the order is different with the different subjects, yet the general
tendency is to place the most complex figures first and the simplest
last. The most simple figure _Y_ never comes in front of the fifth
place except with subject C, who places it first. This exception may be
ascribed to the fact that this subject, on account of his going away,
did not have so many tests. In fact only one day's work of 10 reactions
for each figure is recorded, and it is but natural that some variations
from the standard should occur in his case.
If now, as before, we investigate where each figure occurs in the
series for the different subjects we get the following table:
_Times in_
1_st place_ 2_d place_ 3_d place_ 4_th place_ 5_th place_ 6_th place_
Z 3 3 1 0 0 0
U 3 1 1 2 0 0
V 0 0 4 1 2 0
X 0 2 0 2 1 2
W 0 1 1 2 3 0
Y 1 0 0 0 1 5
Here we again see the large numbers on a line from the upper left-hand
to the lower right-hand corner.
Thus we get the following order from the geometrical figures as
measured by the height and width of the curves:
_Height_ _Z_ _U_ _X_ _V_ _W_ _Y_
_Width_ _Z_ _U_ _V_ _X_ _W_ _Y_
The only difference, it is seen, is that the positions of _V_ and _X_
are reversed in the two series. Such a change would on our principle
be fairly likely to occur, since _V_ and _X_ are figures near to each
other in complexity and the motor effects are very similar.
[Illustration: FIG. 3]
In the same manner, the following tables show the reactions to the
colored figures of different grades of complexity. And first, as
before, is the table of the heights of the curves for the different
subjects, given in millimetres. The numbers given represent the
averages of all reactions made. We will call the figures, for the sake
of reference, _L_, _M_, _N_, _O_, _P_.
_L_ _M_ _N_ _O_ _P_
_Subject_ A 5.75 6.01 5.90 5.82 5.74
B 6.72 5.56 6.35 7.53 4.94
C 10.92 10.90 10.76 10.52 10.99
D 25.49 25.42 26.23 25.89 25.52
E 20.63 20.82 20.37 20.55 20.30
F 15.67 15.23 15.15 15.98 14.51
85.18 83.94 85.26 86.29 82.00
_Average_ 14.20 13.99 14.21 14.38 13.67
Order arranged as before in a descending series according to height of
curve:
_O_ _N_ _L_ _M_ _P_
14.38 14.21 14.20 13.99 13.67
This is exactly, as I should judge, the order of the complexity of the
figures reacted to.
The arrangement by the individual subjects is as follows:
_Subject_ A _M_ _N_ _O_ _L_ _P_
B _O_ _N_ _L_ _M_ _P_
C _P_ _L_ _M_ _N_ _O_
D _N_ _O_ _P_ _L_ _M_
E _M_ _L_ _M_ _N_ _P_
F _O_ _L_ _M_ _N_ _P_
We see that individual differences are stronger here than in the
geometrical figures, but that the same tendency to react more strongly
to the complex is present in nearly every case. This can be brought
to the eye more clearly if we observe the table in which is shown
the position of the different figures in the series of the different
subjects.
_Times in_
1_st place_ 2_d place_ 3_d place_ 4_th place_ 5_th place_
O 2 1 2 0 1
N 1 2 0 3 0
L 0 3 1 2 0
M 2 0 2 1 1
P 1 0 1 0 4
_M_ here presents the principal exception, coming too often in the
first place.
Finally we give the tables for the widths of the curves for the colored
figures; and first the table of the averages of all the subjects for
all the figures:
_L_ _M_ _N_ _O_ _P_
_Subject_ A 25.76 24.27 25.06 24.77 23.14
B 9.42 9.49 9.06 9.84 8.11
C 5.85 5.35 5.62 5.80 5.24
D 13.68 13.53 13.18 13.26 13.38
E 22.06 21.37 22.50 22.17 20.44
F 16.30 15.08 16.65 16.76 15.13
93.07 89.09 92.07 92.60 85.44
_Average_ 15.51 14.85 15.35 15.43 14.24
Order, arranged in a descending series according to width of curve:
_L_ _O_ _N_ _M_ _P_
15.51 15.43 15.35 14.85 14.24
Here the order is not just the same as we got from a measurement of the
heights. The three complex figures have changed places somewhat, but
there is no exchange of a simple and a complex.
The arrangements by the individual subjects are as follows:
_Subject_ A _L_ _N_ _O_ _M_ _P_
B _O_ _M_ _L_ _N_ _P_
C _L_ _O_ _N_ _M_ _P_
D _L_ _M_ _P_ _O_ _N_
E _N_ _O_ _L_ _M_ _P_
F _O_ _N_ _L_ _P_ _M_
The three complex figures have different places with different
subjects, but very seldom is a simple figure found among the complex,
or _vice versa_.
This can be seen easily from the following table:
_Times in_
1_st place_ 2_d place_ 3_d place_ 4_th place_ 5_th place_
_L_ 3 0 3 0 0
_O_ 2 2 1 1 0
_N_ 1 2 1 1 1
_M_ 0 2 0 3 1
_P_ 0 0 1 1 4
A theoretical word may close our report.
The growth of biology and physiology has tended to show that there
is no break in the nervous mechanism. The stimulus goes to the brain
and out through motor channels to muscles, glands, etc. The nervous
current does not wait in the brain for the permission of the mind to
leave on its journey to a muscle nor does it need mental reënforcement.
The nervous current as a whole is a unity. The nervous system is a
physiological instrument for producing the appropriate reaction to
a certain stimulus. In the unicellular organism there is no nervous
system, but the protoplasm receives the stimulus and produces the
reaction. As we go up in the animal series a differentiation is seen
to be present in the organism. Some parts are more concerned with the
receiving of stimuli and others with the approach toward or withdrawal
from the stimulating object. There is a division of labor. The nervous
system is developed as a means of rapid communication between the
different parts, but this communication is a physiological one. The
stimulus sets up a chemical action in the sensory organ which is
transmitted along the nervous path to the motor organ which is caused
to react. As we ascend the animal series the differentiation becomes
greater and greater, and consequently the means of communication must
become more and more complex. So trunk lines are formed which lead to a
centre, and from this centre again go out main lines which divide and
subdivide until the muscles are reached. The centre acts as a kind of
automatic switch-board.
Accepting such a view of the nervous system it must be granted that
different stimulations would produce different reactions. It was my
aim in the experimental work which has been described to show that
this is true. And while much work has already been done in showing
that different kinds, or different amounts of stimulation produce
differences in reactions, it seemed important to demonstrate also
that mere differences in the complexity of the stimulus bring about
differences in the reaction. So the experiment of counting figures of
different complexity was entered upon, and we found that it took longer
to count figures the more complex they were in spite of the fact that
the act of counting seems always the same. The question is how must the
fact that counting becomes slower and slower, as the figures become
more complex, be interpreted?
When we count a row of figures, the eyes do not move along at a regular
uniform rate, but make a quick jump from one figure to the next, halt
a moment, make another jump, and so on. Now, I think the principal
difference comes in with the figures of different complexity in the
time the eye halts at each figure. The halt is longer the more complex
the figure is. It is well known that any visual object which stimulates
the retina is brought by a reflex movement of the eye to the place of
clearest vision. Of two objects stimulating the eye at the same time,
the more pronounced one will produce the reflex and will hold the eye
longer than a weaker stimulus. Similarly here, the more complex figure
produces a stronger reflex and holds the eye longer than the simple
figure. This is repeated at every figure in the series.
The complex figures have more features about them, all of which by way
of the retina and optic nerve are represented in the cortex and thus
more cortical cells are involved, which in turn produce a stronger
stimulation of the muscles which move the eye in the proper way to see
the figure, and thus the eye is held more strongly by the complex than
by the simple figure.
Again in the second experiment, the subject reacts more strongly to
the complex as shown already in explaining the first experiment and
for the same reasons. It might be said that in looking at the colored
figures, _e. g._, that since the same amount of retina is stimulated,
the reaction ought to be the same. But we may presume that the complex
figure, on account of the different shapes and contrasts on its
surface, will more variously affect the same amount of retina and that
the nervous currents sent to the cortex will, many of them, be stronger
than those from the simple figure and will thus cause the cortical
cells to be more strongly excited, or by a process of irradiation the
stimulation will spread to adjoining cells and thus finally more cells
be stimulated. However this may be, the amount of discharge into motor
cells is certainly greater and the muscular reaction, therefore, also
greater.
The interesting side of our results is thus given in the fact that
we have here two activities--counting with highest speed and making
hand-movements of certain length--which are performed every time with
exactly the same intention and with the subjective impression of
equal result, and which yet show marked differences according to the
complexity of the psycho-physical stimuli. It is a new contribution to
our knowledge of the independent motor power of ideas.
ANIMAL PSYCHOLOGY
THE MUTUAL RELATIONS OF STIMULI IN THE FROG RANA CLAMATA DAUDIN[139]
BY ROBERT M. YERKES
I. ANIMAL BEHAVIOR AND THE SENSES
Since the behavior of an animal is conditioned by its senses, it is
extremely important that the comparative psychologist should have
accurate and detailed knowledge of the sense-impressions received by
his subjects. Knowledge of the so-called "special senses" does not
suffice for the satisfactory description of behavior, for there are
several other kinds of sense-data of equal or even greater importance
than those of the five special senses. As investigation of the subject
progresses the banefulness of the notion that all sense-experience is
summed up in "the five special senses" becomes more and more evident.
For the comparative psychologist the senses are not five, six,
eight, or ten, but as numerous as are the kinds of sense-data which
condition animal activities. There can be no doubt that many of the
lower animals are largely dependent upon senses which are not included
in the conventional special sense list. The contention that certain
organs which are commonly recognized as sensory in function, the cristæ
acusticæ of the ear, for instance, are merely reflex control organs,
has little weight in this connection, for every sense-organ is part
of a motor control mechanism, and so far as we are able to judge from
available evidence each has as an accompaniment of its functioning
a mode of sensation. If there are two kinds of peripheral organs in
connection with the afferent nerves, namely, those whose functioning
has sensation for an accompaniment and those in which motor control
is the sole phenomenon, it is high time that the fact were definitely
known.
Even a thoroughly accurate knowledge of the general condition of
the senses in a particular phylum, genus, or species may be of
trifling value in the study of the behavior of a given individual,
for within these groups the state of development and relative
importance of a sense may differ strikingly. Herrick,[140] in his
admirable investigation of the sense of taste in fishes, has rendered
comparative psychology an important service by showing that even a
highly developed sense may be of markedly different value in the
associative life of different species. The cat-fish, according to
Professor Herrick's observations, obtains its food primarily by the aid
of taste-impressions, the hake by the aid of touch, and the sea-robin
chiefly by means of vision. All three of the senses mentioned are
possessed by each of the fishes, yet their values differ so widely
that an understanding of the habits and associative processes of any
one of the species would be impossible except in the light of just
such facts as Herrick has discovered. Clearly, then, we must know the
relative importance of the various sense-impressions received by an
animal before we can discuss its behavior or psychic characteristics
intelligently.
Furthermore, if behavior is to be serviceably described in terms of
stimuli and physiological conditions it is necessary first of all
to recognize that an animal responds to a situation, not to any one
independent and isolated stimulus. Every situation, to be sure, may be
analyzed into its component simple stimuli, but the influence of each
and all of these stimuli is conditioned by the situation. Too often in
our accounts of an animal's behavior we name some one stimulus as the
condition of the reaction and entirely neglect the situation, without
which the stimulus would have been of quite different value to the
animal. For any given stimulus other external and internal stimuli
constitute an environment. The complete description of a reaction
demands knowledge of all the stimuli which enter into the situation and
of their mutual relations of interference or supplementation. A frog
which in its native habitat and undisturbed by an unusual situation
would react violently to the light touch of a stick may give no sign
of reaction to the same stimulus when a human being stands nearby.
The influence of the tactual stimulus has been changed entirely by
the simultaneous appearance of visual, olfactory, and possibly still
other sense-data (man). The animal reacts not to the touch alone, but
to this stimulus as part of a certain situation. The general effect
of a situation we often speak of as excitement, timidity, etc. These
are words for which must be substituted in our accounts of animal
behavior accurate descriptions of the situations. Experimental studies
prove that an animal must become thoroughly accustomed to the general
situation in which it is to be observed before the influence of any
particular condition can be studied to advantage.
Only a few of the important reactions of an animal to either external
or internal stimuli are visible to the casual observer, and many of
them can be detected only by the employment of indirect methods.
Frequently the lack of a visible motor response to a new situation is
good evidence of a fundamentally important reaction. The death-feigning
opossum, crustacean, or insect, truly reacts by becoming motionless. As
Whitman[141] has shown in the case of the leech it is as hazardous to
judge of the degree of sensitiveness of another animal solely on the
basis of our own as it is to maintain that lower animals possess only
the senses which are ours. Varied, indirect and delicate methods are
necessary in the investigation of the senses, as the results of the
experiments to be described below help to prove.
It is my purpose in this paper to call attention to and emphasize
the importance of studying stimuli in their mutual relations of
interference and supplementation. This I shall do by presenting the
results of an investigation of the behavior of the green frog. I shall
discuss briefly, first, the sense-data received by the animal, their
relative importance, significance, and mutual relations, and, next, the
phenomena of reënforcement and inhibition.
II. THE SENSORY REACTIONS OF THE GREEN FROG
The following sensory reactions have been observed in the frog, but
most of them have not been studied with care: olfactory, temperature,
visual, tactual, equilibrational, and auditory. It is my purpose to
investigate each of these senses in such fashion that we shall know the
receptive capacity of the animal. Thus far I have completed only the
work on auditory reactions, but the chemical and temperature senses and
vision will be discussed in similar fashion later.
At present there is little known concerning the chemical senses.
Unpublished observations made by Mr. Sherwin in this laboratory
indicate the existence of olfactory sensitiveness to camphor, iodine,
and several other strong stimuli. The reactions to the stimuli were
slow, however, and there is no reason to believe that the sense of
smell is of great importance to the animal. Of taste barely more is
known than that it is present.
There is marked sensitiveness to variations in temperature, as I
have demonstrated by preliminary test experiments, but the limits,
distribution, and significance of this sensitiveness remain to be
investigated. I am not aware that the existence of temperature spots
has been determined. In connection with a study of the reactions of
frogs to light Torelle[142] discovered that the animals suddenly become
inactive and usually attempt to bury themselves when brought into a
temperature of 8° to 10° C. This reaction is prompt and definite; its
value to an animal which hibernates is evident, yet one would scarcely
anticipate the suddenness and regularity with which it occurs.
My studies of habit-formation and reaction-time[143] have revealed
the importance of vision in the life of the frog. Perception of
movement appears to be of far greater value to the animal than
perception of form or color. The spectral colors are discriminated in
all probability, for the animals react very differently to those of
the blue end than to those of the red. According to Torelle blue is
preferred to red. There is evidence that red has a higher stimulating
value than blue, and the apparent avoidance of red in Torelle's
experiments may be due to this fact. None of the work with which I am
familiar demonstrates that the suspected color-reactions are due to
stimulation of the eye. They may be due to stimulation of the skin, for
Parker[144] has shown that the reactions of _Rana pipiens_ to light are
due to stimulation of the sink as well as of the eyes, or they may even
be due to intensity instead of color.
The tactual-auditory sense series is better known and also, it would
appear, better developed than the chemical series. A large portion of
the body surface of the green frog is keenly sensitive to mechanical
stimulation, and Steinach[145] by measurement of electrical changes in
the nerves of the skin has discovered the existence of "touch spots."
His method, which is ingenious, promises to be of considerable value in
the objective investigation of the senses, but it involves operations
on the subject which inevitably destroy the normal condition of the
sense.
According to Steinach we have in the negative variation in the
electrical condition of nerves during stimulation a phenomenon which
may be used in the determination of the threshold of stimulation as
well as in the investigation of irritability. In a previous paper[146]
I have discussed the associational rôle of tactual impressions as well
as the tactual reaction-time. All my observations lead me to believe
that touch is a highly developed and important sense in the green frog.
Of the senses intermediate between touch and hearing that of
equilibration has been most discussed. Certainly there is good reason
to suppose that the sense-organs of the semicircular canals of the ear
furnish the animal with impressions of position, movement, and possibly
also of direction. Further study of the tactual-auditory senses of
frogs may indicate the existence of conditions similar to those
discovered by Parker in certain fishes, in which, as he remarks, "the
skin, lateral line organs and ears represent, figuratively speaking,
three generations of sense-organs. The oldest is the skin stimulated
by varying pressures, such as are produced by irregular currents, and
capable of initiating equilibrational responses. From the skin have
been derived the lateral line organs stimulated by water vibrations of
low rate, and also significant for equilibration. Finally, from the
lateral line organs have come the ears stimulated by water vibrations
of a high rate and important for equilibration. The ear, unlike the
skin and lateral line organs, is differentiated for its two functions,
the sacculus for hearing, the utriculus for equilibration."[147]
The sense of hearing remains to be considered. My attention was first
drawn to this subject by failure to obtain motor reactions to sounds in
the investigation of the time-relations of the neural processes of the
green frog. Although a large number of sounds of different qualities,
pitches, and intensities were employed, no visible motor reactions
were observed. This led me to seek the significance of what appeared
to be either a surprising lack of sensitiveness to changes in the
environment which would naturally be expected to stimulate the animal,
or an interesting and important case of the inhibition of reaction to
auditory stimuli. This suggested the question, Are frogs deaf, or do
they under certain conditions completely inhibit their usual reactions
to sound?
In the literature on the senses and reactions of frogs I have found
nothing which contributes importantly to our knowledge of the sense
of hearing. Most of the investigations which deal with the ear are
concerned with the equilibrational and orientational functions of the
labyrinth organs, and have nothing whatever to say about hearing.
In the natural histories the existence of a well-developed sense of
hearing is usually assumed, and numerous instances of what are supposed
to be reactions to sound are cited. It is to be noted, however, that
none of the observations in these popular works furnishes satisfactory
proof of the exclusion of the influence of visual stimuli. Among the
few references to frog audition of which I have knowledge, the only
one which seems worthy of special notice is that of Gaupp in his
Anatomie des Frosches. Since his few paragraphs sum up the state of
our knowledge on the subject, while at the same time furnishing an
illustration of the assumption of hearing on the basis of analogy,
I present the substance of them in free and slightly abbreviated
translation.
"The labyrinth organ has an acoustic and non-acoustic (static)
function. For these two functions, according to the leading if not
generally accepted view, entirely different portions of the organ are
in question, and since the non-acoustic is attributed to the three
_Cristae acusticae ampullarum_ and the three _Maculae_ (_M. recessus
utriculi_, _M. sacculi_, _M. lagenae_), there remain for the acoustic
function only the _Papilla basilaris_ and the _Macula neglecta_. It is
not certain, however, that the non-acoustic organs do not participate
in the acoustic function.
"With regard to the acoustic sense of the frog nothing exact is known.
That it exists, and that in good development, is certain. The existence
of the drum and columella, and the fact that frogs have a voice are
unmistakeable proofs of hearing. The participation of the _Papilla
basilaris_ in acoustic functions is rendered certain by comparative
anatomical studies: the _Papilla basilaris_ is the nerve end-organ from
which, in the mammalia, the undoubtedly acoustic organ of Corti arises.
From analogy of structure we may also infer an acoustic function in
the _Macula neglecta:_ on this, as on the _Papilla basilaris_, there
is a simple tectorial membrane, and further the _Pars neglecta_, like
the _Pars basilaris_, has a strong thick wall which only in a limited
region, namely, where it approaches a part of the perilymphatic space,
is markedly thinner." (For fish Breuer (1891) has already stated that
if they really hear--which is proved--the _Macula neglecta_ alone can
come into consideration in connection with the function, for there is
no _Papilla basilaris_ in fishes, and the six other nerve end-organs
apparently serve the non-acoustic function.)[148]
As the green frog does not respond visibly to sounds under experimental
conditions, I found it necessary to employ indirect methods in the
study of audition. By observing the influence of sounds on respiration
and on the reactions to certain electrical, tactual, and visual
stimuli, I obtained results which, since they have already been
described in detail elsewhere,[149] may be summarized here as follows:
1. Observation of frogs in their natural habitat shows that they are
stimulated by sounds, but the sense of hearing apparently serves rather
as a warning sense which modifies reactions to other simultaneous
or succeeding stimuli than as a control for definite auditory motor
reactions.
2. Experimental tests prove that sounds modify the frog's reactions
to visual and tactual stimuli. When the sound accompanies the visual
or tactual stimulus it serves to reënforce the reaction to the other
stimulus, but when given alone it never causes a motor reaction.
3. The green frog responds to sounds made in the air, whether the
tympana be in the air or in water. There is some evidence that
the influence of auditory stimuli is most marked when the drum is
half-submerged in water. The influence of sounds upon tactual reactions
is evident when the frog is submerged in water to a depth of 4 cm.
4. Sounds varying in pitch from those of 50 to 10,000 vibrations per
second affect the frog. The most striking results were obtained by
the use of an electric bell with a metal gong. With this sound in
connection with a weak tactual stimulus a maximum reaction may often be
obtained even when either stimulus alone causes no perceivable reaction.
5. Sounds modify the reactions of the frog after tympana and columellæ
are removed. Cutting of the eighth cranial nerves causes disappearance
of the influence of sound. It is clear, then, that the reactions to
sounds are really auditory reactions and that the sense of hearing in
the frog is fairly well developed, although there is little evidence of
such a sense in the motor reactions of the animal.
6. Experiments during the spring months show marked influence of sounds
for both males and females, whereas experiments made during the winter
indicate a much diminished sensitiveness to auditory stimuli in both
sexes, but especially in the male.
III. THE MUTUAL RELATIONS OF STIMULI
In studying the various influences of complication of stimuli in the
frog, I have used two methods: the measurement of reaction-time and of
the amount of reaction. The reaction-time results will be presented
first.
Reaction-time to electric stimulation of the skin was studied with
special attention to the influence of other stimuli which were given
in definite temporal relation to the electric stimulus. A Hipp
chronoscope, controlled by a Cattell's falling screen, served as a
time-measuring apparatus. The other essentials of the apparatus were
a reaction-box, and devices for giving the stimuli and indicating the
reaction. On the bottom of the reaction-box a series of wires were so
placed that an electric stimulus could be given to the frog resting
upon them by the closing of a key in the hands of the experimenter.
In preparation for each experiment the frog was placed upon these
open circuit wires in such a position that the weight of its body
pressed upon a delicate spring in the floor of the box, thus causing
the chronoscope circuit to be completed. The forward jump of the frog
in response to stimulation caused the breaking of this circuit by the
release of the spring upon which the animal rested. When all was in
readiness for an experiment the chronoscope was started, and a key
closed which simultaneously gave an electric stimulus to the frog and
completed a circuit which caused the chronoscope record to begin. The
stimulus consisted of a current from one or more "Mesco" dry cells.
The motor reaction of the frog broke the chronoscope circuit, thus
causing the chronoscope record to stop. It was then possible for the
experimenter to read from the dials of the chronoscope the time, in
thousandths of seconds, intervening between stimulus and reaction
(reaction-time). In case of additional stimuli in connection with the
electric, various simple devices were introduced to meet the demands
of the experiments. These will be described in connection with the
statement of results in each case.
_Electric and photic stimuli._ A photic stimulus was given from one
to two seconds before the electric stimulus by the turning on of a
sixteen-candle-power incandescent light, which was placed thirty cm. in
front of the frog in the case of one series of experiments and fifteen
cm. above it in another. The light uniformly inhibited reaction to the
electric stimulus, as is shown by the results of Table 1.
TABLE 1
Title of investigation,--Electric-Visual (Red Light).
Experimented on,--Green Frog No. 4.
Harvard Psychological Laboratory,--9.40 A.M., Feb. 28, 1902.
Chronoscope control average, 189σ,--Electric stimulus, 1 Cell.
NO LIGHT.
_Number of_
_Experiment._ _Reaction-time._
1 152σ
2 145
3 221
4 327
5 263
6 271
7 329
8 215
9 225
10 216
LIGHT BEFORE ELECTRIC STIM.
11 No reaction.
12 No reaction.
13 No reaction.
14 No reaction.
15 No reaction.
NO LIGHT.
16 216
The inhibitory influence of light depends upon the intensity of the
electric stimulus. Even a very strong light will not cause much
retardation of reaction to a three or four cell current. As the
strength of the electric stimulus decreases the delay of reaction
increases, until finally there is complete inhibition. At this point,
an electric stimulus, to which the frog would react almost invariably
when there is no disturbing condition, will fail to cause reaction in
the presence of a sudden increase in light intensity.
Merzbacher[150] states that the leg reflex of a frog, so placed that
its legs hang free in the air, is greater in response to a given
cutaneous stimulus in darkness than in daylight.[151]
_Electric and visual stimuli_ (moving object). For the purpose of
determining the effect upon reaction-time to an electric stimulus
of stimulation of the eye by a rapidly moving object, experiments
were made in which, as in the case of electric and photic stimuli,
reactions to electric stimulus alone and to the visual and electric
were observed alternately. Thus in the case of each pair of reactions
it was possible to note whether the visual stimulus shortened or
lengthened the reaction-time. The visual stimulus was given by quickly
moving a finger before a window in the reaction-box.
Two series of twenty pairs of reactions each were taken with each of
two frogs. In the first series the finger was suddenly moved across the
window and the electric stimulus was given either simultaneously or
a small fraction of a second later. It was impossible to arrange for
accurate measurement of the temporal relations of the two stimuli in
the case of these tests. In the second series the finger was moved back
and forth before the opening in the reaction-box for an interval of at
least a second before the electric stimulus was given.
These experiments, which were in the nature of preliminary tests,
yielded the following results. When the stimuli were given almost
simultaneously the visual reënforced the electric as was indicated by a
shortening of the reaction-time. As appears in the upper part of Table
2, the average time of forty reactions, twenty for each frog, to the
electric stimulus was 148^{σ},[152] and to the same stimulus when it
followed the visual 128^{σ}. Furthermore, examination of the several
pairs of reactions shows, as is indicated in the table, that there were
twenty-seven cases in which the visual stimulus caused shortening of
the reaction-time (reënforcement of the electric stimulus) to thirteen
in which it caused lengthening (inhibition). When the visual stimulus
preceded the electric by at least a second, the reaction-time to the
electric stimulus was greatly lengthened. The averages are 150^{σ}
for the electric stimulus alone, 178^{σ} when it was preceded by the
visual. In this series there are twenty-five cases of inhibition to
fourteen of reënforcement.
_Electric and visual stimuli_ (moving red disc). The indications
of the importance of the temporal relations of stimuli, so far
as reaction-time results are concerned, furnished by these crude
preliminary observations led to a more accurate study of the subject. A
revolving disc, which moved at the rate of one revolution per minute,
was so arranged that at a certain point it closed an electric circuit
in which a magnet had been placed. This magnet attracted a steel arm
at the end of which a disc of red cardboard 12 mm. in diameter was
suspended. With the making of the circuit the steel arm was drawn
downward suddenly and the red disc, by reason of the vibrations of
the arm moved rapidly back and forth in front of a window in the
reaction-box. In this way the moving object was exposed to view about
ten cm. to the right and three cm. in front of the right eye of the
frog. The revolving disc, a fraction of a second later, completed the
electric stimulus circuit. Thus both stimuli were given automatically,
at such an interval apart as the experimenter desired. In the two
series of results now to be described the intervals were 0.1 and 0.5
second respectively.
TABLE 2
Reaction-time to Electric Stimulation Alone, and to the Same when
preceded for 0.1, 0.5, or 1.0 Second by Visual Stimulus.
Key:
1 = Frog.
2 = Electric Alone.
3 = Visual 0.1" before elect.
4 = Number Inhibited.
5 = Number Reënforced.
6 = Number Equal.
7 = Visual 1.0" before elect.
1 2 3 4 5 6 2 7 4 5 6
Preliminary Series. Visual Stimulus Moving Finger.
Averages for 20 reactions.
No. 5. 179ˢ 158ˢ 6 14 0 163ˢ 206ˢ 14 6 0
No. 6. 116 98 7 13 0 136 150 11 8 1
Gen.
Aver. 148 128 13 27 0 150 178 25 14 1
Visual Stimulus Moving Red Disc.
Visual 0.1" before electric. Visual 0.5" before electric.
Series I. Averages for 25 reactions.
No. 5. 177 163 10 15 0 170 255 15 9 1
No. 6. 148 112 6 19 0 115 178 18 7 0
Series II. Averages for 25 reactions.
No. 5. 135 120 7 18 0 155 259 24 1 0
No. 6. 128 111 6 19 0 132 227 17 7 1
Gen.
Aver. 147 126 29 71 0 143 230 74 24 2
These series consisted of twenty-five pairs of reactions each, with
two animals. The results of the series are presented separately, in
the lower half of Table 2, because the experiments which constitute
them were separated by a period of three weeks. It is to be noted
that these results agree fully with those of the preliminary series.
The visual stimulus of a moving red disc, given 0.1 second before a
2 cell electric stimulus, reënforces the electric reaction, _i. e._,
it shortens the time of reaction. The same visual stimulus given 0.5
second before tends to inhibit the electric reaction, _i. e._, it
lengthens the time of reaction.
_Tactual and auditory stimuli._ Since in the frog auditory stimuli
under experimental conditions seldom if ever cause visible motor
reactions, the study of the influence of this mode of stimulation
upon the reactions to other simultaneous or succeeding stimuli
is of special interest. In the investigation of the relations of
auditory stimulation to other forms of reaction _amount_ of reaction
instead of _reaction-time_ was taken as a measure of the influence
of the stimulus. By a method the details of which may be most easily
understood by reference to the plan of the apparatus in Figure 1, the
influence of auditory stimuli on the leg-movement induced by tactual
stimulation was observed.
In these experiments the frog sat astride a wooden support, held in
position by linen bands over the back and a wire screen cap over
the head. The hind legs hung free, and any movement of one of them
in response to a stimulus could be read in millimetres by reference
to a scale on the wooden support. This method of measuring the
value of a stimulus in terms of leg-reflex has been used by several
investigators--most recently by Merzbacher.[153] I have found it
desirable, as did Merzbacher, to observe the movements of a shadow of
the leg on the scale and thus read the amount of movement, rather than
to watch the leg itself and attempt to project it upon the scale.
As is indicated in Fig. 1, the auditory and tactual stimuli were given
automatically by means of a swinging pendulum, _P_, which was held in
position by the magnet _a_ until released by the experimenter. Early in
its swing the pendulum turned the key, _m_, thus completing a circuit
which caused the auditory stimulus to be given; later in the swing the
key, _n_, was turned, and the tactual stimulus thus given through the
magnetic release of the lever, _l_. The interval between the auditory
and the tactual stimuli could be varied from 0 to 2" by changing the
position of the key, _n_. For intervals over 1" it was necessary to
arrange this key so that the tactual stimulus was given at some time
during the return swing of the pendulum.
The auditory stimulus used was either the sound of a quick hammer blow
(momentary stimulus of Series I), or the ringing of an electric bell
for a certain length of time (prolonged stimulus of Series II). In Fig.
1 the bell is shown. It was placed eighty cm. from the frog, and in
order that the influence of vibration of the experiment table might
be avoided it was suspended from the pendulum frame. When the hammer
was used it was placed sixty cm. from the frog, on the pendulum table.
The holder for the frog and the tactual apparatus occupied a separate
table which was not disturbed by the jars of the pendulum table.
[Illustration: Figure 1. Auditory-tactual Reënforcement-Inhibition
apparatus. _P_, pendulum; _p_, contact point of _P;_ _b_, attachment
for electro-magnet, _a;_ _m_, key for circuit of electric bell, _B;_
_n_, key for magnet circuit of tactual apparatus; _K_, hand-key for
release of pendulum and temporary closing of electric bell circuit;
_k_{1}_, _k_{2}_, _k_{3}_, keys in circuits; _e_, _f_, _g_, magnetic
release for tactual apparatus; _l_, pivoted lever, bearing rubber cone,
_T_, and weights, _w_. (Drawn by Dr. Wm. E. Hocking.)]
The tactual stimulus was given by a rubber cone, _T_, two mm. in
diameter at its apex. This rubber point, after the electric release
of the lever to which it was attached, struck the frog at the middle
point of a line drawn between the posterior margins of the tympana. The
intensity of the stimulus could be varied by weighting the lever, _l_,
at _w_.
All experiments were made with the green frog, _Rana clamata_ Daudin.
The reactions were taken regularly at half-minute intervals in pairs:
first, a tactual stimulus reaction, then an auditory-tactual reaction.
Ten, fifty, or one hundred pairs constituted a series. So far as
the condition of the frog is concerned there seems to be nothing
undesirable in long series, for fatigue does not appear, and so long
as the animal is kept moist and in an unconstrained position, it
continues to react normally, and without frequent struggles to escape.
The advantage for the purposes of this investigation of taking the
reactions in pairs, rather than taking separate series of reactions
for each stimulus or combination of stimuli, is obvious. It enables us
to compare directly the reactions of each pair, in other words those
reactions which took place under most nearly identical conditions, and
to note at once whether the auditory stimulus reënforced or inhibited
the tactual reaction.
During a series the intensity of the tactual stimulus was changed as
conditions demanded, but for any one pair of reactions it was always
the same. It not infrequently happened that an intensity which at
first caused merely a slight movement of the leg, later in the series
uniformly brought about a maximal contraction, or the reverse might
be true, and inasmuch as a maximal reaction to the tactual stimulus
alone left no opportunity for judging of the influence of the auditory
stimulus, when it was given in addition to the tactual, it was always
necessary in such cases so to alter the intensity of the tactual
stimulus that a medium reaction resulted.
The frogs, after being placed in the saddle-like holder and held firmly
for a few seconds, seldom struggled very much, but if bound tightly
they became irresponsive to the stimuli.[154] It was, therefore,
necessary after they had quieted down to loosen the bands which held
them in position. For the purpose of excluding the influence of visual
stimuli a wire screen cap covered with black cloth was put over the
head; this served to keep the animal in position as well as to exclude
visual stimulation.
_a. Momentary auditory stimulation._ Four frogs were used for a study
of the influence of the momentary sound produced by a hammer blow,
and for each of these animals fifty pairs of reactions were recorded
in series each day. The temporal relation of the stimuli was changed
daily during a week of experimentation: the results therefore consist
of fifty pairs of reactions with each frog for each of the following
seven intervals: (1) Auditory and tactual stimuli simultaneous, (2)
auditory .25" before tactual, (3) auditory .45" before, (4) auditory
.15" before, (5) auditory .65" before, (6) auditory .35" before, (7)
auditory .90" before. The intervals were used in the experiments in the
above order to avoid the formation of the definite habits of reaction
which regular increase in the interval would have favored.
Typical of the results with all the animals are the following (Table
3) which were obtained with No. 1, a male. The figures in each case
indicate the average of fifty reactions. Reënforcement and inhibition
are expressed in terms of the tactual reaction, _i. e._, the
auditory-tactual reaction is so many per cent greater (reënforcement)
or less (inhibition) than the tactual. In the tables reënforcement is
indicated by the + sign; inhibition by the - sign. In the last column
of the table is given the number of reactions that were reënforced or
inhibited. This was determined by comparing directly the reactions
of each pair. Cases in which the two reactions were the same were
distributed equally between the two classes: tactual reactions
reënforced by auditory stimulus, and tactual reactions inhibited by
auditory stimulus. Assuming that the auditory stimulus was without
effect upon the tactual reaction, the number of reactions in these two
classes would be approximately the same, hence all auditory-tactual
reactions over half in a series, _i. e._, over twenty-five, which
are greater than the corresponding tactual reactions, are reënforced
reactions, and can be taken as a measure of the reënforcing influence
of the auditory stimulus. In the same manner all reactions over half
which show inhibition can be taken as a measure of the inhibitory value
of the auditory stimulus.
As preliminary tests described in an earlier paper[155] furnished
evidence of sex-differences, it is worth while to compare the results
given by the males and females in these experiments with momentary
auditory stimulation. For purposes of comparison I have presented
in Table 4 the reënforcement-inhibition values given by the males
and females for each interval. Column one contains the value of the
auditory-tactual reaction in terms of the tactual reaction; column two,
the number of reactions in excess of half which were reënforced or
inhibited.
TABLE 3. FROG NO. 1. MOMENTARY AUDITORY STIMULUS, HAMMER BLOW. WEIGHT
USUALLY 5 OR 10 GRAMS
Reaction Reaction Amount of Number of
to to Auditory Reënforc'm't reactions
Tactual and Tactual or Reënforced
Interval. Stim. Stim. Inhibition. or Inhibited.
0" 6.84mm. 11.08mm. +62.0% +17.0
.15 22.22 28.96 +30.3 +17.0
.25 16.30 21.72 +33.3 +13.0
.35 24.90 25.32 + 1.7 + 0.5
.45 17.56 13.64 -22.3 -10.0
.65 17.46 15.72 -10.0 - 6.0
.90 31.26 31.48 + 0.7 + 0.5
TABLE 4. MOMENTARY AUDITORY STIMULUS, HAMMER BLOW
_Males_ _Females_
Nos. 1 and 3. Nos. 2 and 4.
Per centum No. of Per centum No. of
Interval Diff. Reacts. Diff. Reacts.
0" +82.5% (Reënf't) +17.5 +58.0% +12.7
.15 +58.1 +17.0 +25.4 + 8.5
.25 +32.3 +12.7 +39.8 +12.7
.35 + 4.0 + 1.2 - 9.7 - 3.2
.45 -13.5 (Inhibition) - 7.2 -13.9 - 7.2
.65 -12.5 - 6.2 -11.8 - 7.2
.90 - 0.7 - 1.5 - 2.6 - 0.5
In these results two striking differences between the males and females
appear: first, the reënforcement is not so great for the females as
for the males; second, inhibition appears earlier and continues longer
with the females than with the males. The average reënforcement with
simultaneous stimuli is 82.5% for the males against 58.0% for the
females. Inhibition begins to appear in case of the females when the
interval between the stimuli is .25" to .35"; in case of the males it
appears between .35" and .45". Finally at .90" interval inhibition is
slightly greater for the females.
Although the exact significance of these facts is unknown, it is
not improbable that they are indicative of fundamentally important
sex-differences in reaction to sound. The males among frogs are
usually the vocalists, although in some species the females also
croak. Moreover, in case of the green frog the tympanum of the male
is much larger than that of the female. The results presented would
seem to indicate that certain sounds stimulate the males to activity,
whereas they inhibit activity in the females.
Graphically represented, the results of the momentary auditory stimulus
experiments with frogs Nos. 1, 2, 3, and 4 are as follows:
[Illustration: FIG. 2. Reënforcement-Inhibition curves for momentary
auditory stimulation, based upon amount of reaction. Male No. 1 ----
Male No. 3 ....]
[Illustration: FIG. 3. Reënforcement-Inhibition curves for momentary
auditory stimulation, based upon amount of reaction. Female No. 2 ----
Female No. 4 ....]
The curves are all plotted by the method which will now be
described in connection with Fig. 2. This figure presents the
reënforcement-inhibition curves for the males No. 1 (solid line in
the figure) and No. 3 (broken line). If in this figure we let the
zero-point on the ordinates represent the value of the reaction to the
tactual stimulus when given alone, then the value of the reaction to
the auditory-tactual stimuli would be represented at some point above
the zero-point if this reaction was greater than the tactual reaction
(reënforcement), and below the zero-point if the reaction was less
than the tactual (inhibition). Since one of our chosen measures of
reënforcement and inhibition is the amount, in per cent of tactual
reaction, by which the auditory-tactual reaction exceeds or falls short
of the tactual reaction, such a curve of reënforcement-inhibition
as that of Fig. 2 (solid line) can be constructed at once from the
data given in column four of Table 3. Here the auditory stimulus,
when simultaneous with the tactual, caused 62% reënforcement, as is
indicated in the figure. The figures in the left-hand margin of the
curves indicate amount of reënforcement or inhibition in per cent of
tactual reaction; those at the bottom of the curves mark the intervals.
On the curves dots indicate the intervals used in the experiments. Each
of the curves is plotted on the basis of 700 reactions.
[Illustration: FIG. 4. Reënforcement-Inhibition curves for momentary
auditory stimulation, based upon number of reactions. Male No. 1. ----
Male No. 3 ....]
[Illustration: FIG. 5. Reënforcement-Inhibition curves for momentary
auditory stimulation, based upon number of reactions. Female No. 2 ----
Female No. 4 ....]
In every way comparable with the curves for the males No. 1 and No.
3 in Fig. 2 are those for the females No. 2 and No. 4 of Fig. 3. The
similarity of the two curves in each figure is noteworthy. Inasmuch as
the conditions of experimentation were the same for all the animals
this would seem to indicate sex-differences which are worthy of further
investigation. The curves show clearly the greater reënforcement in the
males, and the greater inhibition in the females.
Figures 4 and 5 are the reënforcement-inhibition curves for the same
series of experiments plotted on the basis of the _number_ of reactions
in excess of half that were reënforced or inhibited. As there were
fifty pairs of reactions with each frog for each interval, uniform
reënforcement would be represented by twenty-five reactions above
the base-line; uniform inhibition by twenty-five reactions below the
base-line. The number of reactions is indicated by the figures in
the left margin; the intervals, by those below the base-line. As an
illustration of the application of the method of plotting, the curve
for male No. 1 (solid line) of Fig. 4 is constructed from the data
of column five of Table 3. With simultaneous stimuli 17 reactions in
excess of half, _i. e._, 17 + 25, or 42, were reënforced; at .35"
interval .5 of a reaction was the average amount of reënforcement;
at .45" interval 10 reactions in excess of half, _i. e._, 35, were
inhibited, therefore the curve falls to 10 below the base-line.
[Illustration: FIG. 6. Composite Reënforcement-Inhibition curve for
momentary auditory stimulation, based upon amount of reaction. Frogs
Nos. 1, 2, 3, 4. (Males and females.)]
[Illustration: FIG. 7. Composite Reënforcement-Inhibition curve for
momentary auditory stimulation, based upon number of reactions. Frogs
Nos. 1, 2, 3, 4. (Males and females.)]
Just as Figures 2 and 3 permit of direct comparison of the results
of the measurement of the _amount_ of reënforcement and inhibition
for males and females, so Figures 4 and 5 make possible comparison in
similar fashion of the _number_ of reënforced and inhibited reactions
for the sexes. It is to be noted that the two sets of curves, plotted
on the bases of _amount_ and _number_ of reaction, agree in all
important respects.
Figure 6 is the composite curve of amount of reënforcement-inhibition
for the four animals; Figure 7 is the composite curve of the number of
reactions reënforced and inhibited.
Summarily stated, the results of the experiments thus far described
are: (1) The auditory stimulus of a quick hammer blow produces the
maximum amount of reënforcement of tactual reaction when it is given
simultaneously with the tactual stimulus; (2) as the interval between
the auditory and the tactual stimulus approaches .35″ the amount of
reënforcement gradually decreases; (3) when given .35″ before the
tactual stimulus the auditory is practically without effect upon the
tactual reaction; (4) as the interval increases above .35″ inhibition
begins to appear; (5) the inhibitory influence of the auditory stimulus
is greatest when the interval is about .45″; (6) when the interval is
as long as .90″ the auditory stimulus is again ineffective. It thus
appears that the reënforcement-inhibition curve of this particular
stimulus under the conditions described is representative of a neural
process which completes itself, in passing through two phases, a
positive phase (reënforcement) and a negative phase (inhibition), in
about one second.
_b. Prolonged auditory stimulation._ The experiments previously
described have proved that a momentary auditory stimulus, which when
given alone never produces a visible motor reaction, either reënforces
or inhibits the reaction to a tactual stimulus which it accompanies or
precedes. The experiments now to be described were made for the purpose
of ascertaining whether reënforcement and inhibition occur in the same
way if the auditory stimulus is prolonged, instead of momentary.
In a trial series of experiments with frog No. 1, one hundred pairs
of reactions were recorded for each of six intervals of auditory
stimulation. The auditory stimulus was given by the ringing of an
electric bell. For all intervals the ringing of the bell continued
until the tactual stimulus was given. When the two stimuli were given
simultaneously the auditory stimulus was necessarily momentary, as
in the foregoing experiments, but for all other relationships of
the stimuli the bell rang for a certain length of time before the
tactual stimulus was given. The six relations of the stimuli were:
(1) simultaneous, (2) bell .2″ before and until tactual, (3) bell .6″
before, (4) bell 1.05″ before, (5) bell 1.5″ before, and (6) bell 2.0″
before. The other conditions of these experiments were the same as
those previously described, except that the auditory stimulus was here
given by the opening of the key which released the pendulum, instead
of being given by the turning of a key in the course of the pendulum
swing. This method of giving the auditory stimulus as the pendulum was
released was found unsatisfactory because of the irregularity of the
magnetic release; at one time the pendulum would start immediately, at
another time there would be a delay of as much as .1″.
The reënforcement-inhibition curve plotted on the basis of the 1200
reactions in this series is presented in Fig. 8. Before stopping to
consider the important features of this curve we should note the
results of certain more accurate experiments with prolonged auditory
stimulation.
[Illustration: FIG. 8. Reënforcement-Inhibition curve for prolonged
auditory stimulation, based upon amount of reaction. Frog No. 1.]
With two animals, No. 2, a female, and No. 3, a male, fifty pairs
of reactions were taken for nine different intervals (see Table 5)
of auditory stimulation. Each of the curves of Figures 9 and 10
is therefore based upon 900 reactions. The conditions for these
experiments were the same as those for the momentary stimulation
series, save that the electric bell took the place of the electrically
actuated hammer, as the mechanism for auditory stimulation.
[Illustration: FIG. 9. Reënforcement-Inhibition curves for prolonged
auditory stimulation, based upon amount of reaction. Female No. 2 ----
Male No. 3 ....]
The important facts exhibited by the results of these prolonged
auditory stimulation experiments in contrast with those with momentary
auditory stimulation are: (1) That whereas for the momentary
auditory stimulus of a hammer blow the reënforcement is greatest for
simultaneous stimuli, in case of the prolonged stimulation with the
electric bell, reënforcement increases during an interval of .25″
of auditory stimulation. Hence, the two conditions of stimulation
give us different types of reënforcement-inhibition curve. For the
momentary stimulus the maximum reënforcement appears at simultaneity,
and for the prolonged stimulus at .25″; (2) that the transition from
reënforcement to inhibition occurs at 1.2″ in the prolonged stimulation
curves, while in the momentary stimulation curves it occurs at .35″;
(3) that the maximum inhibition which appears in the curves under
discussion at about 1.5″ is less in comparison with the amount of
reënforcement than that of the momentary stimulation curves; (4) that
the auditory stimulus becomes ineffective when the interval during
which it continues before tactual stimulation is 2.0″. The curves of
Figures 8, 9, and 10 are then representations of a neural process which
passes through a positive and a negative phase in about 2″. The effect
of prolongation of the auditory stimulation interval is to lengthen
the period of reënforcement; the period of inhibition shows little
modification.
For the purpose of showing in greater detail the nature of the results
of this work the data from which the curves of Figures 9 and 10 were
constructed are presented in the accompanying Table 5.
[Illustration: FIG. 10. Reënforcement-Inhibition curves for prolonged
auditory stimulation, based upon number of reactions. Female No. 2 ----
Male No. 3 ....]
Having now presented the results of my own investigation I wish to
call attention to certain of their relationships to the work of other
investigators, and to discuss briefly their significance.
TABLE 5. PROLONGED AUDITORY STIMULATION (ELECTRIC BELL)
_Frog No. 2._ _Female._ _Weight usually 25 grams._
Auditory Number of
and Amount of Reactions,
Tactual Tactual Reënforcement Reënforced
Interval. Stimulation. Stimulation. or Inhibition. or Inhibited.
0″ 9.20 mm. 12.12 mm. + 31.7% +10.0
.25 4.56 11.88 +160.5 +22.5
.45 8.94 16.94 + 89.5 +18.5
.65 17.18 22.50 + 31.0 +15.5
.90 9.42 13.32 + 41.4 +14.5
1.20 10.54 9.64 - 8.5 - 2.5
1.40 24.00 20.64 - 14.0 - 6.0
1.58 19.16 17.80 - 7.1 - 7.0
1.95 14.50 15.40 + 6.2 + 5.5
_Frog No. 3._ _Male._ _Weight usually 5 or 10 grams._
0″ 14.92 mm. 25.20 mm. + 68.9% +11.0
.25 15.88 38.54 +142.7 +24.0
.45 13.48 26.02 + 92.9 +13.5
.65 18.30 27.94 + 52.6 +13.0
.90 20.94 29.06 + 38.8 + 9.5
1.20 21.90 30.58 + 39.6 + 1.0
1.40 19.18 18.34 - 4.4 - 5.5
1.58 32.24 26.30 - 18.1 - 3.0
1.95 13.86 14.14 + 2.0 + 2.0
VI. DISCUSSION OF LITERATURE AND RESULTS
The literature on reënforcement and inhibition is large, and even
that portion of it which deals especially with the importance of the
temporal relations of stimuli in connection with reënforcement and
inhibition is so extensive that it does not seem worth while to attempt
to give a systematic résumé of it for the purposes of this paper. I
shall therefore call attention merely to those investigations which
have contributed directly to the solution of the problems with which we
are now concerned.
Bowditch and Warren[156] discovered that knee-jerk in the human subject
is reënforced when an auditory, a visual, or a tactual stimulus
precedes the tendon blow by .1″ to .5″, whereas the same stimuli have
an inhibitory influence when they are given from .5″ to 1.0″ before the
tendon blow.
At the suggestion of Bowditch, Cleghorn[157] undertook to investigate
the influence of complication of stimuli upon voluntary movements. In
this research graphic records taken in connection with an ergograph
indicated (1) that "a sensory stimulus" applied just as the muscle was
beginning to contract (voluntarily) caused an increase in the height of
the contraction, and (2) that the relaxation following a contraction
with intercalated sensory stimulus is quicker and more complete than
when no stimulus is given (p. 344). Cleghorn did not give special
attention to the significance of the temporal relations of the stimuli
which he employed, and his work was limited to the phenomenon of
reënforcement of voluntary action by reason of the appearance, during
the progress of his research, of an excellent paper on the interference
of stimuli by Hofbauer.[158]
Hofbauer covered thoroughly the ground which Cleghorn had planned to
work over. The ergographic method was employed also by Hofbauer in
his very careful study of the interference of impulses in the central
nervous system of man. It was noticed that while the subject was
rhythmically contracting a certain group of muscles in response to
some prearranged signal (_e.g._, the sound of a metronome) the report
of a pistol caused the contraction which immediately followed it to be
much greater than the average of the rhythmic series, while the next
contraction was correspondingly less than the average. It thus appeared
that the sudden sound caused, first, reënforcement of the voluntary
movement, then, inhibition. The reënforcement is greatest, according
to Hofbauer, when the voluntary movement occurs immediately after the
pistol report. When the report precedes the metronome signal by .2″
reënforcement is still marked, but thereafter it decreases rapidly in
amount, until finally at .5″ inhibition appears. When the interval
between the two stimuli is 1.0″ the first stimulus has practically
no effect upon the voluntary movement in response to the second.
(Hofbauer, p. 558.)
What Bowditch and Warren, not to mention other students of the subject,
have described for reflex action in man, Hofbauer, Cleghorn, and others
have shown to hold true also of voluntary movements. Unfortunately my
own investigation was completed up to the point of the writing of this
paper before I read Hofbauer's work, so I have not followed methods
of dealing with my data which would make our results directly and
easily comparable. But, whatever may be the relations of our results in
detail, there can be no doubt that what he has demonstrated for man is
true in its important aspect of the reënforcement-inhibition phenomena
for the frog.
Important in their bearings upon the phenomena of reënforcement and
inhibition which we are now considering, are the various studies of
refractory period and rhythm of nerve cell and fibre. The existence of
a refractory period in neural substance, similar to that demonstrated
for certain kinds of muscle by Marey,[159] Englemann,[160] Kaiser,[161]
Cushny and Matthews,[162] Woodworth,[163] and many others, has been
proved by Broca and Richet.[164]
Broca and Richet found that in the normal dog the refractory period
of the nerve substance is too short to be easily detectable, they
therefore experimented with animals which were lightly chloralized and
kept at a temperature of 30 to 34° (the mean normal temperature of the
dog is about 39.5°). Under these conditions a dog, when two identical
stimuli (quality and intensity the same) were applied to the cerebral
cortex successively, exhibited the following reactions: (1) When the
stimuli were separated by .01″ they reënforced one another (addition);
(2) when the interval was .1″ they inhibited the reaction partially
(subtraction).
Concerning this phenomenon Richet writes in his dictionary of
physiology (p.5): "Marey showed, in 1890, that the heart of the frog,
at certain moments of systole, was inexcitable. Now our experiments
prove that the cerebral apparatus, a certain time after the excitation,
also ceases to be excitable: it then has a refractory phase, and this
refractory phase is much more prolonged than that of the cardiac
muscle." In a later publication Richet[165] makes the somewhat
startling statement that a refractory period is not exhibited by the
nerves of cold-blooded animals. In the tortoise, according to his
results, reënforcement occurs so long as the interval between the
two stimuli is not greater than 2″, while for longer intervals each
stimulus to all appearances works independently. Richet seems to have
generalized from a study of the tortoise. That his generalization is
unwarranted seems to me highly probable in the light of the results
of this paper, for there are many reasons for supposing that the
reënforcement-inhibition phenomena with which we have been dealing
in case of the frog are manifestations of the existence of the same
process in the nervous system which under somewhat different conditions
of experimentation exhibits itself in the so-called refractory period.
The researches of Richet and his students indicate that the time of
the process which conditions the phenomena of reënforcement-inhibition
is about .1″. Stimuli given at .1″ intervals do not interfere with one
another. That the process underlying the refractory period and the
reënforcement-inhibition phenomena of our experiments is a rhythmic
double-phase process is made still more probable by the following
results. Horsley and Schäfer[166] found that the rate of response of
the monkey to cortical stimulation was 12 per second, and Schäfer[167]
discovered that the maximum rate of volitional impulses in man is 10 to
12 per second.
It was shown by Exner that certain movements of the foot of a rabbit
could be produced by stimulating either the cortex or the skin of the
foot. Simultaneous stimulation of both regions gives reënforcement.
Stimulation of the cortex, if given not more than 3″ before subliminal
stimulation of the skin, renders the latter effective. When both
stimuli are subliminal each makes the subsequent one effective if
the interval between them is not over 1/8″ (Schäfer[168]). Similarly
for the dog Exner[169] proved that cortical and cutaneous stimuli
reënforced one another, when both were subliminal, if the interval
between them was not greater than .6″. Cortical and auditory stimuli,
and auditory and cutaneous (of the skin of foot) gave similar results.
Physiologists have long been familiar with several aspects of the
phenomena of reënforcement and inhibition in the frog, but I know of no
detailed study of the significance of the temporal relations of stimuli
in this connection. Goltz[170] called attention to the inhibition of
the croaking reflex by peripheral stimulation, as well as to several
similar phenomena. Nothnagel,[171] Lewisson,[172] and Wydensky[173]
further contributed to our knowledge of the interference effects of
stimuli in the frog. Wydensky proved that the application of an induced
current to a nerve-muscle preparation may result in either contraction
or relaxation of the muscle, according to the frequency of stimulation.
More recently Merzbacher[174] has dealt with the influences of
complication of stimuli in the frog with the purpose of ascertaining
the relations of the sense-organs to the reflex movements of the
animal. His first paper is concerned especially with the functional
importance of the eye in connection with reflexes. Unfortunately for
the demands of this research, he did not attend particularly to the
temporal relations of his stimuli. That a visual and a cutaneous
stimulus were given either "at the same time or within a short interval
of one another" (p. 250) is not the sort of information our problems
demand.
According to Merzbacher's very interesting results a visual stimulus
reënforces the reaction to a cutaneous stimulus. As the results of this
paper show, this is only half a truth, for the two stimuli may either
reënforce or inhibit one another's reactions. As Merzbacher observed
no evidences of reaction to auditory stimulation he presumably did not
attempt to study the influences of the ear in connection with reflexes.
There can be no doubt that the words reënforcement and inhibition
as at present used in connection with the functions of the nervous
system cover a multitude of widely differing phenomena. We can at
once distinguish at least two important kinds of reënforcement or
inhibition: first, that which is due to the functioning of special
augmentary or inhibitory portions of the nervous system; second,
that which is the result of the complication of stimuli. Any and
every process in the nervous system may have either a reënforcing or
an inhibiting influence upon simultaneous or succeeding processes;
doubtless most processes or impulses at various times have both
effects. The nervous system is constantly being modified by impulses
from many sources, which suppress or strengthen one another according
to their relative intensity, their temporal relations, and the motor
relations of the portions of the organism which they affect.
The existence of the so-called refractory period in brain cortex and
nerve indicates that every stimulus causes certain fundamentally
important changes in the condition of the neural substance. These
changes we may for convenience of illustration describe as modification
of excitability, or of the functional capacity of central or peripheral
tissues. Every stimulus causes a portion of the neural substance
to pass from its normal state through a condition of increased
excitability, which we may designate the positive phase, to a condition
of diminished excitability, the negative phase. There is first an
increase in the functional capacity of the tissues, then a decrease.
If during the course of the change produced by a given stimulus a
second stimulus becomes effective its result in reaction is determined
by the particular phase of the tissues upon which it intrudes. If the
nervous system is in the condition of increased excitability, and
the two stimuli act upon sensory regions whose motor connections are
not antagonistic, the reaction will be reënforced, as we say, by the
previous stimulus; if, however, the second stimulus falls upon the
negative phase of the nerve substance, the reaction will be partially
or totally inhibited.
The facts which are most prominent as the result of this investigation
are, first, that the temporal relation of stimuli is an important
condition of certain forms of reënforcement and inhibition; second,
that the interference effects of two stimuli cannot be studied to
advantage without attention to the relations of the forms of reaction
which are appropriate to each stimulus.
V. SUMMARY
1. Motor reactions of the green frog to electric stimuli are inhibited
either partially or wholly by photic stimuli. The visual stimulus of a
moving object has a like effect. It has been found, furthermore, that
the same visual stimulus may either inhibit or reënforce the motor
reaction in response to electric stimulation. When the two stimuli are
given simultaneously reënforcement occurs, when the visual stimulus
precedes the electric by half a second or more inhibition appears.
2. An auditory stimulus, which does not produce any visible reaction
when given alone, modifies respiration and the reactions to other
stimuli when given in connection with them.
3. The momentary auditory stimulus of a quick hammer blow when
simultaneous with tactual stimulation reënforces the reaction to the
latter stimulus. This reënforcement, or increase in the amount of
reaction, ranges from 50 to 100% of the average reaction to the tactual
stimulus alone. When the auditory stimulus is given before the tactual
reënforcement occurs in gradually decreasing amount until the interval
between the two stimuli reaches .35″; at this point the auditory
stimulus has no apparent effect upon the tactual reaction. As the
interval is still further increased inhibition appears and continues
for intervals between .35″ and .9″. Reënforcement is greatest when the
two stimuli are simultaneous; inhibition is greatest when the momentary
auditory stimulus precedes the tactual by .4″ to .6″. When the interval
reaches .9″ the first stimulus does not affect the reaction to the
second.
4. Reënforcement is greater for the males than for the females;
inhibition appears sooner and lasts longer in case of the females.
This apparently indicates that the males are stimulated to activity by
certain auditory stimuli, whereas the females are rendered passive by
similar sounds.
5. Prolonged auditory stimulation by means of an electric bell causes
reënforcement and inhibition, according to the temporal relations
of the stimuli, as does momentary auditory stimulation, with the
following differences: The maximum reënforcement occurs when the
tactual stimulus is given about .25″ after auditory stimulation has
begun; reënforcement continues for a period of 1.2″, _i. e._, when the
electric bell continues to ring until the tactual stimulus is given, it
reënforces the tactual reaction from simultaneity to 1.2″. Inhibition
then appears, and continues until 1.8″. Both momentary and prolonged
auditory stimulation cause first reënforcement, then inhibition of the
appropriate reaction to a tactual stimulus.
6. The reënforcement-inhibition curves for the frog are very similar to
those for man.
7. In case of the several pairs of stimuli whose interference effects
have been studied reënforcement-inhibition appears. The first stimulus
reënforces reaction to the second so long as the interval between them
is not more than about .4″, while it inhibits the reaction when the
interval is longer. Whether this reënforcement-inhibition curve as
given in the experiments described may similarly be obtained for any
and every pair of stimuli, no matter what their relation to reactions,
remains to be determined.
8. In connection with the study of the mutual relations of stimuli of
which this paper gives an account certain facts concerning the sense
of hearing have been discovered. A summary statement of the results on
hearing may be found on page 551.
FOOTNOTES:
[Footnote 139: The results brought together in this paper have been
published in part in connection with other work in the following
papers: Inhibition and Reënforcement of Reaction in the Frog, Jour. of
Comp. Neurol. and Psychol., vol. 14, p. 124, 1904. Bahnung und Hemmung
der Reactionen auf tactile Reize durch akustische Reize beim Frosche,
Arch. f. d. ges. Physiol., vol. 107, p. 207, 1905. The Sense of Hearing
in Frogs, Jour. of Comp. Neurol. and Psychol., vol. 15, p. 279, 1905.]
[Footnote 140: The Organ and the Sense of Taste in Fishes, Bulletin
U.S. Fish Commission for 1902, pp. 237-272.]
[Footnote 141: Animal Behavior, Woods Hole Lecture Series, p. 300,
1899.]
[Footnote 142: The Response of the Frog to Light, American Journal of
Physiology, vol. 9, p. 476, 1903.]
[Footnote 143: The Instincts, Habits and Reactions of the Frog, Harvard
Psychological Studies, vol. 1, p. 590, 1903.]
[Footnote 144: The Skin and the Eyes as Receptive Organs in the
Reactions of Frogs to Light, American Journal of Physiology, vol. 10,
p. 31, 1903.]
[Footnote 145: Ueber die electromotorischen Erscheinungen an
Hautsinnesnerven bei adaequater Reizung, Archiv für d. ges.
Physiologie, vol. 63, p. 503, 1896.]
[Footnote 146: Harvard Psychological Studies, vol. 1, p. 592, 1903.]
[Footnote 147: Abstract of paper read before Section F of American
Association for the Advancement of Science in Philadelphia, 1904.
Science, vol. 21, p. 265, 1905. See also Bulletin of the U. S. Fish
Commission for 1902, pp. 45-64, and the same for 1904, pp. 183-207.]
[Footnote 148: Anatomie des Frosches, VI, Lehre von Integument und von
den Sinnesorganen, pp. 751, 752, 1904.]
[Footnote 149: Journal of Comparative Neurology and Psychology, vol.
15, pp. 279-304, 1905.]
[Footnote 150: Ueber die Beziehungen der Sinnesorgane zur den
Reflexbewegungen des Frosches, Arch. f. d. ges. Physiol., vol. 81, pp.
222-262, 1900.]
[Footnote 151: "Blendung oder blosse Lichtentziehung erhoht die
Erregbarkeit für mechanische Reize" (p. 253).]
[Footnote 152: Thousandths of a second.]
[Footnote 153: Arch. f. d. ges. Physiol., vol. 81, p. 227, 1900.]
[Footnote 154: A case of inhibition.]
[Footnote 155: Arch. f. d. ges. Physiol., vol. 107, p. 213, 1905.]
[Footnote 156: Journal of Physiology, vol. 9, pp. 60, 61, 1890.]
[Footnote 157: American Journal of Physiology, vol. 1, p. 336, 1898.]
[Footnote 158: Arch. f. d. ges. Physiol., vol. 68, p. 546, 1897.]
[Footnote 159: Travaux du Laboratoire de Marey, 1876.]
[Footnote 160: Arch. f. d. ges. Physiol., vol 59, p. 309, 1894.]
[Footnote 161: Zeitschr. f. Biol., vol. 32, p. 1, 1895.]
[Footnote 162: Journal of Physiology, vol. 21, p. 213, 1897.]
[Footnote 163: American Journal of Physiology, vol. 8, p. 213, 1902.]
[Footnote 164: Comptes rendus, vol. 124, p. 573, 1897.]
[Footnote 165: Nature, vol. 60, p. 629, 1899.]
[Footnote 166: Journal of Physiology, vol. 7, p. 101, 1886.]
[Footnote 167: Journal of Physiology, vol. 7, p. 111, 1886.]
[Footnote 168: Text-book of Physiology, p. 841, London, 1900.]
[Footnote 169: Arch. f. d. ges. Physiol., vol. 28, p. 495, 1882.]
[Footnote 170: Beiträge zur Lehre von den Functionen der Nervencentren
des Frosches, p. 41, Berlin, 1869.]
[Footnote 171: Centralb. f. d. med. Wissensch., vol. 7, p. 211, 1869.]
[Footnote 172: Arch. f. Anat., u. Physiol., p. 259, 1869.]
[Footnote 173: Arch. d. Physiol. norm. et pathol., vol. 4, p. 690,
1892.]
[Footnote 174: Arch. f. d. ges. Physiol., vol. 81, p. 222, 1900.]
THE TEMPORAL RELATIONS OF NEURAL PROCESSES
BY ROBERT M. YERKES
Muscle contraction-time, according to the determinations of several
investigators, varies about .0035".[175] Sanderson states that the
time for direct stimulation of the muscle is approximately .0035" and
for indirect stimulation, by means of the nerve, .007". The rate of
nerve-transmission in the frog ranges from 25 to 35 metres per second.
Reflex reaction-time, as might be expected, varies widely with the
nature of the reaction elicited by a stimulus, the condition of
the animal, and the quality and strength of the stimulus. For many
of the simple motor reactions of the frog it ranges between 20 and
60^{σ}.[176] Whether reflex reaction-time is to be sharply contrasted
with instinctive and voluntary reaction-times, or whether they
indistinguishably merge into one another is a question of considerable
interest and importance for the student of the evolution of activity.
Voluntary reaction-time may be as short as 150^{σ} or as long as life,
in an animal capable of profiting by experience as does the frog. It is
preëminently the delayed type of reaction-time.
So much concerning the temporal relations of neural processes in the
frog being well established, the purpose of the present paper is
to call attention to some experimental results which indicate the
existence of clearly defined types of reaction, and suggest possible
values of reaction-time as a sign of mind.
The specific problems to be considered are: (1) Do reaction-times, in
any given animal, range with equal frequency of occurrence from short
to long, or are there certain modes (most frequented classes) which
indicate definite types of reaction, such, for example, as the reflex,
instinctive, etc.? (2) If there is distribution of the reaction-times
about one or more modes, what are the types of reaction indicated
thereby? (3) Finally, is reaction-time of service as a sign or measure
of consciousness?
I wish especially to call attention to the fact that this paper deals
with the reactions of the frog, not with animal reactions in general.
REACTIONS TO ELECTRICAL STIMULATION AND TYPES OF REACTION
Two years ago in connection with a discussion of the reaction-time of
the green frog to electrical and tactual stimuli,[177] I presented a
curve showing the distribution of 277 reaction-times to an electrical
stimulus. The curve exhibited two clearly defined modes: one at between
60 and 70^{σ} and the other at about 160^{σ}. There was further a group
of delayed reactions ranging about 500^{σ}. This form of distribution
was interpreted, at the time, as indicative of three types of reaction,
called, respectively, the reflex, the instinctive, and the delayed.
I have since obtained and examined with reference to form of
distribution the further data which are presented in this paper. The
reactions are all those of the green frog to electrical stimulation.
The stimulus was applied by means of wires on the reaction-board
on which the frog rested during the experiments. When reaction
occurred in response to the electrical stimulus a circuit through the
time-measuring apparatus was broken by the release of a delicate spring
which had been held in place up to the instant of reaction by the
weight of the frog. A Hipp chronoscope, controlled by a Cattell falling
screen, served as a time-measuring mechanism. Three intensities of
stimulus were used: (1) A current from one Mesco dry cell, (2) from two
cells, and (3) from four cells.
Of the reactions whose time was measured there are three series. Series
I is constituted by the recorded reaction-times in response to a
one-cell stimulus, Series II, those in response to a two-cell stimulus,
and Series III, those in response to a four-cell stimulus. The number
of reactions, range and mode of each series are as follows:
Number of reactions Range Mode
Series I 193 161-798^{σ} 235^{σ}
Series II 288 41-647 235
Series III 256 61-178 105
The distribution of the 481 reaction-times of Series I and II is shown
by Figure 1; that of the 256 reaction-times of Series III, by Figure
2. For both of these distribution polygons the reaction-times were
arranged in ten^{σ} classes, beginning with the class 41-50^{σ}[178] in
the case of the combined Series I and II and with the class 61-70^{σ}
in the case of Series III.
Series I exhibits a primary mode at 235^{σ}. There are no reflex
reactions in this series, unless it be maintained that the reflex
reactions of the frog may have a reaction-time of over 160^{σ},
but there are a number of delayed reactions, some of which have
reaction-times as long as 798^{σ}. This intensity of stimulation
(one cell) may be said to call forth prompt reactions, which we may
provisionally call instinctive, and delayed reactions, which have all
the appearances of voluntary acts. There are no reactions which come
within the range commonly considered as the reflex range of the frog
(20-60^{σ}), and there are relatively few delayed reactions: almost all
centre about the mode 235^{σ}.
[Illustration: FIG. 1, LOWER. FIG. 2, UPPER.]
Series II, in contrast with Series I, exhibits a secondary
mode at 65^{σ} in addition to the primary mode at 235^{σ}. The
stimulus-intensity of this series (roughly twice as great as that for
Series I) induces a variety of short reaction, which did not appear in
the case of the one-cell stimulus, and at the same time fewer delayed
reactions. The range of the reaction-times for the two series is about
the same, but the lower limits are markedly different.
Observation of the subjects during the experiments revealed two methods
of reaction to the two-cell stimulus: a locomotor reaction (jump)
which at once removed the animal from the source of stimulation,
and a twitch of the hind legs which was instantly followed by the
above-mentioned locomotor reaction. The leg reactions constitute the
reflex group of Fig. 1, the usual prompt locomotor reactions, the
instinctive group, and the slow locomotor reactions, the delayed or
voluntary group.
It is to be noted that the instinctive reaction-time mode is the same
for the two intensities of stimulation. This apparently indicates
that change in intensity of stimulation causes a change in the type
of reaction, not merely a gradual change in the position of the
mode. For example, the modal reaction-time of 235^{σ} given by a
one-cell stimulus did not shift to 200^{σ} or lower, as might have
been expected, but instead there appeared a new type of reaction. The
average reaction-times for the two series indicate a decrease in time
with increase in intensity of stimulation, but they give no indication
of the really important difference in the two series of reactions. The
great importance of the distribution of the data, in addition to the
common statistical quantities, is manifest.
Series III, whose reactions occurred in response to a very strong
stimulus, differs in several important respects from the other series.
Its range is much narrower, only 117^{σ}. Delayed reactions are
lacking, and so also, curiously enough, are the reflex reactions of
Series II. Instead of either or both of the modes of Series II, there
appears in Series III an intermediate mode at 105^{σ}.
Our interpretation of these facts is facilitated by results of
observation of the reacting subject. The leg reflex which frequently
occurred in response to the two-cell stimulus never appeared in
response to the four-cell stimulus. This in part explains the lack
of the short reaction-time mode of Series II; it does not, however,
account for the lack of delayed reactions. The latter fact may be
referred to the intensity of the stimulus. Another difficulty in
interpretation appears in connection with the intermediate mode,
105^{σ}. Is this to be considered an instinctive mode, as were those
at 235^{σ}, or a reflex mode? Where is the line between reflex
and instinctive action to be drawn? These results very clearly
indicate that no line can be drawn, except quite arbitrarily. Reflex
reaction-time, in the case of the frog, is continuous with instinctive,
yet for any given situation the reflex, instinctive, and delayed
(voluntary?) modes are likely to appear, as, for example, in the case
of the data of this paper. Our conclusion must be, therefore, that
although types of reaction are indicated by reaction-time results,
the mode for a given type varies too much in position with different
conditions to make it possible to say that a particular reaction-time
is that of a certain type.
We may safely say, then, that for any given subject, the muscle
contraction-time, nerve transmission-time, and simple sensory
reaction-time to the constant stimulus in question being known, we
should be able safely to interpret reaction-time records in terms of
reaction types. For reflex, instinctive, and voluntary are terms which
designate modes of reaction, albeit not isolated classes, for they
intergrade.
Whether there are more types of reaction than are indicated by the
data of this report does not concern us at present, for the practical
as well as the theoretical bearings of our conclusions depend upon the
existence of types, and not upon their number.
REACTION-TIME AS AN INDICATION OF CONSCIOUSNESS
Hesitation in reaction is commonly accepted as an important sign of
volitional consciousness in man; consequently delayed reactions in
lower animals are supposed to be indicative of psychic processes.
Granting this much, reaction-time may be used as a sign of
consciousness. It cannot be denied that the longer the reaction-time
of a given animal the greater the probability that the reaction is
conditioned by mental processes. Such a statement, it is true, has a
basis neither better nor worse than that of most of our inferences
concerning the nature of the actions of our fellow beings. As I have
already attempted to show in a discussion of criteria of consciousness
in animal psychology,[179] there is no one criterion of consciousness
which can be used alone satisfactorily, but instead there are numerous
signs of mind each of which has value according to the number and
variety of our observations concerning its occurrence in connection
with states of consciousness. The more of such signs we discover
and learn to evaluate properly in relation to consciousness in its
different grades and to one another, the safer will be our inferences
concerning the existence of mental processes in animals.
Reaction-time is presented in this paper as an additional sign of
mind. Like all other signs it is of value only if used as one of a
series of indications of mental life. For if we attempt to judge of
consciousness by reference to reaction-time alone, we may be seriously
misled, whereas if we use it in connection with docility, variability,
neural specialization, and other recognizedly valuable signs, we may
be greatly aided in our inference. As in juristic procedure judgment
is not based upon one bit of evidence nor even upon the evidence of
a single witness, but upon evidence accumulated from all available
sources, so in our attempts to judge of the existence of consciousness,
it matters not whether the being be human or infra-human, we should
make use of all phenomena which are recognized as signs of mind. The
chief task of comparative psychology at present is the discovery and
evaluation of signs of mind.
Reaction-time data, however, furnish another sign, or, as I prefer
to call it in this case, measure of the intensity of consciousness;
for variability of the time of reaction as well as its duration is
significant. Reflex reaction-time is relatively constant, instinctive
varies considerably, and the variability of voluntary reaction-time is
extremely large. Degree of variability of reaction-time may be used
as an indication of consciousness in the same way that variability in
the form of reaction is used. The higher the power of consciousness
the greater the variety in form of reaction and the variability of the
reaction-time.
Reaction-time studies, as well as introspection and the investigation
of animal behavior, indicate the importance of three activity concepts:
automatism, instinct, and will. The automatic act is quick and
relatively constant in form as well as reaction-time, while all signs
lead us to infer that consciousness, when it accompanies the act, is a
sequent phenomenon and not a condition of the act. The instinctive act
is both slower and more variable in form and time than the automatic:
consciousness is indicated as an accompaniment, and apparently it is
at times a condition of the act. The will-act is extremely variable,
unique in form, and almost without limits of reaction-time, for the
conscious organism may react to the present situation in a fifth of a
second, a day, or a year. Will is experience in action: it is our name
for individually acquired control, and voluntary action is above all
consciously conditioned activity.
Reaction-time, with respect to its two aspects of duration and
variability, may be used as a sign or criterion of consciousness, for
in accordance with the nature of these two sets of facts we classify
acts as reflex, instinctive, or voluntary.
FOOTNOTES:
[Footnote 175: Sanderson: Journal of Physiology, vol. 18, p. 147.
Tigerstedt: Archiv f. Physiologie, p. 111, 1895. Boruttau: Archiv f.
Physiologie, p. 454, 1892.]
[Footnote 176: ς = thousandths of a second.]
[Footnote 177: Yerkes: Harvard Psychological Studies, vol. I, p. 609,
1903.]
[Footnote 178: The last four classes of Fig. 1 are 100^{ς} classes,
401-500, 501-600, 601-700, 701-800.]
[Footnote 179: Yerkes: Journal of Philosophy, Psychology, and
Scientific Method, vol. 2, p. 143.]
THE MENTAL LIFE OF THE DOMESTIC PIGEON
AN EXPERIMENTAL STUDY OF CERTAIN EMOTIONAL AND ASSOCIATIVE PROCESSES
BY JOHN E. ROUSE
I. INTRODUCTION
Naturalists have observed the habits of pigeons, and physiologists
since Flourens have subjected them to numerous experiments, but so
far they seem to have received little psychological study. As a
contribution to this interesting field the present paper reports an
investigation of certain emotional and associative processes of the
domestic pigeon. Since the literature of the subject is meagre, I shall
state at the beginning a few related facts which I have gathered from
various sources; then I shall discuss in detail the problems, methods,
and results of my several experiments.
The brain of the pigeon is well developed, although the hemispheres
are unconvoluted. When they are removed, the animal retains unaltered
its reflex and vital activities, but ceases for a time at least to
show evidence of mental life, for example, memory and will.[180] In
the normal animal sight and hearing are acute, and touch seems keen,
although the claws are not used for grasping and eating, as in the case
of more intelligent birds, especially, parrots. There is considerable
sensitiveness to temperature changes. Taste, and probably smell, appear
to be deficient.[181] The "sense of support" is marked, even in the
young.[182]
Since the pigeon seems to dream and also to miss its absent mate, some
observers believe that imagery is present. There is certainly local
memory, and also capacity to observe. Various intelligent acts have
been reported.[183] The remarkable homing habits of the carrier pigeon
have received no satisfactory explanation. While Cyon[184] suggests the
stimulation of the nasal organs by air currents, Thauzièr[185] holds
to the electrical theory; they agree, however, that certain higher
psychical processes are probably involved.
Graber's[186] tests indicate that pigeons have no color-preference.
Beebe's[187] statement concerning birds in general is peculiarly true
of pigeons: "There are few species which do not show the emotions of
love and sympathy, and ... one will sometimes pine and die of grief at
the loss of its mate." After referring to their patient care of the
young, he adds: "Indeed, sympathy is the keynote in the development
of the higher mental faculties." These birds communicate, but their
language consists of comparatively few sounds. As in many other birds,
the play-instinct is highly developed.
II. PROBLEMS AND METHODS
My study of the pigeon's _emotional life_ had for its object certain
respiratory "expressions." These were investigated by means of a
pneumographic tracing, secured while the animal was comfortably
fastened in a shallow nest, partially open below. A small box was
placed over the bird, and apparatus was so arranged that the time
of giving various stimuli was recorded automatically on the smoked
paper of the kymograph drum, below the breathing-curve. A third line
indicated rate of drum movement. Although some interesting results
were obtained, the chief significance of the research consists in its
demonstration of the fact that this method of studying animal mind is
valuable.
In the study of _association_ I sought to determine the sense-data
which the process involves, its method of formation (with due regard
to social conditions), its rapidity, permanence, and modifiability,
and also its probable degree of complexity. Material contributing to
the subject was secured by observing the behavior of the animal when
seeking to obtain food by overcoming such obstacles as labyrinths with
wire passages, and latches, when the food was left in view, or by
finding it when out of sight. In the latter case it was placed in a box
occupying a customary place in a group of exactly similar boxes, or
else in a box of color or form unlike the other members of the group
and variously arranged, from time to time, with respect to them. When
the animals were learning the labyrinth habits, various stimulations
were given them; later the character of some of these was altered, and
the resulting changes in behavior were noted. After the habits had
been thoroughly learned, the birds were given a rest for some weeks,
and then tested again under the old conditions. A few trials were
arranged with special reference to the study of imitation; the animals
here were tested as to their ability to execute simple but unfamiliar
acts, after having only seen them performed by an animal previously
trained. Throughout the associational tests the animals very seldom
received food in their cages; but as they were tested daily and allowed
to satisfy their hunger completely at the last test, they were never
in a state of "utter hunger"--a condition which most experimenters
think best to avoid. A series of tests, given at the conclusion of the
investigation, indicated that the odor of the food had not assisted the
animals in reaching it.
In the two series of experiments (emotion and association) thirty-five
animals in all were used. They were confined in large cages in a fairly
well lighted and ventilated room, and were fed wheat, cracked corn,
and occasionally fruit, and kept well supplied with fresh water and
sand. They generally remained in a healthy condition throughout the
tests, especially during the winter. To exclude, as far as possible,
the disturbing influence of fear, they were usually handled only
after the room had been darkened. As the noise made by the curtains
was objectionable, the birds were tested with the room illuminated by
incandescent lamps; the light was turned off before the birds were
placed in position for the trials, and again before they were removed
from the apparatus to the cages. It is generally agreed that an
experimenter should be out of sight when giving a test. I am convinced
that it is important to avoid being seen by the animals at any time.
This involves great inconvenience, especially when one employs the
pneumographic method, but better results are thus obtained.
For practical suggestions as to apparatus and methods I am
greatly indebted to Dr. Robert MacDougall, at the beginning of my
investigation, and to Dr. Robert M. Yerkes, throughout. I also owe
much to the researches of Zoneff and Meumann,[188] Thorndike,[189]
Mills,[190] Small,[191] and Kinnaman.[192] Porter's[193] interesting
study of sparrows was made almost simultaneously with the investigation
here reported. Fewer animals were used by him, but in some instances
more tests were given.
III. INVESTIGATION OF EMOTION[194]
1. _Respiration in general._ The normal breathing-curve in pigeons
is quite similar in contour to that of the human subject, although
the rhythm is more rapid and the pauses are less pronounced. When
acoustical, visual, olfactory, or tactual stimuli are given, various
modifications appear, for example, quickening, deepening, and minor
irregularities. It was noticed that meaningless stimuli (pistol-shots)
quickly lose their disturbing influence, whereas the breathing remains
sensitive to those of a significant character, such as the noises
made by other birds. It was also found that a stimulus which no
longer affects the breathing will sometimes occasion disturbance if
accompanied by a second stimulus of another order, although of a weak
intensity (summation).
2. _Respiratory reactions to light._ As the easy control of conditions
makes vision an excellent field in which to work, light reactions were
investigated in detail. Two distinct series of tests were given. One
sought to determine the relation between quality of light and reaction;
the other, between intensity of light and reaction. Four colors of one
intensity and three intensities of one color, respectively, were used.
In the first series four stimuli, one for each of the colors, red,
yellow, green, and blue, were given daily; in the second series five
daily stimuli were given, of the same intensity for any one day, and
one minute apart; this made it possible to observe also the effect of
repetition. Each stimulus was given at the beginning of a respiration
and continued two seconds. When the tracings were studied, various
modifications were noted, but special attention was paid to alterations
in rate of breathing. In the case of both sets of trials an immediate
quickening usually occurred after stimulation, and occasionally
shallowing[195] and minor irregularities of contour.
In the first set of tests ten animals were used for twenty-five days.
Average results indicated that red and yellow are less stimulating
than green and blue. To secure data that would assist in the
interpretation of these results, an investigation was made of the
animals' color-preference. This was done by recording, at thirty-minute
intervals, the position of the birds when confined, singly, in a
box one half of which was illuminated (from the side) by light of
one color, and one half by light of different color but of the same
intensity. A water screen excluded the heat rays. After nine records
had been taken the colored glasses were interchanged, and the animal's
position relatively to the two colors was observed as before. This was
repeated with the other colors until each of the four had been used
with each of the other three. There were far more choices of green and
blue than of red and yellow, though none of the colors was avoided.
It seemed a question of _degree of liking_, rather than of liking or
disliking. As stated, Graber's experiments indicated that pigeons have
no color-preference, but his results are probably untrustworthy, since
he tested several animals at once and apparently was not careful to
change the colored glasses regularly. Putting together our two sets of
data (the latter stated first) we have the following comparison:
R Y G B
Color choices of }
5 animals } 72 129 167 172
Breathing-rise of}
10 animals } 9.94% 10.39% 10.41% 12.11%
Although the proportions do not hold, there is a direct correspondence
between the two series of responses; hence it would seem that
_increased respiratory activity is an expression of agreeable feeling_
in pigeons, and this especially since the breathing, when varying at
all in amplitude, usually became shallower, and also showed certain
minor irregularities of contour, as often occurs in human respiration
during moderate stimulation of a pleasant character.[196]
In the second series of respiratory tests four animals were used
for fifteen days. Average results showed nothing as to the relation
between intensity of stimulus and amount of quickening, since the three
intensities used, 1, 2, and 4, produced reactions, respectively, as
follows: 6.6%, 4.3%, and 6.4%. This may have been because the three
intensities were employed each on different days. When the reactions
are averaged according to daily succession, without regard to the
intensities of the stimuli, we get the following results: first
reaction, 8.0% rise in rate; second, 3.7%; third, 4.1%; fourth, 5.7%;
and fifth, 6.9%. We should have expected the second daily response to
be less vigorous than the first, since the animals were perhaps better
prepared for the second stimulation. That the reactions increased
thereafter was probably due, partially to summation, and partially
to the fact that the short illuminations occasioned mental action
(_perception_ of interior of box, increased _desire_ to escape,
etc.) which involved heightened, rather than depressed, breathing
activity, and thus worked directly against the dulling tendency of
repetition.[197]
IV. INVESTIGATION OF ASSOCIATION
1. _Labyrinth experiments._ Four labyrinths in all were used (L, M, H,
O). Each was constructed by attaching moveable wire partitions in a
wooden box, covered with chicken wire. The pigeon was admitted through
a small entrance compartment which was fastened at one end of the box,
and which communicated with it by means of a lifting door, operated by
pulling a cord from behind the observation curtain. Food was placed
within the maze, and usually at the opposite end. Before beginning the
tests the bird was allowed to become thoroughly familiar with the box
without the partitions. After a few trials it learned to go to the food
immediately upon entering the box. The partitions were then put into
position, and the bird was tested as to the time required (except in
the case of labyrinth O) and as to the method employed in reaching the
food. The time was measured by means of a stop-watch, and the bird's
horizontal movements were recorded on a small plot of the labyrinth;
other general observations were added.
A. _Habits in Labyrinth L_
[Illustration: FIG. 1. Labyrinth L. BB, box 6 in. high; E, entrance;
F, feeding-place; a, b, c, and d, moveable partitions 6 in. apart; NN,
edge of pigeon cage; D, observation screen. The lamps which illuminated
the room hung directly above the apparatus.]
In this labyrinth (Fig. 1) six animals were tested once daily for
thirty days, and five of these again after two and six weeks,
respectively. On entering the labyrinth with the partitions in place
the first time, a bird started on its usual direct course toward the
food-box; running against the first partition it made vigorous efforts
to push through, flying at the wire and often clinging to it for a
short time; some of these random movements eventually brought it to
the left of the compartment, and thence, through the opening, into
the second compartment, and so on through the others, until finally
it reached the food by a series of fortunate accidents.[198] The same
general reaction was shown in case of the next few tests, except that
fewer and fewer useless movements were made, and that the right ones
were carried out with greater and greater precision. Later the animal
had no difficulty in reaching the food; it did not run against the
partitions, enter the blind alley, nor display such general signs of
uncertainty as pausing and looking about. The process of learning in
this case was obviously one of "trial and error," or the selection
of useful movements. From the mass of random movements constituting
the reaction to the unfamiliar environment, only those which enabled
the bird to reach the food were retained and improved; the others
gradually disappeared until finally the path taken became the shortest
one possible, and was entered upon and pursued without hesitation by
each animal as soon as it was allowed to enter the labyrinth. The time
required for the tests is given in Table I.
It will be seen at a glance that the absolute time required for
reaching the food varied for the individuals (see especially the
results given by different birds in the case of test 1), but that the
several periods for any one bird were relatively similar to those
for another; and also that the time was long at first, but rapidly
shortened from test to test, thus showing a steady advance in the
learning process. Various lapses occurred (for example, A, 8; C, 13; E,
10) after the habit had been fairly well fixed.
In tests 18-22 the time-shortening was due principally to quickening of
movements which had already become well defined. The great importance
of visual data is brought out by the abrupt lengthening of the periods
in the case of tests 23-25, and 26-30, where the light intensities
were decreased. The lengthening was roughly proportional to the change
of illumination. In the relative darkness the birds had to re-acquire
the habits. The same mistakes were made as at first (running against
partitions, and into the blind alley), yet here, as before, there was
a ready adjustment. That the food was out of sight, or at least very
much less visible, probably made no difference, since it was found
that the birds would readily go to the old place after both food and
food-box had been removed. In order to exclude the light entirely
without making their movements invisible to me, I blindfolded the birds
by means of a thin black hood, comfortably adjusted over their eyes and
top of head; as a result, none was able to make the course in twenty
minutes. The first turn, however, was usually made naturally, perhaps
because associated with certain non-visual sense-data (sound of the
lifting door, and perhaps tactual impressions of the close entrance
compartment, etc.). Rats[199] seem far less dependent upon visual data
than do pigeons. The great permanence of the pigeons' habits is shown
by comparing the periods for tests 31-3, given after two and six weeks
of rest, respectively, with those for tests 18-22.
TABLE I. TIME REQUIRED TO REACH FOOD IN LABYRINTH L
_Animals_
_Trials_, A B C D E F _Average_
1 daily.
' " ' " ' " ' " ' " ' " ' "
{ 1) 28:50 :59 42:20 49:04 22:13 4:04 24:35
{ 2) 7:22 :22 25:47 10:17 :48 2:02 7:46
{ 3) 1:18 :12 8:29 12:35 :12 1:41 4:05
{ 4) :32 :21 10:51 1:26 :19 :52 2:24
{ 5) :24 :28 2:36 2:18 :12 1:33 1:15
{ 6) :25 :26 1:10 :55 :12 1:50 :50
{ 7) :15 :24 :28 :32 :15 2:09 :41
{ 8) 1:05 :23 :33 1:19 :17 1:46 :54
1 { 9) :16 :24 :57 :58 :10 :26 :32
{10) :24 :32 1:15 :51 2:12 :31 :58
{11) :12 :21 1:40 :30 :17 :54 :39
{12) :16 :32 :49 1:34 :22 1:18 :49
{13) :13 :18 2:30 :18 :10 :42 :42
{14) :29 :32 :27 :31 :25 :36 :30
{15) 1:00 :24 :30 :31 :12 :35 :32
{16) :19 :52 1:10 :22 :17 :24 :34
{17) :14 :14 :31 :39 :13 :57 :28
{18) :13 :09 :29 :13 :16 :29 :18
{19) :10 :10 :36 :26 :07 :14 :17
2 { 20) :11 :15 :34 :17 :07 :10 :16
{21) :13 :14 :34 :16 :09 :21 :18
{22) :09 :16 :26 :14 :08 :11 :14
{23) :12 :42 1:29 :39 5:53 :13 1:31
{24) :20 :17 1:31 :33 :15 :13 :31
3 { 25) :15 :24 :40 :28 :19 :20 :24
{26) 1:21 16:29 1:22 13:54 3:51 1:36 6:26
{27) 3:36 4:45 :44 1:03 1:09 2:59 2:23
4 { 28) :51 1:24 :46 1:04 :56 1:09 1:02
{29) :51 :41 1:10 :40 2:17 :11 :58
{30) 2:04 :18 :41 :14 :07 :19 :37
5 { 31) :08 :33 :25 :07 :07 :16
6 { 32) :09 :15 :20 :08 :31 :17
1: With 18-candle-power illumination of the room.
2: Same illumination; tests given after the animals had heard four
other pigeons pecking in the labyrinth.
3: With 2-candle-power illumination, other conditions the same.
4: With a slight illumination through single curtain, other conditions
the same.
5: After two weeks' rest, conditions as in 2.
6: After six weeks' rest, conditions as before.
Let us now notice the gradual progress of learning in three important
parts of the maze, as shown in Fig. 2. It will be seen that in the
beginning the animals started upon their usual course and pressed
against the first partition (stage 1), but that later they touched
it less and less (stages 2 and 3), and that finally they avoided it
entirely (stage 4). The adjustment here was fairly simple: the sound
made by the opening of the entrance door, and the glimpse thus given
of the labyrinth, gradually came to be conditions of the movements of
turning to the left, on emerging from the entrance, and passing along
the compartment toward the opening, where impressions, mostly visual,
in the same manner determined the movements of turning to the right and
entering compartment 2.
The blind alley was naturally a decided obstacle. The pigeons
learned to avoid this compartment by going around it only after many
unsuccessful attempts to go through it. During the first test the
animals entered it many times (stage 1); on emerging they returned to
the second or the first compartment, only to encounter the pen again
when they re-advanced toward the food; finally, on reëmerging from the
annoying enclosure, perhaps for the eighth or tenth time, they might
happen to turn to the right instead of going forward as usual toward
the entrance of the box, and thus make their way along the new passage
and reach the food. For the next few tests they usually entered the
blind alley, but less frequently, and they remained for shorter periods
(stages 2-4). Later they merely entered (stage 5); and still later they
passed very near the opening without entering, or only paused a moment
before it (stage 6); and finally they passed it without the slightest
hesitation, walking briskly, but with well-directed movements, midway
between the partitions (stages 7-8). The act of turning seemed to be an
especially important factor in this habit. We notice that it was a turn
to the right (most probably accidental) which first enabled the animals
to get beyond the opening of the blind alley; that this same act was
repeated in each successive trial until, by the gradual shortening of
the loop forming the path taken by the animals in passing into, and
from, the labyrinth it was finally reduced to a mere pause (stage 6);
and that this later disappeared entirely, leaving only the left turn,
which instead was now conditioned by the visual data at that part of
the labyrinth and carried the animal past the entrance of the blind
alley.
[Illustration: FIG. 2. Stages in learning Labyrinth L. The curves
indicate the pigeon's horizontal movements. Pauses are represented by
heavy dots.]
The animals did not come in contact with the second partition until
they had almost learned to pass the opening of the blind alley (see
stage 6). This was probably because the turn to the left which was
made on approaching _x_ was associated with visual data derived from
points farther along the course (_y_), and when the animals reached
_z_, compartment 2, these same data were received and were sufficient
to occasion the turn to the left there also, thus bringing the birds
against the partition. The adjustment was made principally on the basis
of new sense-data arising from running against the wire, looking at it
more closely, etc. For a few trials the birds made the turn at _x_ too
quickly, and thus failed to reach the third compartment. One of my most
intelligent subjects made this mistake in the third test, and again in
the sixth and seventh, and retained the act almost unchanged through
the tenth, eleventh, twelfth, thirteenth, fourteenth, sixteenth,
seventeenth, twentieth, and twenty-first tests, so strong was the
tendency to continue a movement once begun, though it was really
disadvantageous.[200]
[Illustration: FIG 3. Labyrinth M. E, entrance; F, food-box; height of
large box and width of passages the same as in Labyrinth L.]
After reaching the food and satisfying their hunger, the animals
often returned to the maze passages, seeking an exit; but they never
"explored" passages or showed other evidence of "free curiosity"
and "desire to know all their new surroundings," as Small reports
concerning rats.[201]
B. _Habits in Labyrinth M_
Five of the animals previously used were next tested twice daily,
forenoon and afternoon, for five days, in a larger, more complicated
maze. It had two blind alleys, and the food-box was near the centre
(see Fig. 3). The animals' general behavior was similar to that before
observed. The periods are given in Table II.
TABLE II. TIME REQUIRED TO REACH FOOD IN LABYRINTH M
_Animals_
_Trials_, A B C E F _Average_
2 daily. ' " ' " ' " ' " ' " ' "
(1) 16:25 2:55 6:33 3:26 4:11 6:42
(2) :55 4:10 2:24 3:36 :23 2:18
(3) 1:12 :55 8:27 9:06 2:07 4:21
(4) :48 :44 2:31 :34 1:04 1:08
(5) :27 :16 :14 :16 :14 :17
(6) :18 :32 :25 :15 :27 :23
(7) :14 :11 :31 :44 :30 :26
(8) :12 :16 :57 :16 :48 :30
(9) :12 :18 :23 :16 :16 :17
(10) :10 :19 :16 :15 :28 :18
Although this maze was much more difficult, it will be seen that
the animals learned the route to the food far more readily than
before. The first period in this series was only about one fourth as
long as the first period in the other, and the course was mastered
sooner (by the fifth trial instead of by the ninth). There was less
pressing against the wire than before, and unsuccessful movements were
sooner discontinued. This improvement was probably due entirely to
experience gained in dealing with the first maze. Thorndike speaks of
the gradually increasing ability of animals to deal with successive
contrivances.[202] The average results given in Tables I and II are
plotted in Fig. 4, next page.
C. _Habits in Labyrinth H_
Since hearing is an important sense in pigeons, we should expect
them to be capable of useful acoustical associations. Several things
occurred in the course of the two preceding experiments which seemed to
indicate that this is true; for example, although I could move about,
rather noisily, in the darkened room, without apparently disturbing any
of the birds, some few showed signs of fright (moving about restlessly)
on hearing the low, grating noises made by lifting the hanging door of
the cage, sounds which had always preceded the handling of the subjects
before experimentation, and which had probably become signs to them of
being taken.
[Illustration: FIG. 4. Learning-curves: A, in Labyrinth L; B, in
Labyrinth M. Divisions of ordinates indicate minutes; of abscissas,
successive daily trials. The rise on curve A for tests 23 and 26 was
due to diminution of illumination of room.]
To investigate this kind of association I constructed a labyrinth (see
Fig. 5) in connection with which sounds could be utilized as one form
of sense-data. The passages were so arranged that along the route
leading to the food there were three blind alleys, which the animals
would surely enter before mastering the course. In another part of the
room were placed, very close to each other, two electric gongs of the
same size, but of different material. One was of metal and gave a clear
ringing sound; the other was of wood and gave a low rattling noise.
When an animal was learning the route (as in the other two mazes) I
sounded the gongs, the metallic, as the bird approached and entered
the blind alleys, by openings _M_, _O_, and _R_, and the wooden,
as it emerged from them and proceeded along the proper course, and
occasionally after it had reached the food. The ringing sound was also
given after the animal passed _P_ and was approaching _Q_. When the
new route was fairly well learned, I changed the order of the sound
stimuli, ringing one gong at the places where the other had previously
been sounded, and compared the records thus obtained with those
obtained when the sounds were given in the original order. As an animal
is liable to become confused by the sounds, or else quickly accustomed
to them, I thought it best to give only a few trials, one trial with
the usual order of sound stimulations, the next immediately following
with the reversed order, and so on till four pairs of records had been
secured, the series of trials being completed in a single day. Four
animals were thus tested. The periods of the various trials are shown
in Table III.
TABLE III. SOUND ASSOCIATION, LABYRINTH H
_Time required to reach food under different sound conditions_
I
_Order of gongs the same as when_
_course was being learned._
_Animals_
_Trials_ A B E G
" " " "
(1) 13 19 24 17
(2) 16 12 13 16
(3) 10 16 20 14
(4) 12 19 10 12
Total, 51 66 67 59 "
243
II
_Order of gongs reversed._
_Animals_
_Trials_ A B E G
" " " "
(1) 14 16 18 27
(2) 37 17 16 27
(3) 14 19 26 11
(4) 27 22 21 14
Total, 92 74 81 79 "
326
In the case of thirteen of the sixteen tests given with reversed
acoustical conditions (see column II) the periods were longer than the
corresponding ones given alternately with them for comparison, and
there was an average time-lengthening of 5.2 seconds per trial, or
34.2%. The following is a short description of the animals' reactions
to the changed conditions. It corresponds to the time-values expressed
in column II of the table.
Bird A: test 1, animal undisturbed; test 2, drew back from _S_, turned
to the left and went toward _R_, but later returned and passed _S_
without pausing; test 3, paused at _O_ for a short interval, but did
not enter the blind alley; test 4, paused at _O_ again, later drew
back from _S_, turned to the left and entered blind alley 3; it soon
escaped, and this time passed _S_ without being disturbed, although it
paused at _T_ and _U_.
Bird B: tests 1 and 2, animal apparently undisturbed; test 3, a few
slight pauses at openings; test 4, drew back from _S_, entered blind
alley 3, but soon escaped and passed _S_ without hesitation.
Bird E: test 1, undisturbed; tests 2 and 4, paused at openings; test
3, turned back from _S_, entered blind alley 3, and paused at several
places later when passing toward F.
Bird G: test 1, many pauses; test 2, turned from _S_ and entered blind
alley 3; test 3, undisturbed; test 4, drew back from _S_, went toward
_R_, but did not enter the blind alley.