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The Dancing Mouse

R >> Robert M. Yerkes >> The Dancing Mouse




[Illustration: FIGURE 20.--Color discrimination apparatus. _A,_ nest-box;
_B,_ entrance chamber; _R, R,_ red filters; _G,_ green filter; _L, L,_
incandescent lamps in light-box; _S,_ millimeter scale on light-box; _I,_
door between _A_ and _B; O, O,_ doors between alleys and _A_.]

[Illustration: FIGURE 21--Ground plan of color discrimination apparatus.
_E, E_, exits from electric-boxes. _LB_, light-box; _R, G, R_, filter
boxes on carrier; _L_, left electric-box; _R_, right electric-box; _IC_
induction apparatus; _C_, electric cell; _K_, key; _S_, millimeter scale.]

In the tests which are now to be reported, three portions of the spectrum
were used: the red end, the blue-violet end, and a middle region, chiefly
green. The red light was obtained by the use of a filter which was made by
placing two plates of ruby glass in one of the glass boxes, filling the
box with filtered water and then sealing it to prevent evaporation. The
blue-violet was obtained by the use of a filter box which contained a 5
per cent solution of copper ammonium sulphate. The green, which, however,
was not monochromatic, was obtained by the use of a filter box which
contained a saturated solution of nickel nitrate. These three sets of
filters were examined spectroscopically both before the experiments had
been made and after their completion.[1] The red filters, of which I had
two for shifting the lights, transmitted only red light. The blue-violet
filters, two also, at first appeared to transmit only portions of the blue
and violet of the spectrum, but my later examination revealed a trace of
green. It is important to note, however, that the red and the blue-violet
filters were mutually exclusive in the portions of the spectrum which they
transmitted. Of all the filters used the green finally proved the least
satisfactory. I detected some yellow and blue in addition to green in my
first examination, and later I discovered a trace of red. Apparently the
transmitting power of the solutions changed slightly during the course of
the experiments. On this account certain solutions are undesirable for
experiments on color vision, for one must be certain of the constancy of
the condition of stimulation. It is to be understood, of course, that each
of the three filters transmitted, so far as the eye is concerned, only the
color named. I consider the red filter perfectly satisfactory, the blue-
violet very good, and the green poor. Henceforth, in testing color vision
in animals, I shall make use of colored glasses as filters, if it is in
any way possible to obtain or have manufactured blue, green, and yellow
glasses which are as satisfactory as the ruby.

[Footnote 1: A Janssen-Hoffman spectroscope was used.]

The apparatus needs no further description, as its other important
features were identical with those of the reflected light experiment box.
The use of artificial light for the illumination of the electric-boxes
made it necessary to conduct all of the following tests in a dark-room.
The method of experimentation was practically the same as that already
described. A mouse which had been placed in _A_ by the experimenter was
permitted to enter _B_ and thence to return to _A_ by entering one of the
electric-boxes, the red or blue or green one, as the case might be.
Mistakes in choice were punished by an electric shock. One further point
in the method demands description and discussion before the results of the
tests are considered, namely, the manner of regulating and measuring the
brightness of the lights.

Regulating brightness with this apparatus was easy enough; measuring it
accurately was extremely difficult. The experimenter was able to control
the brightness of each of the two colored lights which he was using by
changing the position or the power of the incandescent lamps in the light-
box. The position of a lamp could be changed easily between tests simply
by moving it along toward or away from the electric-box in the slit which
served as a lamp carrier. As the distance from the entrances of the
electric-boxes to the further end of the light-box was 120 cm., a
considerable range or variation in brightness was possible without change
of lamps. Ordinarily it was not necessary to change the power of the
lamps, by replacing one of a given candle power by a higher or lower,
during a series of tests. Both the candle power of the lamps and their
distance from the filters were recorded in the case of each test, but for
the convenience of the reader I have reduced these measurements to candle
meters[1] and report them thus in the descriptions of the experiments.

[Footnote 1: The illuminating power of a standard candle at a distance of
one meter.]

But measuring the actual brightness of the red light or the green light
which was used for a particular series of tests, and the variations in
their brightnesses, was not so simple a matter as might appear from the
statements which have just been made. The influence of the light filters
themselves upon the brightness must be taken into account. The two red
filters were alike in their influence upon the light which entered them,
for they were precisely alike in construction, and the same was true of
the two blue-violet filters. The same kind of ruby glass was placed in
each of the former, and a portion of the same solution of copper ammonium
sulphate was put into each of the filter boxes for the latter. But it is
difficult to say what relation the diminution in brightness caused by a
red filter bore to that caused by a blue-violet or a green filter. My only
means of comparison was my eye, and as subjective measurement was
unsatisfactory for the purposes of the experiment, no attempt was made to
equalize the amounts of brightness reduction caused by the several
filters. So far as the value of the tests themselves, as indications of
the condition of color vision in the dancer is concerned, I have no
apology for this lack of measurement, but I do regret my inability to give
that accurate objective statement of brightness values which would enable
another experimenter with ease and certainty to repeat my tests. The
nearest approach that it is possible for me to make to such an objective
measurement is a statement of the composition and thickness of the filters
and of the candle-meter value of the light when it entered the filter. The
distance from this point to the entrance to the electric-box was 20 cm.

To sum up and state clearly the method of defining the brightness of the
light in the following experiments: the candle-meter value of each light
by which an electric-box was illuminated, as determined by the use of a
Lummer-Brodhun photometer and measurements of the distance of the source
of light from the filter, is given in connection with each of the
experiments. This brightness value less the diminution caused by the
passage of the light through a filter, which has been defined as to
composition and thickness of the layer of solution, gives that degree of
brightness by which the electric-box was illuminated.

Tests of the dancer's ability to discriminate green and blue[1] in the
transmitted light apparatus were made with four animals. An incandescent
lamp marked 16-candle-power was set in each of the light-boxes. These
lamps were then so placed that the green and the blue seemed to be of
equal brightness to three persons who were asked to compare them
carefully. Their candle-meter values in the positions selected were
respectively 18 and 64, as appears from the statement of conditions at the
top of Table 22.

[Footnote 1: Hereafter the light transmitted by the blue-violet filter
will be referred to for convenience as blue.]



TABLE 22

GREEN-BLUE TESTS

Brightnesses Equal for Human Eye

Green 18 candle meters Blue 64 candle meters

SERIES DATE NO. 10 NO. 11
1906 RIGHT WRONG RIGHT WRONG
(GREEN) (BLUE) (GREEN) (BLUE)
A and B[1] April 2 10 10 12 8
1 3 6 4 5 5
2 4 5 5 6 4
3 5 5 5 5 5
4 6 5 5 5 5
5 7 7 3 5 5
6 8 7 3 3 7
7 9 7 3 5 5
8 10 3 7 7 3
9 11 5 5 4 6
10 12 5 5 6 4
[Footnote: A single preference series of twenty tests.]



Numbers 10 and 11 exhibited no preference for either of these colors in
the series of 20 tests which preceded the training tests, and neither of
them gave evidence of ability to discriminate as the result of ten series
of training tests. In this case, again, the behavior of the animals was as
strongly against the inference that they can tell green from blue as are
the records of choices which appear in the table. Granted, that they are
unable to discriminate green from blue when these colors are of about the
same brightness for the human eye, what results when they differ markedly
in brightness? Table 23 furnishes a definite answer to this question.
Numbers 5 and 12 were given eight series of green-blue tests with each
light at 18 candle meters. Little, if any, evidence of discrimination
appeared. Then, on the supposition that the difference was not great
enough for easy discrimination, the blue light was reduced almost to 0,
the green being left at 18. The tests (series 9) immediately indicated
discrimination. For series 10 the green was made 64 candle meters, the
blue 18, and again there was discrimination. These results were so
conclusively indicative of the lack of color vision and the presence of
brightness vision, that there appeared to be no need of continuing the
experiment further.

Accepting provisionally the conclusion that the dancers cannot tell green
from blue except by brightness differences, we may proceed to inquire
whether they can discriminate other colors. Are green and red
distinguishable?

Green-red discrimination now was tested by a method which it was hoped
might from the first prevent dependence upon brightness. The light in the
light-box on the left was so placed that it had a value of 18 candle
meters, that in the light-box on the right so that it had a value of 1800
candle meters. Neither light was moved during the first four series of the
green-red tests which were given to Nos. 151 and 152.



TABLE 23

GREEN-BLUE TESTS

Brightnesses Different for Human Eye

Green 18 candle meters Blue 18 candle meters


No. 5 No. 12
DATE
SERIES 1906 RIGHT WRONG RIGHT WRONG
(GREEN) (BLUE) (GREEN) (BLUE)

1 April 10 6 4 5 5
2 11 5 5 7 3
3 12 6 4 7 3
4 13 4 6 7 3
5 14 7 3 5 5
6 15 4 6 6 4
7 16 6 4 8 2
8 17 5 5 4 6



As it was now evident that the intensity difference was not sufficient to
render discrimination easy, the blue was reduced to 0 and the green left
at 18.


9 17 7 3 8 2


Now the brightnesses were made, green 64, blue 18, just the reverse of
those of series of Table 22.


10 17 8 2 8 2



Each of these series consisted of 20 tests instead of 10. As a result of
the arrangement of the lights just mentioned, the green appeared to me
very much brighter than the red when it was on the right and very much
darker when it was on the left. If this were true for the mouse also, it
is difficult to see how it could successfully depend upon brightness for
guidance in its choices. Such dependence would cause it to choose now the
green, now the red.

The first four series of green-red tests so clearly demonstrated
discrimination, of some sort, that it was at once necessary to alter the
conditions of the experiment. The only criticism of the above method of
excluding brightness discrimination, of which I could think, was that the
red at no time had been brighter than the green. In other words, that
despite a value of 1800 candle meters for the red and only 18 candle
meters for the green, the latter still appeared the brighter to the mouse.
To meet this objection, I made the extreme brightness values 1 and 1800
candle meters in some of the later series, of which the results appear in
Table 24. From day to day different degrees of brightness were used, as is
indicated in the second column of the table. Instead of having first one
color and then the other the brighter, after the fourth series I changed
the position of the lights each time the position of the filters was
changed; hence, the table states a certain brightness value for each color
instead of for each electric-box.

Series 5 to 14 so clearly indicated discrimination, that it seemed
necessary to devise some other means than that of changing the
brightnesses of the colored lights themselves to test the assumption that
the animals were choosing the brighter light. I therefore removed the
light filters so that the colors which had been present as conditions of
discrimination were lacking, and arranged the apparatus so that first one
box, then the other, was illuminated the more brightly. The purpose of
this was to discover whether as the result of their green-red training the
mice had acquired the habit of choosing uniformly either the lighter or
the darker box. One series was given under the conditions of illumination
specified in Table 24 with the result that the brighter box was chosen
eight times in ten by No. 151 and every time by No. 152. Since neither of
these individuals had previously been trained by white-black tests to go
to the white, and since, furthermore, the dancers usually manifest a
slight preference for the lower instead of the higher illumination, this
result may be interpreted as indicative of dependence upon brightness in
the previous color tests. It looks very much indeed as if the green had
been chosen, not because of its greenness, but on account of its
relatively greater brightness.

This test of brightness preference was followed by two series, 16 and 17,
under conditions similar to those of the first four series of the table.
For series 16 the value of the light in the left box was 1 candle meter,
that of the light in the right box 1800 candle meters. Discrimination was
perfect. For series 17 the value for the left remained at 1 candle meter,
but that of the right box was decreased to 0. In this series No. 152 was
entirely at a loss to know which box to choose. Of course this was an
entirely new set of conditions for choice, namely, a colored box, the
green or the red as the case might be, beside a dark box, the one which
was not illuminated. If the mice really had been choosing correctly
because of a habit of avoiding the red or of seeking the green, this
method should bring out the fact, for the red box, since with it the
disagreeable electric shock had always been associated, should be a box to
be avoided. For No. 151 this seemed to be the case.

Series 23 to 27 of Table 24 were given as final and crucial tests of the
relation of brightness discrimination to color discrimination. As it is
not possible to express in a simple formula the conditions of the tests, a
sample series which indicates the brightness of the colors in each of the
twenty tests of a series, and in addition the results given by No. 151 in
the first of these final series, is reproduced in Table 25. For an animal
which had presumably learned perfectly to choose green in preference to
red, the record of 8 mistakes in 20 choices as a result of changes in
relative brightness is rather bad, and it renders doubtful the existence
of color discrimination in any of these experiments. No. 152 showed no
ability whatever to choose the green in the first of the series (series 23
of Table 24) of which that of Table 25 is a sample. His record, 10
mistakes in 20 choices, was even poorer than that of No. 151. That both of
these mice learned to choose fairly accurately in these final tests is
shown by the results of series 24, 25, 26, and 27. I must admit, however,
that these records indicate little ability on the part of the animals to
discriminate colors.



TABLE 24

GREEN-RED TESTS

Brightnesses Extremely Different for Human Eye
Intensities are given in candle meters (c.m.)


NO. 151 NO. 152
SERIES DATE CONDITIONS
RIGHT WRONG RIGHT WRONG
(GREEN) (RED) (GREEN) (RED)

1 April 26 18 c.m. on left
1800 c.m. on right 11 9 7 13
2 27 Same 16 4 16 4
3 28 Same 20 0 17 3
4 29 Same 19 1 19 1
5 30 Green 18 c.m.
Red 18 c.m. 9 1 10 0
6 30 Green 64 c.m.
Red 18 c.m. 9 1 8 2
7 May 1 Green 6 c.m.
Red 1500 c.m. 7 3 9 1
8 1 Green 4 c.m.
Red 1500 c.m. 8 2 7 3
9 2 Both varied from
4 to 1500 c.m. 18 2 18 2
10 3 Green 2 c.m.
Red 1800 c.m. 6 4 7 3
11 3 Same 10 0 10 0
12 4 Same 7 3 8 2
13 4 Same 8 2 6 4
14 5 Green 1 c.m.
Red 1800 c.m. 19 1 19 1



Filters were now removed. An illumination of 15 c.m. was established on
one side and an illumination of 0 on the other side, in order to ascertain
whether the mice would choose the brighter box. This was done to test the
assumption that the green in the previous tests had always appeared
brighter to the mice than did the red, and that in consequence they had
chosen the brighter box instead of the green box.


TABLE 24--CONTINUED


No. 151 No. 152

SERIES DATE CONDITIONS RIGHT WRONG RIGHT WRONG
(GREEN) (RED) (GREEN) (RED)

15 May 5 Brighter 15 c.m. 8[1] 2[2] 10[1] 0[2]
Darker 0 c.m.
16 5 1 c.m. on left
1800 c.m. on right 10 0 10 0
17 5 1 c.m. on left
0 c.m. on right 9 1 4 6
18 5 Green 18 c.m.
Red 18 c.m. 19 1 17 3
19 9 Same 9 1 9 1
20 9 Same 10 0 10 0
21 10 Same 10 0 10 0
22 11 Same 10 0 10 0
23 June 1 Both varied from
1 to 1800 c.m. 12 8 10 10
24 2 Same 18 2 14 6
25 June 3 Both varied from
2 to 1800 c.m. 19 1 17 3
26 4 Same 17 3 17 3
27 5 Same 18 2 18 2

[Footnote 1: Brighter.]
[Footnote 2: Darker.]



These long-continued and varied tests with Nos. 151 and 152 revealed three
facts: that the mice depend chiefly upon brightness differences in visual
discrimination; that they probably have something which corresponds to our
red-green vision, although their color experience may be totally unlike
ours; and that the red end of the spectrum seems much darker to them than
to us, or, in other words, that the least refrangible rays are of lower
stimulating value for them than for us.



TABLE 25

GREEN-RED TESTS

June 1, 1906 No. 151

BRIGHTNESS VALUE IN CANDLE RIGHT WRONG
TEST POSITION METERS (GREEN) (RED)
1 Green on left Green 4, Red 448 Right --
2 Green on right Green 448, Red 4 Right --
3 Green on right Green 4, Red 448 Right --
4 Green on left Green 448, Red 4 Right --
5 Green on left Green 3, Red 1800 -- Wrong
6 Green on right Green 1800, Red 3 -- Wrong
7 Green on right Green 3, Red 1800 -- Wrong
8 Green on left Green 1800, Red 3 Right --
9 Green on right Green 5, Red 34 Right --
10 Green on left Green 34, Red 5 Right --
11 Green on right Green 6, Red 74 Right --
12 Green on left Green 74, Red 6 Right --
13 Green on left Green 4, Red 448 -- Wrong
14 Green on right Green 448, Red 4 Right --
15 Green on right Green 4, Red 448 -- Wrong
16 Green on left Green 448, Red 4 Right --
17 Green on right Green 3, Red 1800 -- Wrong
18 Green on left Green 1800, Red 3 -- Wrong
19 Green on right Green 1800, Red 3 -- Wrong
20 Green on left Green 3, Red 1800 Right --

Totals 12 8



So many of the results of my color experiments have indicated the all-
important role of brightness vision that I have hesitated to interpret any
of them as indicative of true color discrimination. But after I had made
all the variations in brightness by which it seemed reasonable to suppose
that the mouse would be influenced under ordinary conditions, and after I
had introduced all the check tests which seemed worth while, there still
remained so large a proportion of correct choices that I was forced to
admit the influence of the quality as well as of the intensity of the
visual stimulus.

The first of the facts mentioned above, that brightness discrimination is
more important in the life of the mouse than color discrimination, is
attested by almost all of the experiments whose results have been
reported. The second fact, namely, that the dancer possesses something
which for the present we may call red-green vision, also has been proved
in a fairly satisfactory manner by both the reflected and the transmitted
light experiments. I wish now to present, in Table 26, results which
strikingly prove the truth of the statement that red appears darker to the
dancer than to us.

The brightness conditions which appeared to make the discrimination
between green and red most difficult were, so far as my experiments permit
the measurement thereof, green from 1 to 4 candle meters with red from
1200 to 1600. Under these conditions the red appeared extremely bright,
the green very dark, to the human subject.

According to the description of conditions in Table 26, Nos. 2 and 5 were
required to distinguish green from red with the former about 3 candle
meters in brightness and the latter about 1800 candle meters. In the
eighth series of 20 tests, each of these animals made a perfect record. As
it seemed possible that they had learned to go to the darker of the two
boxes instead of to the green box, I arranged the following check test.
The filters were removed, the illumination of one electric-box was made 74
candle meters, that of the other 3, and the changes of the lighter box
from left to right were made at irregular intervals. In February, No. 2
had been trained to go to the black in black-white tests, and at the same
time No. 5 had been trained to go to the white in white-black tests. The
results of these brightness check tests, as they appear in the table,
series 8 _a_, are indeed striking. Number 2 chose the darker box each
time; No. 5 chose it eight times out of ten. Were it not for the fact that
memory tests four weeks after his black-white training had proved that No.
2 had entirely lost the influence of his previous experience (he chose
white nine times out of ten in the memory series), it might reasonably be
urged that this individual chose the darker box because of his experience
in the black-white experiment. And what can be said in explanation of the
choices of No. 5? I can think of no more reasonable way of accounting for
this most unexpected result of the brightness tests than the assumption
that both of these animals had learned to discriminate by brightness
difference instead of by color.

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