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The Dancing Mouse

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[Footnote 1: No preference tests were given.]


It is now necessary to justify the interpretation of these results as
evidence of brightness discrimination by proving that all other conditions
for choice except brightness difference may be excluded without
interfering with the animal's ability to select the right box. We shall
consider in order the possibility of discrimination by position, by odor,
and by texture and form of the cardboards.

The tendency which the dancer has in common with many, if not all, animals
to perform the same movement or follow the same path under uniform
conditions is an important source of error in many habit-formation
experiments. This tendency is evident even from casual observation of the
behavior of the dancer. The ease with which the habit of choosing the box
on the left or the box on the right is formed in comparison with that of
choosing the white box or the black box is strikingly shown by the
following experiment. Five mice were given one series of ten trials each
in the discrimination box of Figure 14 without the presence of cardboards
or of other means of visual discrimination. The electric shock was given
whenever the box on the left was entered. Thus without other guidance than
that of direction, for the boxes themselves were interchanged in position,
and, as was proved by additional tests, the animals were utterly unable to
tell one from the other, the mouse was required to choose the box on its
right. Only one of the five animals went to the box on the left after once
experiencing the electric shock. The results of the series are given in
Table 11.



TABLE 11

CHOICE BY POSITION
Choices of Choices of
Box on Right Box on Left
First mouse 9 1
Second mouse 8 2
Third mouse 9 1
Fourth mouse 9 1
Fifth mouse 9 1



This conclusively proves that the habit of turning in a certain direction
or of choosing by position can be formed more readily than a habit which
depends upon visual discrimination. A rough comparison justifies the
statement that it takes from six to ten times as long for the dancer to
learn to choose the white box as it does to learn to choose the box on the
right. Since this is true, it is exceedingly important that the
possibility of choice by position or direction of movement be excluded in
the case of tests of brightness discrimination. To indicate how this was
effectively accomplished in the experiments, the changes in the position
of the cardboards made in the case of a standard set of white-black series
are shown in Table 12. The number of the series, beginning at the top of
the table with the two lettered preference series, is given in the first
column at the left, the number of the tests at the top of the table, and
the position of the white cardboard, left or right, is indicated below by
the letters l (left) and r (right).


TABLE 12

POSITION OF WHITE CARDBOARDS FOR A SET OF 150 TESTS


SERIES 1 2 3 4 5 6 7 8 9 10

Preference
A l r l r l r l r l r
B r l r l r l r l r l

1 r l r l r l r l r l
2 l l r r l r l l r r
3 r r l r l l r l r l
4 l l l r r r l r r l
5 r l r l r l r l r l
6 l l r l r r l r l r
7 r l l l r r r l r l
8 r r l l r l r l r l
9 r r r l l l r l r l
10 l l l l r r r r l r
11 r l r r r l l l r l
12 r l r l r r l l r l
13 r l r l l l r r r l
14 l l l l r r r r l r
15 r l r r r l l l r l


It is to be noted that in the case of each series of ten tests the white
cardboard was on the left five times and on the right five times. Thus the
establishment of a tendency in favor of one side was avoided. The
irregularity of the changes in position rendered it impossible for the
mouse to depend upon position in its choice. It is an interesting fact
that the dancer quickly learns to choose correctly by position if the
cardboards are alternately on the left box and on the right.

The prevalent, although ill-founded, impression that mice have an
exceedingly keen sense of smell might lead a critic of these experiments
to claim that discrimination in all probability was olfactory rather than
visual. As precautions against this possibility the cardboards were
renewed frequently, so that no odor from the body of the mouse itself
should serve as a guiding condition, different kinds of cardboard were
used from time to time, and, as a final test, the cardboards were coated
with shellac so that whatever characteristic odor they may have had for
the dancer was disguised if not totally destroyed. Despite all these
precautions the discrimination of the boxes continued. A still more
conclusive proof that we have to do with brightness discrimination, and
that alone, in these experiments is furnished by the results of white-
black tests made with an apparatus which was so arranged that light was
transmitted into the two electric-boxes through a ground glass plate in
the end of each box. No cardboards were used. The illumination of each box
was controlled by changes in the position of the sources of light. Under
these conditions, so far as could be ascertained by critical examination
of the results, in addition to careful observation of the behavior of the
animals as they made their choices, there was no other guiding factor than
brightness difference. Nevertheless the mice discriminated the white from
the black perfectly. This renders unnecessary any discussion of the
possibility of discrimination by the texture or form of the cardboards.

Since a variety of precautionary tests failed to reveal the presence, in
these experiments, of any condition other than brightness difference by
which the mice were enabled to choose correctly, and since evidence of
ability to discriminate brightness differences was obtained by the use of
both reflected light (cardboards) and transmitted light (lamps behind
ground glass), it is necessary to conclude that the dancer possesses
brightness vision.



CHAPTER VIII

THE SENSE OF SIGHT: BRIGHTNESS VISION (Continued)

Since the ability of the dancer to perceive brightness has been
demonstrated by the experiments of the previous chapter, the next step in
this investigation of the nature of vision is a study of the delicacy of
brightness discrimination, and of the relation of the just perceivable
difference to brightness value. Expressed in another way, the problems of
this portion of the investigation are to determine how slight a difference
in brightness enables the dancer to discriminate one light from another,
and what is the relation between the absolute brightnesses of two lights
and that amount of difference which is just sufficient to render the
lights distinguishable. It has been discovered in the case of the human
being that a stimulus must be increased by a certain definite fraction of
its own value if it is to seem different. For brightness, within certain
intensity limits, this increase must be about one one-hundredth; a
brightness of 100 units, for example, is just perceivably different from
one of 101 units. The formulation of this relation between the amount of a
stimulus and the amount of change which is necessary that a difference be
noted is known as Weber's law. Does this law, in any form, hold for the
brightness vision of the dancing mouse?

Two methods were used in the study of these problems. For the first
problem, that of the delicacy of brightness discrimination, I first used
light which was reflected from gray papers, according to the method of
Chapter VII. For the second, the Weber's law test, transmitted light was
used, in an apparatus which will be described later. Either of these
methods might have been used for the solution of both problems. Which of
them is the more satisfactory is definitely decided by the results which
make up the material of this chapter, Under natural conditions the dancer
probably sees objects which reflect light more frequently than it does
those which transmit it; it would seem fairer, therefore, to require it to
discriminate surfaces which differ in brightness. This the use of gray
papers does. But, on the other hand, gray papers are open to the
objections that they may not be entirely colorless (neutral), and that
their brightness values cannot be changed readily by the experimenter. As
will be made clear in the subsequent description of the experiments with
transmitted light, neither of these objections can be raised in connection
with the second method of experimentation.

To determine the delicacy of discrimination with reflected light it is
necessary to have a series of neutral grays (colorless) whose adjacent
members differ from one another in brightness by less than the threshold
of discrimination of the animal to be tested. A series which promised to
fulfill these conditions was that of Richard Nendel of Berlin. It consists
of fifty papers, beginning with pure white, numbered 1, and passing by
almost imperceptible steps of decrease in brightness through the grays to
black, which is numbered 50. For the present we may assume that these
papers are so nearly neutral that whatever discrimination occurs is due to
brightness. The differences between successive papers of the series are
perceptible to man. The question is, can they, under favorable conditions
of illumination, be perceived by the dancer?

On the basis of the fact that the dancer can discriminate between white
and black, two grays which differed from one another in brightness by a
considerable amount were chosen from the Nendel series; these were numbers
10 and 20. It seemed certain, from the results of previous experiments,
that the discrimination of these papers by brightness difference would be
possible, and that therefore the use of papers between these two extremes
would suffice to demonstrate the delicacy of discrimination. In Figure 16
we have a fairly accurate representation of the relative brightness of the
Nendel papers Nos. 10, 15, and 20.

[Illustration: FIGURE 16. Three of Nendel's gray papers: Nos. 10, 15, and
20. To exhibit differences in brightness.]

Pieces of the gray papers were pasted upon cardboard carriers so that they
might be placed in the discrimination box as were the white and black
cardboards in the tests of brightness vision previously described. Mice
which had been trained to choose the white box by series of white-black
tests were now tested with light gray (No. 10) and dark gray (No. 20), my
assumption being that they would immediately choose the brighter of the
two if they were able to detect the difference. As a matter of fact this
did not always occur; some individuals had to be trained to discriminate
gray No. 10 from gray No. 20. As soon as an individual had been so trained
that the ability to choose the lighter of these grays was perfect, it was
tested with No. 10 in combination with No. 15. If these in turn proved to
be discriminable, No. 10 could be used with No. 14, with No. 13, and so on
until either the limit of discrimination or that of the series had been
reached.

That it was not necessary to use other combinations than 10 with 20, and
10 with 15 is demonstrated by the results of Table 13. Mouse No. 420,
whose behavior was not essentially different from that of three other
individuals which were tested for gray discrimination, learned with
difficulty to choose gray No. 10 even when it was used with No. 20. Two
series of ten tests each were given to this mouse daily, and not until the
twentieth of these series (200 tests) did he succeed in making ten correct
choices in succession. Immediately after this series of correct choices,
tests with grays No. 10 and No. 15 were begun. In the case of this amount
of brightness difference twenty series failed to reveal discrimination.
The average number of right choices in the series is slightly in excess of
the mistakes, 5.8 as compared with 4.2.

From the experiments with gray papers we may conclude that under the
conditions of the tests the amount by which Nendel's gray No. 10 differs
in brightness from No. 20 is near the threshold of discrimination, or, in
other words, that the difference in the brightness of the adjacent grays
of Figure 16 is scarcely sufficient to enable the dancer to distinguish
them.


TABLE 13

GRAY DISCRIMINATION

The Delicacy of Brightness Discrimination

No. 420



GRAYS NOS. 10 GRAYS NOS. 20
AND 20 AND 15
SERIES DATE DATE
NO. 10 NO. 2 NO. 10 NO. 15
(RIGHT) (WRONG) (RIGHT) (WRONG)

1 Jan. 26 5 5 Feb. 6 8 2
2 27 8 2 6 5 5
3 28 6 4 7 9 1
4 28 2 8 7 7 3
5 29 1 9 8 5 5
6 29 6 4 8 6 4
7 30 9 1 9 5 5
8 30 7 3 9 6 4
9 31 6 4 10 8 2
10 31 4 6 10 3 7
11 Feb. 1 7 3 11 4 6
12 1 8 2 11 4 6
13 2 7 3 12 7 3
14 2 8 2 12 7 3
15 3 9 1 13 6 4
16 3 9 1 13 4 6
17 4 6 4 14 4 6
18 4 9 1 14 5 5
19 5 6 4 15 5 5
20 5 10 0 15 8 2

Averages 6.6 3.4 5.8 4.2




This result of the tests with gray papers surprised me very much at the
time of the experiments, for all my previous observation of the dancer had
led me to believe that it is very sensitive to light. It was only after a
long series of tests with transmitted light, in what is now to be
described as the Weber's law apparatus, that I was able to account for the
meager power of discrimination which the mice exhibited in the gray tests.
As it happened, the Weber's law experiment contributed quite as
importantly to the solution of our first problem as to that of the second,
for which it was especially planned.

For the Weber's law experiment a box similar to that used in the previous
brightness discrimination experiments (Figure 14) was so arranged that its
two electric-boxes could be illuminated independently by the light from
incandescent lamps directly above them. The arrangements of the light-box
and the lamps, as well as their relations to the other important parts of
the apparatus, are shown in Figure 17. The light-box consisted of two
compartments, of which one may be considered as the upward extension of
the left electric-box and the other of the right electric-box. The light-
box was pivoted at A and could be turned through an angle of 180° by the
experimenter. Thus, by the turning of the light-box, the lamp which in the
case of one test illuminated the left electric-box could be brought into
such a position that in the case of the next test it illuminated the right
electric-box. The practical convenience of this will be appreciated when
the number of times that the brightnesses of the two boxes had to be
reversed is considered. The light-box was left open at the top for
ventilation and the prevention of any considerable increase in the
temperature of the experiment box. In one side of each of the compartments
of the light-box a slit (B, B of the figure) was cut out for an
incandescent lamp holder. A strip of leatherette, fitted closely into inch
grooves at the edges of the slit, prevented light from escaping through
these openings in the sides of the light-box. By moving the strips of
leatherette, one of which appears in the figure, C, the lamps could be
changed in position with reference to the bottom of the electric-box. A
scale, S, at the edge of each slit enabled the experimenter to determine
the distance of the lamp from the floor of the electric-box. The front of
the light-box was closed, instead of being open as it appears in the
figure.

[Illustration: Figure l7.--Weber's law apparatus for testing brightness
discrimination. Lower part, discrimination box similar to that of Figure
14. Upper part, rotatory light-box, pivoted at A, and divided into two
compartments by a partition P in the middle. L, L, incandescent lamps
movable in slits, B, B, in which a narrow strip of leatherette, C, serves
to prevent the escape of light. S, scale.]

This apparatus has the following advantages. First, the electric-boxes,
between which the mouse is expected to discriminate by means of their
difference in brightness, are illuminated from above and the light
therefore does not shine directly from the lamps into the eyes of the
animal, as it approaches the entrances to the boxes. Choice is required,
therefore, between illuminated spaces instead of between two directly
illuminated surfaces. Second, the amount of illumination of each electric-
box can be accurately measured by the use of a photometer. Third, since
the same kind of lamp is used in each box, and further, since the lamps
may be interchanged at any time, discrimination by qualitative instead of
quantitative difference in illumination is excluded. And finally, fourth,
the tests can be made expeditiously, conveniently, and under such simple
conditions that there should be no considerable error of measurement or of
observation within the range of brightness which must be used.

It was my purpose in the experiment with this apparatus to ascertain how
great the difference in the illumination of the two electric-boxes must be
in order that the mouse should be able to choose the brighter of them.
This I attempted to do by fixing an incandescent lamp of a certain known
illuminating power at such a position in one compartment of the light-box
that the electric-box below it was illuminated by what I call a standard
value, and by moving the incandescent lamp in the other compartment of the
light-box until the illumination of the electric-box below it was just
sufficiently less than that of the standard to enable the dancer to
distinguish them, and thereby to choose the brighter one. The light which
was changed from series to series I shall call the _variable_, in contrast
with the _standard_, which was unchanged.

The tests, which were made in a dark-room under uniform conditions, were
given in series of fifty each; usually only one such series was given per
day, but sometimes one was given in the morning and another in the
afternoon of the same day. To prevent choice by position the lights were
reversed in position irregularly, first one, then the other, illuminating
the right electric-box. For the fifty tests of each initial series the
order of the changes in position was as follows: standard (brighter light)
on the _l_ (left), _l, r_ (right), _r, l, l, r, r, l, r, l, r, l, l, r, r,
l, l, r, r, l, l, l, r, r, r, l, r, l, r, r, r, l, l, l, r, r, r, l, l, r,
l, r, l, r, l, r, l, r, l_. Twenty-five times in fifty the standard light
illuminated the right electric-box, and the same number of times it
illuminated the left electric-box. When a second series was given under
the same conditions of illumination, a different order of change was
followed.

In order to discover whether Weber's law holds in the case of the
brightness vision of the dancer it was necessary for me to determine the
just perceivable difference between the standard and the variable lights
for two or more standard values. I chose to work with three values, 5, 20,
and 80 hefners, and I was able to discover with a fair degree of accuracy
how much less than 5, 20, or 80 hefners, as the case might be, the
variable light had to be in order that it should be discriminable from the
other. For the work with the 5 hefner standard I used 2-candle-power
lamps,[1] for the 20, 4-candle-power, and for the 80, 16-candle-power.

[Footnote 1: I give merely the commercial markings of the lamps. They had
been photometered carefully by two observers by means of a Lummer-Brodhun
photometer and a Hefner amyl acetate lamp previous to their use in the
experiment. For the photometric measurements in connection with the
Weber's law tests I made use of the Hefner lamp with the hope of attaining
greater accuracy than had been possible with a standard paraffine candle,
in the case of measurements which I had made in connection with the
experiments on color vision that are reported in Chapters IX and X. The
Hefner unit is the amount of light produced by an amyl acetate lamp at a
flame height of 40 mm. (See Stine's "Photometrical Measurements.") A
paraffine candle at a flame height of 50 mm. is equal to 1.2 Hefner
units.]

For reasons which will soon appear, Weber's law tests were made with only
one dancer. This individual, No. 51, had been thoroughly trained in white-
black discrimination previous to the experiments in the apparatus which is
represented in Figure 17. Having given No. 51 more than two hundred
preliminary tests in the Weber's law apparatus with the electric-boxes
sufficiently different in brightness to enable her to discriminate
readily, I began my experiments by trying to ascertain how much less the
value of the illumination of one electric-box must be in order that it
should be discriminable from a value of 20 hefners in the other electric-
box. In recording the several series of tests and their results hereafter,
I shall state in Hefner units the value of the fixed or standard light and
the value of the variable light, the difference between the two in terms
of the former, and the average number of wrong choices in per cent.

With the lamps so placed that the difference in the illumination of the
two electric-boxes was .53 of the value of the standard, that is about one
half, No. 51 made twenty wrong choices in one hundred, or 20 per cent.
When the difference was reduced to .36 (one third) the number of errors
increased to 36 per cent, and with an intermediate difference of .48 there
were 26 per cent of errors (see Table 14).

Are these results indicative of discrimination, or are the errors in
choice too numerous to justify the statement that the dancer was able to
distinguish the boxes by their difference in brightness? Evidently this
question cannot be answered satisfactorily until we have decided what the
percentage of correct choices should be in order that it be accepted as
evidence of ability to discriminate, or, to put it in terms of errors,
what percentage of wrong choices is indicative of the point of just
perceivable difference in brightness. Theoretically, there should be as
many mistakes as right choices, 50 per cent of each, when the two
electric-boxes are equally illuminated (indiscriminable), but in practice
this does not prove to be the case because the dancer tends to return to
that electric-box through which in the previous test it passed safely,
whereas it does not tend in similar fashion to reënter the box in which it
has just received an electric shock. The result is that the percentage of
right choices, especially in the case of series which have the right box
in the same position two, three, or four times in succession, rises as
high as 60 or 70, even when the visual conditions are indiscriminable.
Abundant evidence in support of this statement is presented in Chapters
VII and IX, but at this point I may further call attention to the results
of an experiment in the Weber's law apparatus which was made especially to
test the matter. The results appear under the date of May 27 in Table 14.
In this experiment, despite the fact that both boxes were illuminated by
80 hefners, the mouse chose the standard (the illumination in which it was
not shocked) 59 times in 100. In other words the percentage of error was
41 instead of 50. It is evident, therefore, that as low a percentage of
errors as 40 is not necessarily indicative of discrimination. Anything
below 40 per cent is likely, however, to be the result of ability to
distinguish the brighter from the darker box. To be on the safe side we
may agree to consider 25 wrong choices per 100 as indicative of a just
perceivable difference in illumination. Fewer mistakes we shall consider
indicative of a difference in illumination which is readily perceivable,
and more as indicative of a difference which the mouse cannot detect. The
reader will bear in mind as he examines Table 14 that 25 per cent of wrong
choices indicates the point of just perceivable difference in brightness.

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