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

The column of figures at the left indicates the number of reactions at

any point. Below the base line are the classes. For convenience of

plotting the reactions have been grouped into classes which are

separated by 25[sigma]. Class 1 includes all reactions between

1[sigma] and 25[sigma], class 2 all from 25[sigma] to 50[sigma], and

so on to 400[sigma], thereafter the classes are separated by

100[sigma]. It is noticeable that there is one well-marked mode at

75[sigma]. A second mode occurs at 175[sigma]. This is the primary and

in our present work the chiefly significant mode, since it is that of

the quick instinctive reaction to a stimulus. At 500[sigma] there is a

third mode; but as such this has little meaning, since the reactions

are usually pretty evenly distributed from 300[sigma] on to

2000[sigma]; if there is any grouping, however, it appears to be about

500[sigma] and 800[sigma].

The first mode has already been called the reflex mode. The short

reactions referred to usually lie between 40[sigma] and 80[sigma], and

since experiment has shown conclusively that the spinal reflex

occupies about 50[sigma], there can be little doubt that the first

mode is that of the reflex reaction time.

The second mode represents those reactions which are the result of

central activity and control. I should be inclined to argue that they

are what we usually call the instinctive and impulsive actions. And

the remaining reactions represent such as are either purely voluntary,

if any frog action can be so described, or, in other words, depend

upon such a balancing of forces in the brain as leads to delay and

gives the appearance of deliberate choice.

Everything points to some such classification of the types as follows:

(1) Stimuli strong enough to be injurious cause the shortest possible

reaction by calling the spinal centers into action, or if not spinal

centers some other reflex centers; (2) slightly weaker stimuli are not

sufficient to affect the reflex mechanism, but their impulse passes on

to the brain and quickly discharges the primary center. There is no

hesitation, but an immediate and only slightly variable reaction; just

the kind that is described as instinctive. As would be expected, the

majority of the frog’s responses are either of the reflex or of this

instinctive type. (3) There is that strength of stimulus which is not

sufficient to discharge the primary center, but may pass to centers of

higher tension and thus cause a response. This increase in the

complexity of the process means a slower reaction, and it is such we

call a deliberate response. Precisely this kind of change in neural

action and in reaction time is at the basis of voluntary action. And

(4) finally, the stimulus may be so weak that it will not induce a

reaction except by repetition. Just above this point lies the

threshold of sensibility, the determination of which is of

considerable interest and importance.

Group 2 of the electrical reactions consists of three series taken

to determine the relation of strength of stimulus to reaction time.

The conditions of experimentation differed from those for group 1 in

the following points: (1) The stimulus was applied directly by the

making of a circuit through wires upon which the subject rested (Fig.

9); (2) the thread was attached to the right hind leg; (3) the thread,

instead of being kept at the tension given by the 5-gram weight as in

the former reactions, was slackened by pushing the upright lever of

the reaction key one eighth of an inch toward the animal. This was

done in order to avoid the records given by the slight twitches of the

legs which precede the motor reaction proper. For this reason the

reactions of group 2 are not directly comparable with those of group

1. Fig. 9 is the plan of the bottom of a reaction box 15 cm. at one

end, 30 cm. at the other, 60 cm. long and 45 cm. deep. On the bottom

of this, at one end, a series of interrupted circuits were arranged as

shown in the figure. The wires were 1.2 cm. apart, and an animal

sitting anywhere on the series necessarily touched two or more, so

that when the stimulus key, X, was closed the circuit was completed by

the animal’s body; hence, a stimulus resulted. The stimulus key, X,

was a simple device by which the chronoscope circuit, c, c, was

broken at the instant the stimulus circuit, s, c, was made.

Cells of ‘The 1900 Dry Battery’ furnished the current used as a

stimulus. Three different strengths of stimulus whose relative values

were 1, 2 and 4, were employed in the series 1, 2 and 3. Careful

measurement by means of one of Weston’s direct-reading voltmeters gave

the following values: 1 cell, 0.2 to 0.5 volt, 0.00001 to 0.00003

ampère. This was used as the stimulus for series 1. 2 cells, 0.5 to

1.0 volt, 0.00003 to 0.00006 ampère. This was used for series 2. 4

cells, 1.2 to 1.8 volt, 0.00007 to 0.0001 ampère. This was used for

series 3.

[Illustration: Fig. 9. Ground Plan of Reaction Box for Electrical

Stimuli (Group 2). IC, interrupted circuits; CC, chronoscope

circuit; X, key for making stimulus circuit and breaking chronoscope

circuit; B, stimulus battery; S, string from reaction key to

animal. Scale 1/2.]

The reactions now under consideration were taken in sets of 24 in

order to furnish evidence on the problem of fatigue. The stimulus was

given at intervals of one minute, and the subject was moistened at

intervals of ten minutes. To obtain 24 satisfactory reactions it was

usually necessary to give from thirty to forty stimulations. Five

animals, numbers 1, 2, 4, 5, and 6, served as subjects. They were

green frogs whose size and sex were as follows:

Length. Weight. Sex.

Number 1 7.5 cm. 35 grams. Male.

Number 2 7.3 ” 37 ” Male.

Number 4 8.2 ” 50.4 ” Female?

Number 5 7.1 ” 25 ” Female.

Number 6 7.8 ” 42 ” Male.

For most of these frogs a one-cell stimulus was near the threshold,

and consequently the reaction time is extremely variable. In Table X.

an analysis of the reactions according to the number of repetitions of

the stimulus requisite for a motor reaction has been made. Numbers 1

and 5 it will be noticed reacted most frequently to the first

stimulus, and for them 48 satisfactory records were obtained; but in

case of the others there were fewer responses to the first stimulus,

and in the tabulation of series 1 (Table XI.) averages are given for

less than the regular sets of 24 reactions each.

TABLE X.

ANALYSIS OF REACTIONS TO ONE-CELL STIMULUS.

Frog. Reactions to To 2d. To 3d. To 4th. To 5th. More. Total No.

first Stimulus. of Reactions.

1 53 2 1 0 0 1 57

2 20 12 5 5 4 12 58

4 31 15 1 0 2 8 57

5 51 11 1 2 0 1 66

6 45 15 6 3 1 5 75

Totals, 200 55 14 10 7 27 313

Table XI. is self-explanatory. In addition to the usual averages,

there is given the average for each half of the sets, in order that

the effect of fatigue may be noted. In general, for this series, the

second half is in its average about one third longer than the first

half. There is, therefore, marked evidence of tiring. The mean

reaction time for this strength of stimulus is difficult to determine

because of the extremely great variations. At one time a subject may

react immediately, with a time of not over a fifth of a second, and at

another it may hesitate for as much as a second or two before

reacting, thus giving a time of unusual length. Just how many and

which of these delayed responses should be included in a series for

the obtaining of the mean reaction time to this particular stimulus is

an extremely troublesome question. It is evident that the mode should

be considered in this case rather than the mean, or at least that the

mean should be gotten by reference to the mode. For example, although

the reaction times for the one-cell stimulus vary all the way from

150[sigma] to 1000[sigma] or more, the great majority of them lie

between 200[sigma] and 400[sigma]. The question is, how much deviation

from the mode should be allowed? Frequently the inclusion of a single

long reaction will lengthen the mean by 10[sigma] or even 20[sigma].

What is meant by the modal condition and the deviation therefrom is

illustrated by the accompanying curve of a series of reaction times

for the electric stimulus of group I.

__________________________________________________________________________

8|______________________________________________________________________

7|_____________________________________|________________________________

6|_____________________________________|________________________________

5|_____________________________________|________________________________

4|________________________________|____|____|___________________________

3|____________|___________________|____|____|___________________________

2|_______|____|____|_________|____|____|____|____|______________________

1|__|____|____|____|_________|____|____|____|____|____|____|____|____|__

100 110 120 130 140 150 160 170 180 190 200 210 220 230

The column of figures at the left indicates the number of reactions;

that below the base line gives the reaction times in classes separated

by 10[sigma]. Of thirty-one reactions, seven are here in the class

170[sigma]. This is the model class, and the mean gotten by taking the

average of 31 reactions is 162[sigma]. If the mode had been taken to

represent the usual reaction time in this case, there would have been

no considerable error. But suppose now that in the series there had

occurred a reaction of 800[sigma]. Should it have been used in the

determination of the mean? If so, it would have made it almost

30[sigma] greater, thus removing it considerably from the mode. If

not, on what grounds should it be discarded? The fact that widely

varying results are gotten in any series of reactions, points, it

would seem, not so much to the normal variability as to accidental

differences in conditions; and the best explanation for isolated

reactions available is that they are due to such disturbing factors as

would decrease the strength of the stimulus or temporarily inhibit the

response. During experimentation it was possible to detect many

reactions which were unsatisfactory because of some defect in the

method, but occasionally when everything appeared to be all right an

exceptional result was gotten. There is the possibility of any or all

such results being due to internal factors whose influence it should

be one of the objects of reaction-time work to determine; but in view

of the fact that there were very few of these questionable cases, and

that in series I, for instance, the inclusion of two or three

reactions which stood isolated by several tenths of a second from the

mode would have given a mean so far from the modal condition that the

results would not have been in any wise comparable with those of other

series, those reactions which were entirely isolated from the mode and

removed therefrom by 200[sigma] have been omitted. In series I alone

was this needful, for in the other series there was comparatively

little irregularity.

The results of studies of the reaction time for the one-cell electric

stimulus appear in Table XI. The first column of this table contains

the average reaction time or mean for each subject. Nos. 2 and 4

appeared to be much less sensitive to the current than the others, and

few responses to the first stimulus could be obtained. Their time is

longer than that of the others, and their variability on the whole

greater. Individual differences are very prominent in the studies thus

far made on the frog. The one-cell stimulus is so near the threshold

that it is no easy matter to get a mean which is significant. Could

the conditions be as fully controlled as in human reaction time it

would not be difficult, but in animal work that is