The Dancing Mouse by Robert M. Yerkes

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I shall now report in detail the results of my own study of the sense of
hearing in the dancer. As the behavior of the young differs greatly from
that of the adult, by which is meant the sexually mature animal, I shall
present first the results of my experiments with adults and later, in
contrast, the results obtained with mice from one to twenty-eight days
old.

My preliminary tests were made with noises. While carefully guarding
against the interference of visual, tactual, temperature, and olfactory
stimuli, I produced noises of varying degrees of loudness by clapping the
hands together suddenly, by shouting, whistling, exploding pistol caps,
striking steel bars, ringing an electric bell, and causing another mouse
to squeak. To these sounds a common mouse usually responds either by
starting violently, or by trembling and remaining perfectly quiet for a
few seconds, as if frightened. The adult dancers which I have tested, and
I have repeated the experiment scores of times during the last three years
with more than a hundred different individuals, have never given
unmistakable evidence of hearing. Either they are totally deaf or there is
a most surprising lack of motor reactions.

Precisely the same results were obtained in tests made with the Galton
whistle throughout its range of pitches, and with Appuun whistles which,
according to their markings, ranged from 2000 Vs. (C_4) to 48,000 (G_9),
but which undoubtedly did not correspond at all exactly to this range, and
with a series of Koenig tuning forks which gave tones varying in pitch from
1024 to 16,382 complete vibrations.

I am willing to trust these experimental results the more fully because
during all the time I have had adult dancers under observation I have
never once seen a reaction which could with any fair degree of certainty
be referred to an auditory stimulus. Never once, although I have tried
repeatedly, have I succeeded in arousing a dancer from sleep by producing
noises or tones, nor have I ever been able to observe any influence of
sounds on the dance movements. All of Cyon's signs have failed with my
mice. Occasionally what looked like a response to some sound appeared, but
critical observation invariably proved it to be due to some other cause
than the auditory stimulus. A sound produced above the animal is very
likely to bring about a motor reaction, as Cyon claims; but I have always
found it to be the result of the currents of air or odors, which usually
influence the animal when the experimenter is holding any object above it.
I do not wish to maintain that Cyon's conclusions are false; I merely
emphasize the necessity for care in the exclusion of other stimuli. The
mice are extremely sensitive to changes in temperature, such, for example,
as are produced by the breath of the experimenter, and one must constantly
guard against the misinterpretation of behavior.

In a single experiment with mice over a month old, I observed what might
possibly indicate sensitiveness to sound. While holding a mouse, thirty-
five days old, in my hand I pursed my lips and made a very shrill sound by
drawing in air; the mouse seemed to start perceptibly according to the
indications given by my sense of touch. I repeated the stimulus several
times and each time I could see and feel the animal start slightly. With
two other individuals which I tested the reaction was less certain, and
with several others I failed to get any indication of response. This would
seem to prove that the three individuals which responded happened to be
sensitive to that particular tone at the age of five weeks. The test is
unsatisfactory because the vibrations from my own body may have brought
about the reaction instead of the air vibrations produced by my lips, and
I therefore merely mention it in the enumeration of the various
experimental tests which I have made.

If we should conclude from all the negative evidence that is available, or
that could be obtained, that the dancer is totally deaf, it might fairly
be objected that the conclusion is unsafe, since an animal does not
necessarily respond to stimuli by a visible change in the position or
relations of its body. Death feigning may fairly be considered a response
to a stimulus or stimulus complex, yet there may be no sign of movement.
The green frog when observed in the laboratory usually gives no indication
whatever, by movements that are readily observable, that it hears sounds
which occur about it, but I have been able to show by means of indirect
methods of study that it is stimulated by these same sounds.[1] Its rate
of respiration is changed by the sounds, and although a sound does not
bring about a bodily movement, it does very noticeably influence movements
in response to other stimuli which occur simultaneously with the sound. I
discovered that under certain rather simple experimental conditions the
green frog would regularly respond to a touch on the back by drawing its
hind leg up toward the body. Under the same conditions the sound of an
electric bell caused no visible movement of the leg, but if at the instant
the back was touched the bell was rung, the leg movement was much greater
than that brought about by the touch alone. This suggests at once the
desirability of studying the sense of hearing in the dancer by some
indirect method. The animal may be stimulated, and yet it may not give any
visible sign of the influence of the auditory stimulus.

[Footnote 1: "The Sense of Hearing in Frogs." _Journal of Comparative
Neurology and Psychology_, Vol. XV, p. 288, 1905.]

Were not the dancing so extremely variable in rapidity and duration, it
might be used as an index of the influence of auditory stimuli. Cyon's
statements would indicate that sounds interfere with the dancing, but as I
obtained no evidence of this, I worked instead with the following indirect
method, which may be called the method of auditory choice.

The apparatus which was used is described in detail in Chapter VII.
Figures 14 and 15 will greatly aid the reader in understanding its
essential features. Two small wooden boxes, identical in form and as
closely similar as possible in general appearance, were placed in a larger
box in such positions that a mouse was forced to enter and pass through
one of them in order to get to the nest-box. On the bottom of each of
these small boxes was a series of wires through which an electric current
could be made to pass at the will of the experimenter. The boxes could
readily be interchanged in position. At one side of the large wooden box
and beyond the range of vision of the mouse was an electric bell which
could be caused to ring whenever the mouse approached the entrance to one
of the small boxes. The point of the experiment was to determine whether
the dancer could learn to avoid the box-which-rang when it was approached.
The method of conducting the tests was as follows. Each day at a certain
hour the mouse was placed in that part of the large box whence it could
escape to the nest-box only by passing through one of the small boxes. If
it approached the wrong box (whether it happened to be the one on the
right or the one on the left depended upon the experimenter's decision),
the bell began to ring as a warning against entering; if it approached the
other box, all was silent. As motives for the choice of the box-which-did-
not-ring both reward and punishment were employed. The reward consisted of
freedom to return to the nest-box _via_ the passage which led from the
box-which-did-not-ring; the punishment, which consisted of a disagreeable
electric shock, was given whenever the mouse entered the wrong box, that
is, the one which had the sound as a warning. Entering the wrong box
resulted in a disagreeable stimulus and in the necessity of returning to
the large box, for the exit to the nest-box by way of the passage from
this box was closed. My assumption, on the basis of extended study of the
ability of the dancer to profit by experience, was that if it could hear
the sound of the bell it would soon learn to avoid the box-that-rang and
enter instead the one which had no sound associated with it.

Systematic tests were made with No. 4 from the 3d to the 12th of February,
inclusive, 1906. Each day the mouse was permitted to find his way to the
nest-box through one of the small boxes ten times in succession. Usually
the experimenter rang the bell alternately for the box on the left and the
box on the right. The time required for such a series of experiments
varied, according to the rapidity with which the mouse made his choice,
from ten to thirty minutes. If in these experiments the animal approached
and entered the right, or soundless box, directly, the choice indicated
nothing so far as ability to hear is concerned; if it entered the wrong,
or sounding box, despite the ringing of the bell, it indicated either the
lack of the influence of experience or inability to hear the sound; but if
it regularly avoided the box-which-sounded it thus gave evidence of
ability to hear the sound of the bell. The purpose of the test was to
determine, not whether the mouse could learn, but whether it could hear.

For ten successive days this experiment was carried on with No. 4 without
the least indication of increasing ability to avoid the wrong box by the
association of the sound of the bell with the disagreeable electric shock
and failure to escape to the nest-box. In fact, the experiment was
discontinued because it became evident that an impossible task had been
set for the mouse. Day by day as the tests were in progress I noticed that
the animal became increasingly afraid of the entrances to the small boxes;
it seemed absolutely helpless in the face of the situation. Partly because
of the definiteness of the negative results obtained with No. 4 and partly
because of the cruelty of subjecting an animal to disagreeable conditions
which it is unable to avoid, the experiment was not repeated with other
individuals. I have never conducted an experiment which gave me as much
discomfort as this; it was like being set to whip a deaf child because it
did not learn to respond to stimuli which it could not feel.

By a very similar method No. 18 was tested for his sensitiveness to the
noise and jar from the induction apparatus which was used in connection
with many of my experiments on vision and the modifiability of behavior.
In this experiment the wrong box was indicated by the buzzing sound of the
apparatus and the slight vibrations which resulted from it. Although No.
18 was tested, as was No. 4, for ten successive days, ten trials each day,
it gave no evidence of ability to avoid the box-which-buzzed.

Since both direct and indirect methods of testing the hearing of the
dancer have uniformly given negative results, in the case of mice more
than five weeks old, I feel justified in concluding that they are totally
deaf and not merely irresponsive to sounds.

Rawitz's statements, and the fact that what may have been auditory
reactions were obtained with a few individuals of five weeks of age,
suggest that the mice may be able to hear at certain periods of life. To
discover whether this is true I have tested the young of twenty different
litters from the first day to the twenty-eighth, either daily or at
intervals of two or three days. In these tests Koenig forks, steel bars,
and a Galton whistle were used. The results obtained are curiously
interesting.

During the first two weeks of life none of the mice which I tested gave
any visible motor response to the various sounds used. During the third
week certain of the individuals responded vigorously to sudden high tones
and loud noises. After the third week I have seen only doubtful signs of
hearing. I shall now describe in detail the method of experimentation, the
condition of the animals, and the nature of the auditory reactions.

Between the twelfth and the eighteenth day the auditory canal becomes open
to the exterior. The time is very variable in different litters, for their
rate of growth depends upon the amount of nourishment which the mother is
able to supply. Without exception, in my experience, the opening to the
ear appears before the eyes are open. Consequently visual stimuli usually
are not disturbing factors in the auditory tests with mice less than
sixteen days old. There is also a sudden and marked change in the behavior
of the mice during the third week. Whereas, for the first fourteen or
eighteen days they are rather quiet and deliberate in their movements when
removed from the nest, some time in the third week their behavior suddenly
changes and they act as if frightened when taken up by the experimenter.
They jump out of his hand, squeak, and sometimes show fight. This is so
pronounced that it has attracted my attention many times and I have
studied it carefully to determine, if possible, whether it is due to some
profound change in the nervous system which thus suddenly increases the
sensitiveness of the animal or to the development of the sexual organs. I
am inclined to think that it is a nervous phenomenon which is intimately
connected with the sexual condition. Within a day or two after it appears
the mice usually begin to show auditory reactions and continue to do so
for three to five days.

I shall now describe the results obtained with a few typical litters. A
litter born of Nos. 151 and 152 gave uniformly negative results in all
auditory tests up to the fourteenth day. On that day the ears were open,
and the following observations were recorded. The five individuals of the
litter, four females and one male, were taken from the nest one at a time
at 7 A.M. and placed on a piece of paper in the bright sunlight. The
warmth of the sun soon quieted them so that auditory tests could be made
to advantage. As soon as an individual had become perfectly still, the
Galton whistle was held at a distance of about four inches from its head
in such a position that it could not be seen nor the currents of air
caused by it felt, and suddenly blown. Each of the five mice responded to
the first few repetitions of this stimulus by movements of the ears,
twitchings of the body, and jerky movements of the legs. The most violent
reactions resulted when the individual was lying on its back with its legs
extended free in the air. Under such circumstances the four legs were
often drawn together suddenly when the whistle was sounded. Similar
responses were obtained with the lip sound already mentioned. Two other
observers saw these experiments, and they agreed that there can be no
doubt that the mice responded to the sound. The sounds which were
effective lay between 5000 and 10,000 complete vibrations.

On the fifteenth day the eyes were just beginning to open. Three of the
mice responded definitely to the sounds, but the other two slightly, if at
all. On the sixteenth day they were all too persistently active for
satisfactory auditory tests, and on the seventeenth, although they were
tested repeatedly under what appeared to be favorable conditions, no signs
of sensitiveness were noted. Although I continued to test this litter, at
intervals of three or four days, for two weeks longer, I did not once
observe a response to sound.

This was the first litter with which I obtained perfectly definite, clear-
cut responses to sounds. That the reactive ability had not been present
earlier than the fourteenth day I am confident, for I had conducted the
tests in precisely the same manner daily up to the time of the appearance
of the reactions. To argue that the mice heard before the fourteenth day,
but were unable to react because the proper motor mechanism had not
developed sufficiently would be short-sighted, for if the response
depended upon the development of such a mechanism, it is not likely that
it would disappear so quickly. I am therefore satisfied that these
reactions indicate hearing.

With another litter the following results were obtained. On the thirteenth
day each of the eight members of the litter responded definitely and
uniformly to the Galton whistle, set at 5 (probably about 8000 complete
vibrations), and to a Koenig steel bar of a vibration rate of 4096 Vs. The
largest individuals, for almost always there are noticeable differences in
size among the members of a litter, appeared to be most sensitive to
sounds.

On the fifteenth day and again on the seventeenth unmistakable responses
to sound were observed; on the eighteenth the responses were indefinite,
and on the nineteenth none were obtained. I continued the tests up to the
twenty-eighth day without further indications of hearing.

Certain individuals in this litter reacted so vigorously to the loud sound
produced by striking the steel bar a sharp blow and also to the Galton
whistle, during a period of five days, that I have no hesitation in saying
that they evidently heard during that period of their lives. Other members
of the litter seemed to be less sensitive; their reactions were sometimes
so indefinite as to leave the experimenter in doubt about the presence of
hearing.

A third litter, which developed very slowly because of lack of sufficient
food, first showed unmistakable reactions to sound on the twenty-first
day. On this day only two of the five individuals reacted. The reactions
were much more obvious on the twenty-second day, but thereafter they
became indefinite.

Still another litter, which consisted of one female and four males, began
to exhibit the quick, jerky movements, already mentioned, on the
fourteenth day. On the morning of the fifteenth day three members of the
litter definitely reacted to the tone of the steel bar, and also to the
hammer blow when the bar was held tightly in the hand of the experimenter.
My observations were verified by another experimenter. Two individuals
which appeared to be very sensitive were selected for special tests. Their
reactions were obvious on the sixteenth, seventeenth, and eighteenth days;
on the nineteenth day they were indefinite, and on the twentieth none
could be detected. Some individuals of this litter certainly had the
ability to hear for at least five days.

A sixth litter of four females and two males first gave indications of the
change in behavior which by this time I had come to interpret as a sign of
the approach of the period of auditory sensitiveness, on the seventeenth
day. I had tested them almost every day previous to this time without
obtaining evidence of hearing. The tests with the steel bar and the Galton
whistle were continued each day until the end of the fourth week without
positive results. To all appearances the individuals of this litter were
unable to hear at any time during the first month of life.

Practically the same results were obtained with another litter of four
females. The change in their behavior was obvious on the eighteenth day,
but at no time during the first month did they give any satisfactory
indications of hearing.

In the accompanying table, I have presented in condensed form the results
of my auditory tests in the case of twelve litters of young dancers.

TABLE 5

PERIOD OF AUDITORY REACTION IN YOUNG DANCERS

PARENTS No. in Change in Ears Auditory Reactions
Litter Behavior Open Appear Disappear

152+151 5 13th day 14th day 14th day 16th day
152+15l 8 (?) 13th day 13th day 17th day
152+151 5 13th day 13th day 13th day 17th day
152+151 4 10th day 12th day 13th day 15th day
410+415 5 14th day 15th day 15th day 19th day
410+415 6 13th day 14th day 14th day 18th day
420+425 2 12th day 14th day 14th day 17th day
210+215 5 17th day 13th day 17th day 19th day
210+215 6 11th day 14th day No reactions
220+225 6 16th day 14th day No reactions
220+225 6 17th day 13th day No reactions
212+211 4 15th day 14th day No reactions

Certain of the litters tested responded definitely to sounds, others gave
no sign of hearing at any time during the first four weeks of life. Of the
twelve litters for which the results of auditory tests are presented in
Table 5, eight evidently passed through an auditory period. It is
important to note that all except one of these were the offspring of Nos.
151 and 152, or of their descendants Nos. 410 and 415 and Nos. 420 and
425. In fact every one of the litters in this line of descent which I have
tested, and they now number fifteen, has given indications of auditory
sensitiveness. And, on the other hand, only in a single instance have the
litters born of Nos. 210 and 215, or of their descendants, given evidence
of ability to hear.

These two distinct lines of descent may be referred to hereafter as the
400 and the 200 lines. I have observed several important differences
between the individuals of these groups in addition to the one already
mentioned. The 200 mice were sometimes gray and white instead of black and
white; they climbed much more readily and danced less vigorously than
those of the 400 group. These facts are particularly interesting in
connection with Cyon's descriptions of the two types of dancer which he
observed.

In criticism of my conclusion that the young dancers are able to hear
certain sounds for a few days early in life, and then become deaf, it has
been suggested that they cease to react because they rapidly become
accustomed to the sounds. That this is not the case, is evident from the
fact that the reactions often increase in definiteness during the first
two or three days and then suddenly disappear entirely. But even if this
were not true, it would seem extremely improbable that the mouse should
become accustomed to a sudden and startlingly loud sound with so few
repetitions as occurred in these tests. On any one day the sounds were not
made more than five to ten times. Moreover, under the same external
condition, the common mouse reacts unmistakably to these sounds day after
day when they are first produced, although with repetition of the stimulus
at short intervals, the reactions soon become indefinite or disappear.

The chief results of my study of hearing in the dancer may be summed up in
a very few words. The young dancer, in some instances, hears sounds for a
few days during the third week of life. The adult is totally deaf. Shortly
before the period of auditory sensitiveness, the young dancer becomes
extremely excitable and pugnacious.

CHAPTER VII

THE SENSE OF SIGHT: BRIGHTNESS VISION

The sense of sight in the dancer has received little attention hitherto.
In the literature there are a few casual statements to the effect that it
is of importance. Zoth, for example (31 p. 149), remarks that it seems to
be keenly developed; and other writers, on the basis of their observation
of the animal's behavior, hazard similar statements. The descriptions of
the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and
Kishi (p.47), apparently indicate that the sense is of some value; they do
not, however, furnish definite information concerning its nature and its
role in the daily life of the animal.

The experimental study of this subject which is now to be described was
undertaken, after careful and long-continued observation of the general
behavior of the dancer, in order that our knowledge of the nature and
value of the sense of sight in this representative of the Mammalia might
be increased in scope and definiteness. The results of this study
naturally fall into three groups: (1) those which concern brightness
vision, (2) those which concern color vision, and (3) those which indicate
the role of sight in the life of the dancer.

Too frequently investigators, in their work on vision in animals, have
assumed that brightness vision and color vision are inseparable; or, if
not making this assumption, they have failed to realize that the same
wave-length probably has markedly different effects upon the retinal
elements of the eyes of unlike organisms. In a study of the sense of sight
it is extremely important to discover whether difference in the quality,
as well as in the intensity, of a visual stimulus influences the organism;
in other words, whether color sensitiveness, as well as brightness
sensitiveness, is present. If the dancer perceives only brightness or
luminosity, and not color, it is evident that its visual world is
strikingly different from that of the normal human being. The experiments
now to be described were planned to show what the facts really are.

[Illustration: Figure 14.--Discrimination box. _W_, electric-box with
white cardboards; B, electric-box with black cardboards. Drawn by Mr. C.H.
Toll.]

As a means of testing the ability of the dancer to distinguish differences
in brightness, the experiment box represented by Figures 14 and 15 was
devised. Figure 14 is the box as seen from the position of the
experimenter during the tests. Figure 15 is its ground plan. This box,
which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as
measured on the outside. The plan of construction and its significance in
connection with these experiments on vision will be clear from the
following account of the experimental procedure. A mouse whose brightness
vision was to be tested was placed in the nest-box, A (Figure 15). Thence
by pushing open the swinging door at _I_, it could pass into the entrance
chamber, _B_. Having entered _B_ it could return to _A_ only by passing
through one of the electric-boxes, marked _W_, and following the alley to
_O_, where by pushing open the swing door it could enter the nest-box. The
door at _I_ swung inward, toward _B_, only; those at _O_, right and left,
swung outward, toward _A_, only. It was therefore impossible for the mouse
to follow any other course than _A-I-B-L-W-E-O_ or _A-I-B-R-W-E-O_. The
doors at _I_ and _O_ were pieces of wire netting of 1/2 cm. mesh, hinged
at the top so that a mouse could readily open them, in one direction, by
pushing with its nose at any point along the bottom. On the floor of each
of the electric-boxes, _W_, was an oak board 1 cm. in thickness, which
carried electric wires by means of which the mouse could be shocked in _W_
when the tests demanded it. The interrupted circuit constituted by the
wires in the two electric-boxes, in connection with the induction
apparatus, _IC_, the dry battery, _C_, and the hand key, _K_, was made by
taking two pieces of No. 20 American standard gauge copper wire and
winding them around the oak board which was to be placed on the floor of
each electric-box. The wires, which ran parallel with one another, 1/2 cm.
apart, fitted into shallow grooves in the edges of the board, and thus, as
well as by being drawn taut, they were held firmly in position. The coils
of the two pieces of wire alternated, forming an interrupted circuit
which, when the key _K_ was closed, was completed if the feet of a mouse
rested on points of both pieces of wire. Since copper wire stretches
easily and becomes loose on the wooden base, it is better to use phosphor
bronze wire of about the same size, if the surface covered by the
interrupted circuit is more than three or four inches in width. The
phosphor bronze wire is more difficult to wind satisfactorily, for it is
harder to bend than the copper wire, and it has the further disadvantage
of being more brittle. But when once placed properly, it forms a far more
lasting and satisfactory interrupted circuit for such experiments as those
to be described than does copper wire. In the case of the electric-boxes
under consideration, the oak boards which carried the interrupted circuits
were separate, and the two circuits were joined by the union of the wires
between the boxes. The free ends of the two pieces of wire which
constituted the interrupted circuit were connected with the secondary coil
of a Porter inductorium whose primary coil was in circuit with a No. 6
Columbia dry battery. In the light of preliminary experiments, made in
preparation for the tests of vision, the strength of the induced current
received by the mouse was so regulated, by changing the position of the
secondary coil with reference to the primary, that it was disagreeable but
not injurious to the animal. What part the disagreeable shock played in
the test of brightness vision will now be explained.

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