Sixteen Experimental Investigations from the Harvard Psychological Laboratory by Hugo Münsterberg (100 books to read .txt) 📕

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 repetition of a stimulus can never mean the

repetition 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 passing 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 knob 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

5¹ 199.8 138.5

6 221.9 164.4

Gen. Avs. 205.7 33.8 172.1 34.3

¹For 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 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 205.7[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

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 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 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