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 assumed 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 205.7σ, while the electrical reaction time which has approximately the same variability is 172.1σ. 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 assumptions (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 | 50σ | 50σ | 10σ | 10σ. |
| 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 assumed 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 assumption. 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 300σ (thousandths of a second) to 2,000σ. (2) A very strong stimulus causes a spinal reflex, whose time is from 50 to 80σ; and (3) a stimulus of intermediate strength causes a quick instinctive reaction of from 150 to 170σ in duration.
3. The reaction time for electric stimuli whose relative values were 1, 2 and 4 were found to be 300.9σ, 231.5σ and 103.1σ.
4. The reaction time of the frog to a tactual stimulus (contact of a rubber point) is about 200σ.