15 minutes, when it was found to average *207 sec. These and other experiments show that a prolonged series of auditory perceptions to which but slight attention is directed produces no sign of fatigue, as judged by the duration of the reaction time. In order to stimulate the same sensory and motor paths, as in the initial and closing reaction time experiments, the finger of the right hand was moved in time with the metronome ticks. The only disagreeable feeling after such a prolonged series of metronome counting was one of boredom, so that the return to a reaction time series was really refreshing. From these and other similar experiments one is led to believe that, so far as reaction time is concerned, it is exceedingly difficult to produce a fatigue of the sensory paths. D. Effect of Change from one Reaction Time Series to Another. Many experiments were carried out in order to find out whether, in changing from one sensory channel to another, there was any sign of

5 Fatigue studied in Reaction Time Experiments 281 recuperation. Thus reaction times for sight were taken for a certain time, followed by a series for hearing, then again a series for sight. The order in which these were taken was frequently altered. Two examples of this class of experiment may be given:- Reaction times (sight) taken for 25 minutes; average, 206 sec. Followed by reaction times (hearing) 7 minutes (i.e. 70 responses); average, *246 sec. Then reaction times (sight) 5 minutes (i.e. 50 responses); average, -271 sec. Another example: Reaction times (sight) taken for 30 minutes; average of the first 40 responses, 193 sec.; average of the last 40 responses, -227 sec. Now reaction times (hearing) taken for 4 minutes (i.e. 40 responses); average, *244 sec. Then reaction times (sight) 4 minutes (i.e. 40 responses); average, *291 sec. In all cases examined in this way it was found that a change from one sensory channel to another did not prove beneficial, but produced rather a more marked fatigue than if the one sensory channel had been stimulated throughout. One must remember that the nervous processes concerned on the two acts of reaction time for sight and hearing are of much the same *character, with the exception that in one case the sensory tract is the visual and in the other the auditory. The central (attention) process is at least of the same general character in each case, while the motor acts in response are identical. Thus resting one sense channel, while attention is directed towards impressions dependent upon changes taking place along another sensory tract, does not lead to such an alteration in the condition of the originally stimulated neural chain as to produce any beneficial effect upon it so far at least as the reaction time response is concerned. On the contrary, a change from one system to the other is provocative of a greater fatigue than would be pro-,duced by stimulation of the original neural chain for the whole period. The person feels the strain to be unalleviated, and attention begins to weaken more and more, although one might expect any slight beneficial effects that interest might call forth to become manifest on passing from one sense channel to another. The same effect was produced if reaction -times for hearing were taken first, then sight, and finally hearing. E. Effect of Throwing in Stimuli at Unexpected Times. If, while a series of reaction times is being taken, the stimulus is thrown in when the person is not expecting it, one naturally finds a marked prolongation of the reaction time, or the response may be omitted altogether. When a person is responding to a sight signal given at the usual more or less definite intervals, if the signal move at the time when the person has no expectation of such a thing occurring he is more likely to respond than if the unexpected signal were a metronome beat, and when the response does take place there is a less marked degree of lengthening of the reaction time than if the unexpected signal affected another sense channel. One also

6 282 Fatigue Studied in Reaction Time Experiments observes that a signal given at an unexpected moment more readily calls forth a response in the case of a person who has been replying more or less regularly to the same signal for a preceding period of 15 to 20 minutes. If it be thrown in at the outset of the series or towards the close of a, very long series, the response is apt to be omitted or at least much delayed. In the former case the delay or omission is probably due to the fact that practice has not yet sufficiently connected up the chain to allow the unexpected stimulus to discharge readily into the motor channel, while in the latter case fatigue has probably disturbed the linkage of the central connections. CONCLUSIONS. The delay in response towards the close of reaction time experiments is evidently due to fatigue affecting some higher central process which is concerned in the maintenance of a. close linkage between the central terminations of the sensory tracts and the so-called voluntary motor centres. This is judged to be the case for the following reasons :- 1. An intermittent and prolonged series of auditory stimuli towards which attention does not require to be sharply directed has no distinct fatiguing effect on succeeding reaction time responses for hearing. Hence, one concludes that the delay in response, which occurs after a prolonged series of reaction times (hearing), is not due to a fatigue of the auditory path. 2. If, in a series of reaction time experiments, the sensory tracts be changed, as in passing from reaction time (sight) to reaction time (hearing) and then back again to sight, one finds that fatigue is apparently more marked at the close than if the stimulated sensory tracts remained the same throughout. At least, there is no sign of recuperation. As in this case the motor channel remains the same while the higher central one only varies to the extent of being directed towards a different " sensory centre," the sensory path alone being changed, the fatigue effects must be due to alterations in the higher central links or in the motor path. They are evidently not due to alterations in the last-mentioned path, as there are no signs of fatigue when this motor path is kept in slight activity of the same type as that existing in reaction time experiments. That is to say, the mere voluntary tapping of the Morse key at the rate of 10 per minute produces no appreciable fatigue. 3. The effects of unexpected stimuli are more easily explained on the supposition that the time interval in reaction time is most largely dependent upon the close central linkage. 4. The effect of disposition upon the duration of reaction time is most readily understood when one recognises the importance of the central linkage. 5. The feelings of fatigue, such as sleepiness, difficulty in keeping the attention fixed, etc., which a person suffers from at the close of a long series, point to a central disturbance affecting the connecting links between sensory and motor channels.