Various - Harvard Psychological Studies, Volume 1

V >> Various >> Harvard Psychological Studies, Volume 1

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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
ampere. This was used as the stimulus for series 1. 2 cells, 0.5 to
1.0 volt, 0.00003 to 0.00006 ampere. This was used for series 2. 4
cells, 1.2 to 1.8 volt, 0.00007 to 0.0001 ampere. 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 impossible. No
attempt has thus far been made to get the reaction time in case of
summation effects except in occasional instances, and in so far as
those are available they indicate no great difference between the
normal threshold reaction and the summation reaction, but on this
problem more work is planned.

There are large mean variations in Table XI., as would be anticipated.
Since the reactions were taken in sets of 24, the means of each set as
well as that of the total are given, and also, in columns 4 and 5, the
means of the first half and the last half of each set.

A comparison of Tables XI., XII. and XIII. makes clear the differences
in reaction time correlated with differences in the strength of an
electric stimulus. For Table XI., series I, the relative value of the
stimulus was I; for Table XII., series 2, it was 2, and for Table
XIII., series 3, it was 4. Throughout the series from I to 3 there is
a rapid decrease in the reaction time and in the variability of the
same. The reaction time for stimulus I, the so-called threshold, is
given as 300.9[sigma]; but of the three it is probably the least
valuable, for reasons already mentioned. The mean of the second
series, stimulus 2, is 231.5[sigma] while that of the third, stimulus
4, is only 103.1[sigma]. This great reduction in reaction time for the
four-cell stimulus apparently shows the gradual transition from the
deliberate motor reaction, which occurs only after complex and varied
central neural activities, and the purely reflex reaction, which takes
place as soon as the efferent impulse can cause changes in the spinal
centers and be transmitted as an afferent impulse to the muscular
system.

TABLE XI.

ELECTRICAL STIMULUS REACTION TIME. SERIES 1.

Average Average of Average Average Mean Var
Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.

1 238.5* 33.3* 216.0* 205.6* 226.7* 33.2*
261.0 248.0 274.1 33.3
2 458.0 219.0 458.0 270.4 643.8 219.0
4 273.4 59.9 273.4 245.7 301.1 59.9
5 263.9 50.5 268.6 244.7 292.5 44.9
259.2 236.0 282.4 56.1
6 271.1 65.1 322.6 273.2 372.0 87.9
219.6 208.5 230.6 42.3
Gen Av. 300.9 85.5 300.9 244.8 356.8 85.5

Totals.
For No. 1 the averages are for 2 sets of 24 reactions each, 48
" 2 " " one set of 12 " " 12
" 4 " " one set of 24 " " 24
" 5 " " two sets of 24 " " 48
" 6 " " two sets of 24 and 12 reactions,
respectively, 36

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

TABLE XII.

ELECTRICAL STIMULUS REACTION TIME. SERIES 2.

Average Average of Average Average Mean Var
Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.

1 227.3* 33.7* 229.4* 209.1* 249.6* 25.5*
225.2 207.3 243.0 42.1
2 240.1 30.9 239.0 222.3 255.1 29.0
241.3 220.2 262.4 32.8
4 270.3 56.5 298.5 285.3 311.4 62.8
242.2 206.0 278.4 50.2
198.5 26.2 195.0 197.5 193.0 33.5
202.0 195.2 209.0 18.8
6 224.4 24.4 221.6 209.7 233.7 23.6
227.2 213.5 241.0 25.1
Gen. Av. 231.5 34.3 231.0 216.6 246.6 34.3

For No. 5 the averages are for two sets of 18 each; for all the
others there are 24 in each set.

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

TABLE XIII.

ELECTRICAL STIMULUS REACTION TIME. SERIES 3.

Average Average Average Average Mean Var.
Frog. of all. Mean Var. of all. of 1st h. of 2d h. of Sets.
1 93.6* 13.5* 91.8* 93.2* 90.4* 13.5*
95.4 91.8 99.0 13.5
2 99.9 12.8 92.2 89.4 95.0 17.4
107.5 105.9 109.0 8.2
4 125.2 16.3 113.5 106.5 120.5 13.6
136.0 135.7 138.2 19.0
5 94.4 8.0 88.6 90.5 88.6 8.2
100.2 97.8 102.7 7.9
6 102.5 12.2 104.2 98.6 109.9 12.8
100.9 101.0 108.3 11.6
Gen. Avs. 103.1 12.5 103.1 101.0 105.9 12.5

For each animal there are two sets of 24 reactions each.

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

The spinal reflex for a decapitated frog, as results previously
discussed show, is approximately 50[sigma]; and every time the
four-cell stimulus is given this kind of a reaction results. There is
a slight twitch of the legs, immediately after which the animal jumps.
Now for all these series the thread was slackened by one eighth of an
inch, but the reflex time was determined without this slack.
Calculation of the lengthening of the reaction time due to the slack
indicated it to be between 20 and 30[sigma], so if allowance be made
in case of the reactions to the four-cell stimulus, the mean becomes
about 70[sigma], or, in other words, nearly the same as the spinal
reflex. The conclusion seems forced, therefore, that when a stimulus
reaches a certain intensity it produces the cord response, while until
that particular point is reached it calls forth central activities
which result in much longer and more variable reaction times. It was
said above that the series under consideration gave evidence of the
gradual transition from the reflex to the volitional in reaction time.
Is this true, or do we find that there are well-marked types, between
which reactions are comparatively rare? Examination of the tables
VII., VIII., IX., XI., XII. and XIII. will show that between 70[sigma]
and 150[sigma] there is a break. (In tables XI., XII. and XIII.,
allowance must always be made for the slack in the thread, by
subtracting 30[sigma].) All the evidence furnished on this problem by
the electrical reaction-time studies is in favor of the type theory,
and it appears fairly clear that there is a jump in the reaction time
from the reflex time of 50-80[sigma], to 140 or 150[sigma], which may
perhaps be taken as the typical instinctive reaction time. From
150[sigma] up there appears to be a gradual lengthening of the time as
the strength of the stimulus is decreased, until finally the threshold
is reached, and only by summation effect can a response be obtained.

The most important averages for the three series have been arranged in
Table XIV. for the comparison of the different subjects. Usually the
reaction time for series 3 is about one half as long as that for
series 2, and its variability is also not more than half as large. In
the small variability of series 3 we have additional reason for
thinking that it represents reflexes, for Table IX. gives the mean
variation of the reflex as not more than 8[sigma], and the fact that
the means of this series are in certain cases much larger is fully
explained by the greater opportunity for variation afforded by the
slack in the thread.

TABLE XIV.

MEANS, ETC., FOR EACH SUBJECT FOR THE THREE SERIES. (TIME IN [sigma])

Mean First Second Mean Frog.
Half. Half. Variation.
Series 1 238.5 226.8 259.4 33.3
Series 2 227.3 208.2 246.3 33.7 No. 1
Series 3 93.6 92.5 94.7 13.5

Series 1 458.0 270.4 643.8 219.0
Series 2 240.1 221.2 258.8 30.9 No. 2
Series 3 99.9 97.6 102.0 12.8

Series 1 273.4 245.7 301.1 59.9
Series 2 270.3 245.6 294.9 56.5 No. 4
Series 3 125.2 121.1 129.3 16.3

Series 1 263.9 240.4 287.4 50.5
Series 2 198.5 196.4 201.0 26.2 No. 5
Series 3 94.4 94.2 94.7 8.0

Series 1 271.1 240.8 301.3 65.1
Series 2 224.4 211.6 237.3 24.4 No. 6
Series 3 102.5 99.8 109.1 12.2

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 repetition 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 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].