The Dancing Mouse by Robert M. Yerkes

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The duration of a discrimination habit having been measured with a fair
degree of accuracy, I undertook the task of ascertaining whether training
whose results have wholly disappeared, so far as memory tests are in
question, influences the re-acquisition of the same habit. Can a habit be
re-acquired with greater facility than it was originally acquired? Is re-
learning easier than learning? To obtain an answer to the question which
may be asked in these different forms, ten individuals were experimented
with in accordance with a method whose chief features are now to be
stated. In each of these ten individuals a perfect white-black habit was
established by the use of the standard series of tests the order of which
is given in Table 12. At the expiration of a rest interval of eight weeks
precisely the same series of tests were repeated as memory and re-training
tests. In this repetition, the preliminary series, _A_ and _B_, served as
memory tests, and the subsequent training series, as re-training series.

[Illustration: FIGURE 32.--Error curves plotted from the data given by ten
dancers in white-black discrimination tests. The solid line ([Symbol:
solid line]) is the error curve of the original learning process; the
broken line (------) is that of the re-learning process, after an interval
of eight weeks.]

The striking results of this investigation of re-learning are exhibited in
the curves of learning and re-learning of Figure 32. These curves make it
appear that the mice re-acquired the white-black discrimination habit much
more readily than they had originally acquired it. But in addition to
furnishing the basis for some such statement as the foregoing, the curves
suggest a serious criticism of the experiment.

In the original tests, the preliminary series indicated a strong
preference for black. In series _A_ it was chosen on the average 5.8 times
in 10, and in series _B_, 5.7 times. This preference was rapidly overcome
by the training series, and at the end of 130 tests discrimination was
perfect. All this appears in the curve of learning (solid line of figure).
On the other hand, these preliminary series when repeated as memory tests,
after a rest-interval of eight weeks, gave markedly different results.
Series _A_ indicated preference for white (5.6 times in 10) instead of
black, and series _B_ indicated only a slight preference for black. In
brief, series _A_ and _B_ show that the preference for black was
considerably stronger at the beginning of the training than at the
beginning of the re-training.

In the light of these facts it is fair to claim that the effects of the
white-black training had not wholly disappeared as the result of eight
weeks of rest, and that the experiment therefore fails to furnish
satisfactory grounds for the statement that re-learning occurs more
rapidly than learning. I accept this criticism as pertinent, although not
necessarily valid, and at the same time I freely admit that the results
have a significance which I had not anticipated. But they are not less
interesting or valuable on that account. Granting, then, that at least
some of the ten individuals which took part in the experiment had not
completely lost the memory of their white-black training at the end of
eight weeks, it is still possible that an examination of the individual
results may justify some conclusion concerning the question which was
proposed at the outset of the investigation. Such an examination is made
possible by Tables 49 and 50, in which I have arranged separately the
results for the males and the females.

TABLE 49

WHITE-BLACK TRAINING. TEN TESTS PER DAY

Males
TRAINING RETRAINING
210 220 230 410 420 AV. 210 220 230 410 420 AV.

A 6 5 6 6 6 5.8 5 4 5 4 3 4.2
B 6 8 8 5 1 5.6 8 4 5 4 6 5.4

1 6 7 6 2 4 5.0 3 3 4 7 3 4.0
2 4 3 1 2 3 2.6 2 4 2 5 3 3.2
3 3 1 4 3 4 3.0 1 4 1 4 1 2.2
4 5 0 3 3 2 2.6 0 1 0 1 2 0.8
5 3 0 4 1 4 2.4 0 2 0 2 0 0.8
6 2 1 4 0 1 1.6 0 1 0 0 2 0.6
7 1 0 3 1 0 1.0 0 0 0 0
8 0 0 1 0 0 0.2 0 0 1 0.2
9 0 0 0 1 0 0.2 0 0 0
10 0 0 0 0 1 0.2
11 0 0 0 0 0
12 0 0 0 0
13 0 0
14
15

Only three of the ten individuals failed to re-acquire the habit of white-
black discrimination more quickly than it had originally been acquired,
and, in the case of these exceptions, No. 220 required exactly the same
number of tests in each case, and No. 420 was placed at a slight
disadvantage in the re-learning series by an interruption of the training
between the seventh and the eighth series. Had his training been completed
by the sixth series he too would have had the same number of tests in
training and re-training. Moreover, and this is of preeminent importance
for a fair interpretation of the results, in several instances even those
individuals which exhibited as strong a preference for the black in the
memory series as in the preliminary series re-learned more quickly than
they had learned. Number 210, for example, although he gave no evidence of
memory, and, in fact, chose the black more frequently in the memory series
than he did in the preliminary series, re-acquired the discrimination
habit in less than half the number of tests which had been necessary for
the establishment of the habit originally.

TABLE 50

WHITE-BLACK TRAINING. TEN TESTS PER DAY

Females

TRAINING RE-TRAINING
215 225 235 415 425 Av. 215 225 235 415 425 Av.
A 8 4 4 8 5 5.8 5 2 7 6 3 4.6
B 8 7 6 6 2 5.8 8 5 6 4 3 5.2
1 7 6 5 6 4 5.6 4 1 5 4 3 3.4
2 5 6 4 2 5 4.4 1 1 1 2 3 1.6
3 3 3 4 3 4 3.4 1 0 3 6 0 2.0
4 2 1 3 3 3 2.4 0 0 3 3 1 1.4
5 1 3 3 3 3 2.6 0 0 died 2 0 0.5
6 2 1 1 1 0 1.0 0 1 0 0.2
7 1 1 2 3 3 2.0 0 0 0
8 0 0 2 2 3 1.4 1 0.2
9 1 0 0 1 1 0.6 0 0
10 0 2 1 0 2 1.0 0 0
11 0 3 0 1 0 0.8 0 0
12 0 0 0 2 0 0.4
13 0 0 0 0 0
14 0 0 0
15 0 0

The facts which have been presented thus far become more significant when
the indices of modifiability for the learning and the re-learning
processes are compared.

INDICES OF MODIFIABILITY

LEARNING RE-LEARNING

Females . . . . . . . 104 42.5
Males . . . . . . . . 72 54

The behavior of the mice in the experiments, the detailed results of
Tables 49 and 50, and the indices of modifiability together justify the
following conclusions. Most of the ten dancers, at the end of a rest
interval of eight weeks, had so far lost the habit of white-black
discrimination that memory tests furnished no conclusive evidence of the
influence of previous training; a few individuals seemed to possess traces
of the habit after such an interval. In the case of each group of
individuals re-training brought about the establishment of a perfect habit
far more quickly than did the original training. This suggests the
existence of two kinds or aspects of organic modification in connection
with training; those which constitute the basis of a definite form of
motor activity, and those which constitute the bases or dispositions for
the acquirement of certain types of behavior. There are several
indications that further study of the modifiability of behavior will
furnish the facts which are necessary to render this suggestion
meaningful.

Closely related to the facts which have been revealed by the re-training
experiments are certain results of the labyrinth experiments. For the
student of animal behavior, as for the human educator, it is of importance
to learn whether one kind of training increases the efficiency of similar
forms of training. Can a dancer learn a given labyrinth path the more
readily because it has previously had experience in another form of
labyrinth?

The answer to this question, which my experimental results furnish, is
given in Table 51. In the upper half of the table have been arranged the
results for six individuals which were trained first in labyrinth B, then
in labyrinth C, and finally in labyrinth D. Below, in similar fashion, are
given the results for six individuals which were trained in the same three
labyrinths in the order C, B, D, instead of B, C, D. My purpose in giving
the training in these two orders was to ascertain whether labyrinth C,
which had proved to be rather difficult for most individuals, would be
more easily learned if the training in it were preceded by training in
labyrinth C.

TABLE 51

THE INFLUENCE OF ONE LABYRINTH HABIT UPON THE FORMATION
OF ANOTHER
LABYRINTH B LABYRINTH C LABYRINTH D
NO. OF NO. OF NO. OF NO. OF NO. OF NO. OF
NO. FIRST COR- LAST OF FIRST COR- LAST OF FIRST COR- LAST OF
RECT TEST FIVE COR- RECT TEST FIVE COR- RECT TEST FIVE COR-
RECT TESTS RECT TESTS RECT TESTS

76 8 14 3 19 4 7
78 5 20 6 14 4 5
86 13 22 5 12 3 9
75 4 15 8 19 4 13
77 7 11 11 29 11 12
87 12 22 9 20 4 9

AV. 8.2 17.3 7.0 18.8 5.0 9.2

LABYRINTH C LABYRINTH B LABYRINTH D

58 16 -- 2 14 7 10
60 17 -- 13 37 10 14
88 25 35 9 22 4 8
49 34 -- 1 5 7 8
57 15 -- 3 20 3 6
85 11 18 2 11 3 4

AV. 19.7 26.5 5.0 18.2 5.7 8.3

The results are sufficiently definite to warrant the conclusion that
experience in B rendered the learning of C easier than it would have been
had there been no previous labyrinth training. Those individuals whose
first labyrinth training was in C made their first correct trip as the
result of 19.7 trials, whereas those which had previously been trained in
labyrinth B were able to make a correct trip as the result of only 7.0
trials. Similarly the table shows that training in C rendered the
subsequent learning of B easier. To master B when it was the first
labyrinth required 8.2 trials; to master it after C had been learned
required only 5 trials. In addition to proving that the acquisition of one
form of labyrinth habit may facilitate the acquisition of others,
comparison of the averages of Table 51 furnishes evidence of the truth of
the statement that no results of training can be properly interpreted in
the absence of knowledge of the previous experience of the organism.

CHAPTER XVII

INDIVIDUAL, AGE, AND SEX DIFFERENCES IN BEHAVIOR

All dancers are alike in certain important respects, but to the trained
observer of animal behavior their individual peculiarities are quite as
evident, and even more interesting than their points of resemblance.
Omitting consideration of the structural marks of individuality, we shall
examine the individual, age, and sex differences in general behavior,
rapidity of learning, memory, and discrimination, which have been revealed
by my experiments. Observations which bear on the subject of differences
are scattered through the preceding chapters, but in no case have they
been given sufficient prominence to force them upon the attention of those
who are not especially interested in individual peculiarities. It has
seemed worth while, therefore, to assemble all the available material in
this chapter for systematic examination and interpretation.

In the pages which follow, individual, age, and sex peculiarities are
discussed in turn. Within each of these three groups of differences I have
arranged in order what Royce has appropriately named the facts of
discriminating sensitiveness, docility, and initiative. Individuals of the
same age and sex no less than those which differ in sex or age exhibit
important differences in ability to discriminate among sense impressions
("discriminative sensitiveness"), in ability to profit by experience
("docility"), and in ability to try new kinds of behavior ("initiative").

Individual differences in sensitiveness to visual, auditory, tactual, and
olfactory stimuli have been revealed by many of my experiments. The
brightness discrimination tests conclusively proved that a degree of
difference in illumination which is easily detectable by one dancer may be
beyond the discriminating sensitiveness of another. Both the tests with
gray papers and those with the Weber's law apparatus furnished striking
evidence of individual differences in the kind of visual sensitiveness
which throughout this book has been called brightness vision. I suspect
that certain of the differences which were observed should be referred to
the experience of the individuals rather than to the capacity of the
visual organs, for training improves visual discrimination to a much
greater extent than would ordinarily be thought possible. To the truth of
this statement the results of the Weber's law experiments with No. 51 bear
witness. Likewise in color discrimination there are individual
differences, examples of which may be discovered by the examination of the
results given in Chapters IX and X.

No differences in auditory sensitiveness appeared in my adult dancers, for
in none of them was there definite response to sounds, but among the young
individuals differences were prominent. I may call attention to the data
on this subject which Table 5, p. 89, contains. The mice in four out of
twelve litters gave no indications of hearing any sounds that I was able
to produce; the remaining individuals responded with varying degrees of
sensitiveness. I made no attempt to measure this sensitiveness, but it
obviously differed from mouse to mouse. I feel justified, therefore, in
stating that the young dancers exhibit extreme individual difference in
sensitiveness to sounds.

My observations of differences in sensitiveness to other forms of
stimulation were made in connection with training tests, and although they
are not quantitative, I venture to call attention to them. Indeed, I am
led by the results of my study of various aspects of the dancer's behavior
to conclude that the race exhibits individual differences in
discriminating sensitiveness to a far greater extent than do most mammals,
not excepting man. The importance of this fact (for I am confident that
any one who carefully examines the detailed results of the various
experiments which are described in this book will agree that it is an
established fact) cannot be overlooked. It alters our interpretation of
the results of training, memory, heredity, and discrimination experiments,
and it leads us to suspect that the dancing race is exceedingly unstable.
I do not venture to make comparison of my own observations of the dancer's
sense equipment with those of Cyon, Rawitz, Zoth, and Kishi, for the
differences are too great in many instances to be thought of as other than
species or variety peculiarities. It has seemed fairer to compare only
individuals of the same breed, or, as I have done and shall continue to do
throughout this chapter, of two lines of descent.

With respect to docility individual differences are prominent. We need
only turn to the various tables of results to discover that in
modifiability of behavior, in memory, in re-learning, not to mention other
aspects of docility, dancers of the same sex and age differed strikingly.
Let me by way of illustration cite a few cases of difference in docility.
Number 1000 learned to discriminate white from black more quickly and
retained his habit longer than any other dancer with which I have
experimented. I should characterize him as an exceptionally docile
individual. Table 44 offers several examples. Numbers 403 and 407, though
they were born in the same litter and were alike in appearance and in
conditions of life, acquired the white-black habit with a difference in
rapidity which is expressed by the indices of modifiability 50 and 100. In
other words, it took No. 407 twice as long to acquire this habit as it
took No. 403. Similarly the ladder-climbing tests revealed important
individual differences in ability to profit by experience. In the tables
of labyrinth tests (38, 39, 40) individual differences are too numerous to
mention. It required forty-nine tests to establish in No. 50 a labyrinth-C
habit which was approximately equal in degree of perfection to that which
resulted from twenty-two tests in the case of No. 52. The figures in this
and other instances do not exaggerate the facts, for repeatedly I have
tested individuals of the same litter, the same sex, and, so far as I
could judge, of the same stage of development, and obtained results which
differ as markedly as do those just cited. If space limits permitted, I
could present scores of similar differences in docility which the problem,
labyrinth, and discrimination methods have revealed.

In examining the detailed individual results of the various tables for
differences of this sort, it is important to bear in mind that sex, age,
and descent should be taken into account, for with each of them, as will
be shown clearly later in this chapter, sensitiveness, docility, and
initiative vary. I have therefore based my statements concerning
individual differences in docility upon the results of comparison of mice
of the same litter, sex, and age. It is safe to say that human beings
similarly selected for comparison do not exhibit greater differences in
ability to profit by experience than did these dancing mice.

The facts concerning individual differences in initiative which I have
discovered are not less definite than those of the preceding paragraphs.
From the beginning of my study of the dancer I observed that what one
individual would readily learn of his own initiative another never
learned. For example, in the ladder-climbing experiment No. 1000
distinguished himself for his initiative, whereas Nos. 4 and 5 never
acquired the habit of escaping from confinement by using the ladder. I
noticed, in this test of the animal's ability to learn, that while one
individual would be scurrying about trying all ways of escape,
investigating its surroundings, looking, sniffing, and dancing by turns,
another would devote all its time to whirling, circling, or washing
itself. One in the course of its activity would happen upon the way of
escape, the other by reason of the limited scope of its activity, not the
lack of it, would fail hour after hour to discover even the simplest way
of getting back to its nest, to food, and to its companions. Hundreds of
times during the past three years I have noticed important individual
differences in initiative in connection with the discrimination
experiments. The swinging wire doors which one dancer learned to push open
before he had been in the box five minutes, another might not become
familiar with through his own initiative for hours or days. In fact, it
was not seldom that I had to teach an individual to pass from one
compartment to the other by gently pushing him against the door until it
opened sufficiently to allow him to squeeze through. Occasionally a mouse
learned to pull the doors open so that he could pass through the openings
in either direction with facility. This was a form of individual
initiative which I had not anticipated and did not especially desire, so I
did not encourage its development, but, nevertheless, at least one fourth
of the mice which I experimented with in the discrimination box learned
the trick. The other three fourths, although they were used in the box day
after day sometimes for weeks, never discovered that they might return to
the nest-box by pulling the swing-door through which they had just passed
as well as by entering one of the electric-boxes.

Another indication of individual initiative in action appeared in the
tendency of certain mice to climb out of the experiment boxes or
labyrinths. It would have been extremely easy for any of the mice to
escape from the labyrinths by scaling the walls of the alleys, for they
were only 10 cm. in height, and when a dancer stood on its hind legs it
could easily reach the top with its nose. But, strange though it will seem
to any one who has not worked with the dancer, not more than one in ten of
the animals which I observed made any attempt to escape in this manner.
They lacked initiative. That it was not due to a lack of the power to
climb, I abundantly demonstrated by teaching a few individuals that a
scramble in one corner meant easy escape from the maze of paths. I do not
think any one of the mice was physically incapable of climbing, but I am
confident that they differed markedly, not only in the willingness to try
new modes of action, but in the readiness with which they could climb. I
have already said that individuals differ noticeably in the scope of their
activity. By this statement I mean that they try a varying number of kinds
of activity. As in the case of men, so in mice, one individual will do a
greater number of things in a few hours than another will in weeks or
months. The dancers differ in versatility, in individual initiative, as do
we, albeit not so markedly.

Important differences which may with certainty be described as age
differences are not so obvious as are such marks of individuality as have
been set forth in the preceding pages. I have noted few changes in
discriminative sensitiveness, other than those with regard to auditory
sensitiveness, which could be correlated with age. In certain instances
adults appeared to be able to discriminate more accurately and more easily
than young mice, but it is difficult to say whether this change belongs
under sensitiveness or docility. I have not made an ontogenetic study of
the senses, and I am therefore unable to describe in detail the course of
their development and decline. Of one important fact I am certain, that
discriminative sensitiveness increases up to a certain point with age and
with training.

Differences in docility which are obviously to be correlated with age
abound. In the prime of its life (from the second to the tenth month) the
dancer is active, full of energy, quick to learn; in its senility (during
the second year) it is inactive, but at times even more docile than during
the period of greatest physical development. Frequently I have noticed in
connection with labyrinth tests that individuals of the age of a year or
more learn much more quickly than do individuals of the age of two or
three months. But, on the other hand, I have contradictory observations,
for now and then I obtained just the opposite result in experiments to
test docility. Evidently this is a matter which demands systematic,
quantitative investigation. Casual observation may suggest conclusions,
but it will not justify them.

Early in my investigation of the behavior of the dancer I conceived the
idea of determining the relation of modifiability of behavior (docility)
to age. The question which was foremost in my mind and for which I first
sought an answer may be stated thus: can the dancer acquire a given habit
with the same facility at different ages? Since the visual discrimination
experiment seemed to be well suited for the investigation of this problem
I planned to train, in the white-black discrimination experiment, five
pairs of dancers at the age of one month, and the same number for each of
the ages four, seven, ten, thirteen, sixteen, and nineteen months.[1]

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