A striking fact is that the averages for the first and last half of sets of reactions differ more for the weak than for the strong stimulus. One would naturally expect, if the increase were a fatigue phenomenon purely, that it would be greatest for the strongest stimulus; but the results force us to look for some other conditions than fatigue. A stimulus that is sufficiently strong to be painful and injurious to an animal forces an immediate response so long as the muscular system is not exhausted; but where, as in series 1 and 2 of the electrical stimulus, the stimulus is not harmful, the reason for a sudden reaction is lacking unless fear enters as an additional cause.

Just as long as an animal is fresh and unfamiliar with the stimulus there is a quick reaction to any stimulus above the threshold, and as soon as a few experiences have destroyed this freshness and taught the subject that there is no immediate danger the response becomes deliberate. In other words, there is a gradual transition from the flash-like instinctive reaction, which is of vast importance in the life of such an animal as the frog, to the volitional and summation responses. The threshold electrical stimulus does not force reactions; it is a request for action rather than a demand, and the subject, although startled at first, soon becomes accustomed to the experience and responds, if at all, in a very leisurely fashion. The reaction time to tactual stimuli, soon to be considered, was determined by giving a subject only three or four stimulations a day; if more were given the responses failed except on repet.i.tion or pressure; for this reason the data on fatigue, or lengthening of reaction time toward the end of a series, are wanting in touch. A few tests for the purpose of discovering whether the time would lengthen in a series were made with results very similar to those of the threshold electrical stimulus; the chief difference lies in the fact that the responses to touch fail altogether much sooner than do those to the electrical stimulus. This, however, is explicable on the ground that the latter is a stimulus to which the animal would not be likely to become accustomed so soon as to the tactual.

First Half. Second Half. Second % Greater.

Series 1 244.8[sigma] 356.8[sigma] 46 per cent Series 2 216.6 246.6 14 "

Series 3 101.0 105.9 5 "

If pure fatigue, that is, the exhaustion of the nervous or muscular system, appears anywhere in this work, it is doubtless in series 3, for there we have a stimulus which is so strong as to force response on penalty of death; the reaction is necessarily the shortest possible, and, as a matter of fact, the motor reaction (jump forward) here occupies little more time than the leg-jerk of a decapitated frog. This probably indicates that the reaction is a reflex, and that the slight increase in its length over that of the spinal reflex is due to occasional cerebellar origin; but of this there can be no certainly from the evidence herewith presented. At any rate, there is no possibility of a voluntary reaction to the strong current, and any changes in the general character of the reaction time in a series will have to be attributed to fatigue of the nervous or muscular systems.

The second halves of the sets of series 3 are 5 per cent. longer than the first, and unless this is due to the partial exhaustion of the nervous system it is hard to find an explanation of the fact. Fatigue of the muscles concerned seems out of the question because the reactions occur at the rate of only one per minute, and during the rest interval any healthy and well-nourished muscle would so far recover from the effect of contraction that it would be able to continue the rhythmic action for long periods.

To the inquiry, Does fatigue in the experiments mean tiring by the exhaustion of nerve energy, or is the lengthening in reaction time which would naturally be attributed to tiring due to the fact that experience has shown quick reaction to be unnecessary? we shall have to reply that there is evidence in favor of both as factors. There can be little doubt that in case of the strong stimuli there is genuine fatigue which makes quick reaction impossible; but at the same time it is certain that the 40 to 50 per cent. increase of the second half of sets in series 1 over the first half can not be due to fatigue, for the strain is here evidently much less than for series 3. Rather, it would seem that habituation instead of exhaustion is the all-important cause of the difference in series 1 and 2. It becomes clear from these considerations that the repet.i.tion of a stimulus can never mean the repet.i.tion of an effect.

VII. TACTUAL REACTION TIME.

In the following work on the reactions to tactual stimulation the subject was placed in a large reaction box with a thread attached to one of its legs and pa.s.sing to a reaction key, as in the experiments already described. The box in which the subject was confined was surrounded by movable cloth curtains to prevent the animal's escape and at the same time permit the experimenter to work without being seen by the frog.

Tactual stimulation was given by means of a hand key[15] similar to that used for electrical stimulation which is represented in Fig. 6.

The touch key ended in a hard-rubber k.n.o.b which could be brought in contact with the skin of the subject. This key was fixed to a handle of sufficient length to enable the operator to reach the animal wherever it chanced to be sitting in the reaction box. Stimulation was given by allowing the rubber point of the touch key to come in contact with the skin in the middle region of the subject's back. As soon as the point touched the animal the chronoscope circuit was broken by the raising of the upper arm of the key.

[15] This apparatus was essentially the same as Scripture's device for the giving of tactual stimulation.

As a precaution against reactions to visual stimuli, which it might well be supposed would appear since the subject could not in every case be prevented from seeing the approaching apparatus, the frog was always placed with its head away from the experimenter so that the eyes could not readily be directed toward the touch apparatus.

Notwithstanding care in this matter, a reaction occasionally appeared which was evidently due to some disturbance preceding the tactual stimulus which served as a warning or preparation for the latter. All such responses were at once marked as questionable visual reactions and were not included in the series of touch reactions proper.

As has been mentioned in connection with the discussion of fatigue, it was found absolutely necessary to have the subjects perfectly fresh and active, and for this purpose it was advisable to give not more than three or four stimulations at any one time. The subject was usually kept in the reaction box from 30 to 45 minutes, dependent upon the success of the experiments. As the work progressed it became evident that the responses to the stimulus were becoming less and less certain and slower, that the subjects were becoming accustomed to the novel experience and no longer suffered the surprise which had been the cause of the prompt reactions at first. It seemed best for this reason not to continue the work longer than two weeks, and as a consequence it was impossible to base the averages on more than twenty reactions for each subject.

So far as the tension of the thread is concerned, the condition for the tactual reaction time was the same as that for the first group of electrical reaction-time experiments. In comparing the tactual with the electrical of series 1, 2 and 3, allowance must be made for the slack in the latter cases.

Selection of the tactual reaction times upon which the mean is based, has been made with reference to the mode for each set of experiments.

Inspection of the curves given by the reactions of each subject indicated that the great majority of the responses lay between 100 and 300[sigma], and that those which were beyond these limits were isolated and, in all probability, exceptional reactions due to some undetected variation in conditions which should throw them out of the regular series. On this account it was thought best to use only reactions between 100 and 300[sigma].

For convenience of comparison, again, the averages for the electrical reaction time of subjects _A_, _B_, _C_, _D_, _E_ and _F_, and the same for the tactual reaction time of subjects 1, 2, 3, 4, 5 and 6 are herewith given together. All averages are for twenty reactions, except for _D_ and 5, for which there are ten.

Besides the usual determination for the tactual reaction-time work on the six subjects named, there is given in Table XVI. the electrical reaction time of these animals to a two-cell current. Comparison of the electrical and tactual results are of interest in this case because the mean variation for each is about 34[sigma], being 34.3[sigma], for the electrical and 33.8[sigma], for the tactual.

TABLE XV.

Average of 20 Electrical Average of 20 Tactual Frog. Reactions. Frog. Reactions.

_A_ 149.5[sigma] 1 188.3[sigma]

_B_ 158.3 2 199.1 _C_ 191.0 3 212.1 _D_ 167.0 4 213.0 _E_ 182.4 5 199.8 _F_ 176.3 6 221.9 Gen. Avs. 167.9 205.7

TABLE XVI.

REACTION TIME FOR TACTUAL AND ELECTRICAL STIMULI.

Tactual Reaction Time. Electrical Reaction Time.

Frog. Average. Mean Variation. Average. Mean Variation.

1 188.3[sigma] 167.3[sigma]

2 199.1 180.1 3 212.1 4 213.0 210.3 5 199.8 138.5 6 221.9 164.4 Gen. Avs. 205.7 33.8 172.1 34.3

For 5 the average of ten instead of twenty is given.

VIII. EQUAL VARIABILITY AS A CRITERION OF COMPARABILITY OF REACTION TIME FOR DIFFERENT KINDS OF STIMULI.

Since variability as indicated in the study of the influence of different strengths of electrical stimulus becomes less as the stimulus increases, parity in variability for different stimuli offers a basis for the comparison of reaction times. Certain it is that there is no use in comparing the reaction times for different senses or different qualities of stimuli unless the relative values of the stimuli are taken into consideration; but how are these values to be determined unless some such index as variability is available? If the reaction time to tactual stimuli as here presented is to be studied in its relation to the electrical reaction time, it will mean little simply to say that the former is longer than the latter, because the electrical reaction time for a one-cell stimulus happens to be somewhat less than that for the particular tactual stimulus used. For it is clear that this tactual reaction time is really shorter than the reaction time to a weak current. In making variability a basis of comparison it must be a.s.sumed that the strength of stimulus is the important factor, and that all other variable conditions are, so far as possible, excluded. If, now, on the basis of parity in variability we compare the tactual and electrical reaction times, it is apparent that the tactual is considerably longer. The tactual average of Table XV. is 205.7[sigma], while the electrical reaction time which has approximately the same variability is 172.1[sigma]. It may well be objected that I have no right to make variability the basis of my comparison in these experiments, because the work for the various kinds of stimuli was done under different conditions. Admitting the force of this objection, and at the same time calling attention to the fact that I do not wish to lay any stress on the results of the comparisons here made, I take this opportunity to call attention to the possibility of this criterion.

The use of variability as a basis of comparison would involve the a.s.sumptions (1) that a certain intensity of every stimulus which is to be considered is capable of producing the shortest possible, or reflex reaction, and that this reaction is at the same time the least variable; (2) that as the strength of a stimulus decreases the variability increases until the threshold is reached.

Suppose, now, it is our desire to compare the results of reactions to different intensities of electrical and tactual stimuli; let the figures be as follows:

Reaction Time. Variability.

Stimulus Strength. Elect. Touch. Elect. Touch.

8 50[sigma] 50[sigma] 10[sigma] 10[sigma].

4 130 155 25 30 2 175 220 40 40 1 300 320 50 60

In the double columns the results for electrical stimuli are given first, and in the second column are the tactual. Stimulus 8 is a.s.sumed to be of sufficient strength to induce what may be designated as forced movement, and whatever the quality of the stimulus this reaction time is constant. I make this statement theoretically, although all the evidence which this work furnishes is in support of it. So, likewise, is the variability of this type of reaction time small and nearly constant. At the other extreme, stimulus 1 is so weak as to be just sufficient to call forth a response; it is the so-called threshold stimulus. Whether all qualities of stimulus will give the same result here is a question to be settled by experimentation. Wundt contends that such is the case, but the observations I have made on the electrical and tactual reactions of the frog cause me to doubt this a.s.sumption. It seems probable that the 'just perceptible stimulus reaction time' is by no means the same thing for different qualities of stimulus. Those modifications of the vital processes which alone enable organisms to survive, make their appearance even in the response to the minimal stimulus. In one case the just perceptible stimulus may cause nothing more than slight local changes in circulation, excretion, muscular action; in another it may produce, just because of the particular significance of the stimulus to the life of the organism, a violent and sudden motor reaction. But grant, if you will, that the threshold reaction time is the same for all kinds of stimuli, and suppose that the variability is fairly constant, then, between the two extremes of stimuli, there are gradations in strength which give reaction times of widely differing variabilities.

If, now, at some point in the series, as, for instance, to stimulus 2, the variability for different kinds of stimuli is the same either with reference to the reaction time (ratio) or absolutely, what interpretation is to be put upon the fact? Is it to be regarded as merely a matter of chance, and unworthy of any special attention, or should it be studied with a view to finding out precisely what variability itself signifies? It is obvious that any discussion of this subject, even of the possible or probable value of variability as a criterion for the comparative study of stimuli, can be of little value so long as we do not know what are the determining factors of variations of this sort. The only suggestion as to the meaning of such a condition (_i.e._, equal variability at some point)--and our studies seem to show it for touch and electrical stimulation--which I feel justified in offering at present, is that parity in variability indicates equality in strength of stimuli, that is, the electrical stimulus which has a reaction time of the same variability as a tactual stimulus has the same effect upon the peripheral nervous system as the tactual, it produces the same amplitude and perhaps the same form of wave, but the reaction times for the two stimuli differ because of the biological significance of the stimuli. The chances are that this is wholly dependent upon the central nervous system.

IX. SUMMARY.

1. This paper gives the results of some experiments on the frog to determine its electrical and tactual reaction time. It is the beginning of comparative reaction-time studies by which it is hoped important information may be gained concerning the significance and modes of action of the nervous system. Comparative physiology has already made clear that the time relations of neural processes deserve careful study.

2. According to the strength of the stimulus, electric stimulation of the frog causes three types of reaction: (1) A very weak or threshold stimulus results in a deliberate or delayed reaction, the time of which may be anywhere from 300[sigma] (thousandths of a second) to 2,000[sigma]. (2) A very strong stimulus causes a spinal reflex, whose time is from 50 to 80[sigma]; and (3) a stimulus of intermediate strength causes a quick instinctive reaction of from 150 to 170[sigma]

in duration.

3. The reaction time for electric stimuli whose relative values were 1, 2 and 4 were found to be 300.9[sigma], 231.5[sigma] and 103.1[sigma].

4. The reaction time of the frog to a tactual stimulus (contact of a rubber point) is about 200[sigma].

5. The variability of reaction times of the frog is great, and increases as the strength of the stimulus decreases.

6. When two kinds of stimuli (_e.g._, electrical and tactual) give reaction times of equal variability, I consider them directly comparable.

7. According to this criterion of comparability the reaction time to electric stimulation which is comparable with that to tactual is 172.1[sigma]; and it is to be compared with 205.7[sigma]. Both of these have a variability of approximately 34[sigma]. On this basis one may say that the tactual reaction time is considerably longer than the electrical.