The Dancing Mouse by Robert M. Yerkes

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TABLE 14

RESULTS OF WEBER'S LAW EXPERIMENTS
Brightness vision

DATE NUMBER STANDARD VARIABLE DIFFERENCE % OF ERRORS
OF TESTS LIGHT LIGHT

May 13 100 20 9.4 .53 20
15 100 20 12.8 .36 36
16 100 20 10.8 .46 26
20 50 80 37.6 .53 6
21 50 80 51.3 .36 10
22 100 80 71.1 .11 35
24 100 80 60.0 .25 21
25 100 80 65.0 .19 25
27 100 80 80 0 41
28 50 5 2.5 .50 18
29 50 5 4.0 .20 14
29 100 5 4.5 .10 25
31 50 5 4.25 .15 20
June 1 50 5 4.85 .03 48
2 50 20 15.0 .25 16
3 50 20 17.4 .13 22
3 100 20 18.0 .10 22
4 100 80 72.0 .10 18
5 100 5 4.5 .10 12
7 100 5 4.67 .067 46
8 50 80 74.67 .067 56
9 50 20 18.67 .067 44

If we apply this rule to the results of the first tests, reported above,
it appears that a standard of 20 hefners was distinguished from a variable
of 9.4 hefners (.53 difference), for the percentage of errors was only 20.
But in the case of a difference of .36 in the illuminations lack of
discrimination is indicated by 36 per cent of errors. A difference of .46
gave a frequency of error so close to the required 25 (26 per cent) that I
accepted the result as a satisfactory determination of the just
perceivable difference for the 20 hefner standard and proceeded to
experiment with another standard value.

The results which were obtained in the case of this second standard, the
value of which was 80 hefners, are strikingly different from those for the
20 hefner standard. Naturally I began the tests with this new standard by
making the differences the same as those for which determinations had been
made in the case of the 20 standard. Much to my surprise only 6 per cent
of errors resulted when the difference in illumination was .53. I finally
discovered that about .19 difference (about one fifth) could be
discriminated with that degree of accuracy which is indicated by 25 per
cent of mistakes.

So far as I could judge from the results of determinations for the 20 and
the 80 hefner standards, Weber's law does not hold for the dancer. With
the former a difference of almost one half was necessary for
discrimination; with the latter a difference of about one fifth could be
perceived. But before presenting additional results I should explain the
construction of Table 14, and comment upon the number of experiments which
constitutes a set.

The table contains the condensed results of several weeks of difficult
experimentation. From left to right the columns give the date of the
initial series of a given set of experiments, the number of experiments in
the set, the value of the standard light in hefners, the value of the
variable light, the difference between the lights in terms of the standard
(the variable was always less than the standard), and the percentage of
errors or wrong choices. Very early in the investigation I discovered that
one hundred tests with any given values of the lights sufficed to reveal
whatever discriminating ability the mouse possessed at the time. In some
instances either the presence or the lack of discrimination was so clear,
as the result of 50 tests (first series), that the second series of 50 was
not given. Consequently in the table the number of tests for the various
values of the lights is sometimes 100, sometimes 50.

After finishing the experiments with the 80 standard on May 27 (see table)
I decided, in spite of the evidence against Weber's law, to make tests
with 5 as the standard, for it seemed not impossible that the lights were
too bright for the dancer to discriminate readily. I even suspected that I
might have been working outside of the brightness limits in which Weber's
law holds, if it holds at all. The tests soon showed that a difference of
one tenth made discrimination possible in the case of this standard. If
the reader will examine the data of the table, he will note that a
difference of .20 gave 14 per cent of mistakes; a difference of .03, 48
per cent. Evidently the former difference is above the threshold, the
latter below it. But what of the interpretation of the results in terms of
Weber's law? The difference instead of being one half or one fifth, as it
was in the cases of the 20 and 80 standards respectively, has now become
one tenth. Another surprise and another contradiction!

Had these three differences either increased or decreased regularly with
the value of the standard I should have suspected that they indicated a
principle or relationship which is different from but no less interesting
than that which Weber's law expresses. But instead of reading 5 standard,
difference one tenth; 20 standard, difference one fifth; 80 standard,
difference one half: or 5 standard, difference one half; 20 standard,
difference one fifth; 80 standard, difference one tenth: they read 5
standard, difference one tenth; 20 standard, difference one half; 80
standard, difference one fifth. What does this mean? I could think of no
other explanation than that of the influence of training. It seemed not
impossible, although not probable, that the mouse had been improving in
ability to discriminate day by day. It is true that this in itself would
be quite as interesting a fact as any which the experiment might reveal.

To test the value of my supposition, I made additional experiments with
the 20 standard, the results of which appear under the dates June 2 and 3
of the table. These results indicate quite definitely that the animal had
been, and still was, improving in her ability to discriminate. For instead
of requiring a difference of about one half in order that she might
distinguish the 20 standard from the variable light she was now able to
discriminate with only 22 per cent of errors when the difference was one
tenth.

As it seemed most improbable that improvement by training should continue
much longer, I next gave additional tests with the 80 standard. Again a
difference of one tenth was sufficient for accurate discrimination (18 per
cent of errors). These series were followed immediately by further tests
with the 5 standard. As the results indicated greater ease of
discrimination with a difference of one tenth in the case of this standard
than in the case of either of the others I was at first uncertain whether
the results which I have tabulated under the dates June 3, 4, and 5 of the
table should be interpreted in terms of Weber's law.

Up to this point the experiments had definitely established two facts:
that the dancer's ability to discriminate by means of brightness
differences improves with training for a much longer period and to a far
greater extent than I had supposed it would; and that a difference of one
tenth is sufficient to enable the animal to distinguish two lights in the
case of the three standard values, 5, 20, and 80 hefners. The question
remains, is this satisfactory evidence that Weber's law holds with respect
to the brightness vision of the dancer, or do the results indicate rather,
that this difference is more readily detected in the case of 5 as a
standard (12 per cent error) than in the case of 20 as a standard (22 per
cent error)?

For the purpose of settling this point I made tests for each of the three
standards with a difference of only one fifteenth. In no instance did I
obtain the least evidence of ability to discriminate. These final tests,
in addition to establishing the fact that the limit of discrimination for
No. 51, after she had been subjected to about two thousand tests, lay
between one tenth and one fifteenth, proved to my satisfaction, when taken
in connection with the results already discussed, that Weber's law does
hold for the brightness vision of the dancer.

In concluding this discussion of the Weber's law experiment I wish to call
attention to the chief facts which have been revealed, and to make a
critical comment. In my opinion it is extremely important that the student
of animal behavior should note the fact that the dancer with which I
worked week after week in the Weber's law investigation gradually improved
in her ability to discriminate on the basis of brightness differences
until she was able to distinguish from one another two boxes whose
difference in illumination was less than one tenth[1] that of the brighter
box. At the beginning of the experiments a difference of one half did not
enable her to choose as certainly as did a difference of one tenth after
she had chosen several hundred times. Evidently we are prone to
underestimate the educability of our animal subjects.

[Footnote 1: Under the conditions of the experiment I was unable to
distinguish the electric-boxes when they differed by less than one
twentieth.]

The reason that the experiments were carried out with only one mouse must
now be apparent. It was a matter of time. The reader must not suppose that
my study of this subject is completed. It is merely well begun, and I
report it here in its unfinished state for the sake of the value of the
method which I have worked out, rather than for the purpose of presenting
the definite results which I obtained with No. 51.

The critical comment which I wish to make for the benefit of those who are
working on similar problems is this. The phosphor bronze wires, on the
bottom of the electric-boxes, by means of which an electric shock could be
given to the mouse when it chose the wrong box, are needless sources of
variability in the illumination of the boxes. They reflect the light into
the eyes of the mouse too strongly, and unless they are kept perfectly
clean and bright, serious inequalities of illumination appear. To avoid
these undesirable conditions I propose hereafter to use a box within a
box, so that the wires shall be hidden from the view of the animal as it
attempts to discriminate.

A brief description of the behavior of the dancer in the brightness
discrimination experiments which have been described may very
appropriately form the closing section of this chapter. For the
experimenter, the incessant activity and inexhaustible energy of the
animal are a never-failing source of interest and surprise. When a dancer
is inactive in the experiment box, it is a good indication either of
indisposition or of too low a temperature in the room. In no animal with
which I am familiar is activity so much an end in itself as in this odd
species of mouse. With striking facility most of the mice learn to open
the wire swing doors from either side. They push them open with their
noses in the direction in which they were intended by the experimenter to
work, and with almost equal ease they pull them open with their teeth in
the direction in which they were not intended to work. In the rapidity
with which this trick is learned, there are very noticeable individual
differences. The pulling of these doors furnished an excellent opportunity
for the study of the imitative tendency.

When confronted with the two entrances of the electric-boxes, the dancer
manifested at first only the hesitation caused by being in a strange
place. It did not seem much afraid, and usually did not hesitate long
before entering one of the boxes. The first choice often determined the
majority of the choices of the preference series. If the mouse happened to
enter the left box, it kept on doing so until, having become so accustomed
to its surroundings that it could take time from its strenuous running
from _A_ by way of the left box to the alley and thence to _A_, to examine
things in _B_ a little, it observed the other entrance and in a seemingly
half-curious, half-venturesome way entered it. In the case of other
individuals, the cardboards themselves seemed to determine the choices
from the first.

The electric shock, as punishment for entering the wrong box, came as a
surprise. At times an individual would persistently attempt to enter, or
even enter and retreat from the wrong box repeatedly, in spite of the
shock. This may have been due in some instances to the effects of fright,
but in others it certainly was due to the strength of the tendency to
follow the course which had been taken most often previously. The next
effect of the shock was to cause the animal to hesitate before the
entrances to the boxes, to run from one to the other, poking its head into
each and peering about cautiously, touching the cardboards at the
entrances, apparently smelling of them, and in every way attempting to
determine which box could be entered safely. I have at times seen a mouse
run from one entrance to the other twenty times before making its choice;
now and then it would start to enter one and, when halfway in, draw back
as if it had been shocked. Possibly merely touching the wires with its
fore paws was responsible for this simulation of a reaction to the shock.
The gradual waning of this inhibition of the forward movement was one of
the most interesting features of the experiment. Could we but discover
what the psychical states and the physiological conditions of the animal
were during this period of choosing, comparative psychology and physiology
would advance by a bound.

If the conditions at the entrances of the two boxes were discriminable,
the mouse usually learned within one hundred experiences to choose the
right box without much hesitation. Three distinct methods of choice were
exhibited by different individuals, and to a certain extent by the same
individual from time to time. These methods, which I have designated
_choice by affirmation_, _choice by negation_, and _choice by comparison_,
are of peculiar interest to the psychologist and logician.

When an individual runs directly to the entrance of the right box, and,
after stopping for an instant to examine it, enters, the choice may be
described as recognition of the right box. I call it choice by affirmation
because the act of the animal is equivalent to the judgment--"this is it."
If instead it runs directly to the wrong box, and, after examining it,
turns to the other box and enters without pause for examination, its
behavior may be described as recognition of the wrong box. This I call
choice by negation because the act seems equivalent to the judgment--"this
is not it." Further, it seems to imply the judgment--"therefore the other
is it." In the light of this fact, this type of choice might appropriately
be called choice by exclusion. Finally, when the mouse runs first to one
box and then to the other, and repeats this anywhere from one to fifty
times, the choice may be described as comparison of the boxes; therefore,
I call it choice by comparison. Certain individuals choose first by
comparison, and later almost uniformly by affirmation and negation.
Whenever the conditions are difficult to discriminate, choice by
comparison occurs most frequently and persistently. If, however, the
conditions happen to be absolutely indiscriminable, as was true, for
example, in the case of the sound tests, in certain of the Weber's law
tests, and in the plain electric-box tests, the period of hesitation
rapidly increases during the first three or four series of tests, then the
mouse seems to lessen its efforts to discriminate and more and more tends
to rush into one of the boxes without hesitation or examination, and
apparently with the expectation of a shock, but with the intention of
getting it over as soon as possible. Now and then under such conditions
there is a marked tendency to enter the same box each time.
Indiscriminable conditions are likely to render the animals fearful of the
experiment; instead of going from _A_ to _A_ willingly, they fight against
making the trip. They refuse to pass from _A_ to _B_; and when in _B_,
they fight against being driven toward the entrances to the electric-
boxes.

In marked contrast with this behavior on the part of the mouse under
conditions which do not permit it to choose correctly is that of the
animal which has learned what is expected of it. The latter, far from
holding back or fighting against the conditions which urge it forward, is
so eager to make the trip that it sometimes has to be forced to wait while
the experimenter records the results of the tests. There is evidence of
delight in the freedom of movement and in the variety of activity which
the experiment furnishes. The thoroughly trained dancer runs into _B_
almost as soon as it has been placed in _A_ by the experimenter; it
chooses the right entrance by one of the three methods described above,
immediately, or after whirling about a few times in _B_; it runs through
_E_ and back to _A_ as quickly as it can, and almost before the
experimenter has had time to record the result of the choice it is again
in _B_ ready for another choice.

CHAPTER IX

THE SENSE OF SIGHT: COLOR VISION

Is the dancing mouse able to discriminate colors as we do? Does it possess
anything which may properly be called color vision? If so, what is the
nature of its ability in this sense field? Early in my study of the mice I
attempted to answer these and similar questions, for the fact that they
are completely deaf during the whole or the greater part of their lives
suggested to me the query, are they otherwise defective in sense
equipment? In the following account of my study of color vision, I shall
describe the evolution of my methods in addition to stating the results
which were obtained and the conclusions to which they led me. For in this
case the development of a method of research is quite as interesting as
the facts which the method in its various stages of evolution revealed.

Observation of the behavior of the dancer under natural conditions caused
me to suspect that it is either defective in color vision or possesses a
sense which is very different from human vision. I therefore devised the
following extremely simple method of testing the animal's ability to
distinguish one color from another. In opposite corners of a wooden box 26
cm. long, 23 cm. wide, and 11 cm. deep, two tin boxes 5 cm. in diameter
and 1.5 cm. deep were placed, as is shown in part I of Figure 18. One of
these boxes was covered on the outside with blue paper (_B_ of Figure 18),
and the other with orange[1] (_O_ of Figure 18). A small quantity of
"force" was placed in the orange box. As the purpose of the test was to
discover whether the animals could learn to go directly to the box which
contained the food, the experiments were made each morning before the mice
had been fed. The experimental procedure consisted in placing the
individual to be tested in the end of the large wooden box opposite the
color boxes, and then permitting it to run about exploring the box until
it found the food in the orange box. While it was busily engaged in eating
a piece of "force" which it had taken from the box and was holding in its
fore paws, squirrel fashion, the color boxes were quickly and without
disturbance shifted in the directions indicated by the arrows of Figure
18, I. Consequently, when the animal was ready for another piece of
"force," the food-box was in the corresponding corner of the opposite end
of the experiment box (position 2, 18, II). After the mouse had again
succeeded in finding it, the orange box was shifted in position as is
indicated by the arrows in Figure 18, II. Thus the tests were continued,
the boxes being shifted after each success on the part of the animal in
such a way that for no two successive tests was the position of the food-
box the same; it occupied successively the positions 1, 2, 3, and 4 of the
figure, and then returned to 1. Each series consisted of 20 tests.

[Footnote 1: These were the Milton Bradley blue and orange papers.]

[Illustration: FIGURE 18.--Food-box apparatus for color discrimination
experiments. _O_, orange food-box; _B_, blue food-box; 1, 2, 3, 4,
different positions of the food-boxes, _O_ and _B_; I, II, III, IV,
figures in which the arrows indicate the direction in which the food-boxes
were moved.]

[Illustration: FIGURE 19.--Food-box apparatus with movable partitions.
_O_, orange food-box; _B_, blue food-box; _X_, starting point for mouse;
_A_, point at which both food-boxes become visible to the mouse as it
approaches them; 1, 2, two different positions of the food-boxes; _T_,
_T_, movable partitions. (After Doctor Waugh.)]

An improvement on this method, which was suggested by Doctor Karl Waugh,
has been used by him in a study of the sense of vision in the common
mouse. It consisted in the introduction, at the middle of the experiment
box, of two wooden partitions which were pivoted on their mid-vertical
axes so that they could be placed in either of the positions indicated in
Figure 19. Let us suppose that a mouse to be tested for color vision in
this apparatus has been placed at _X_. In order to obtain food it must
pass through _A_ and choose either the orange or the blue box. If it
chooses the former, the test is recorded as correct; if it goes to the
blue box first, and then to the orange, it is counted an error. While the
animal is eating, the experimenter shifts the boxes to position 1 of
Figure 19, and at the same time moves the partitions so that they occupy
the position indicated by the dotted lines. The chief advantage of this
improvement in method is that the animal is forced to approach the color
boxes from a point midway between them, instead of following the sides of
the experiment box, as it is inclined to do, until it happens to come to
the food-box. This renders the test fairer, for presumably the animal has
an opportunity to see both boxes from _A_ and can make its choice at that
point of vantage.

Two males, A and B, of whose age I am ignorant, were each given seventeen
series of tests in the apparatus of Figure 18. A single series, consisting
of twenty choices, was given daily. Whether the animal chose correctly or
not, it was allowed to get food; that is, if it went first to the blue
box, thus furnishing the condition for a record of error, it was permitted
to pass on to the orange box and take a piece of "force." No attempt was
made to increase the animal's desire for food by starving it. Usually it
sought the food-box eagerly; when it would not do so, the series was
abandoned and work postponed. "Force" proved a very convenient form of
food in these tests. The mice are fond of it, and they quickly learned to
take a flake out of the box instead of trying to get into the box and sit
there eating, for when they attempted the latter they were promptly pushed
to one side by the experimenter and the box, as well as the food, was
removed to a new position.

The results of the tests appear in Table 15. No record of the choices in
the first two of the 17 series was kept. The totals therefore include 15
series, or 300 tests, with each individual. Neither the daily records nor
the totals of this table demonstrate choice on the basis of color
discrimination. Either the dancers were not able to tell one box from the
other, or they did not learn to go directly to the orange box. It might be
urged with reason that there is no sufficiently strong motive for the
avoidance of an incorrect choice. A mistake simply means a moment's delay
in finding food, and this is not so serious a matter as stopping to
discriminate. I am inclined, in the light of result of other experiments,
to believe that there is a great deal in this objection to the method.
Reward for a correct choice should be supplemented by some form of
punishment for a mistake. This conclusion was forced upon me by the
results of these preliminary experiments on color vision and by my
observation of the behavior of the animals in the apparatus. At the time
the above tests were made I believed that I had demonstrated the inability
of the dancer to distinguish orange from blue, but now, after two years'
additional work on the subject, I believe instead that the method was
defective.

The next step in the evolution of a method of testing the dancer's color
vision was the construction of the apparatus (Figures 14 and 15) which was
described in Chapter VII. In connection with this experiment box the basis
for a new motive was introduced, namely, the punishment of mistakes by an
electric shock. Colored cardboards, instead of the white, black, or grays
of the brightness tests, were placed in the electric-boxes.

TABLE 15

ORANGE-BLUE TESTS, WITH FOOD-BOX

MOUSE A MOUSE B
SERIES DATE
1904 RIGHT WRONG RIGHT WRONG
(ORANGE) (BLUE) (ORANGE) (BLUE)

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