Various - Harvard Psychological Studies, Volume 1

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For convenience of comparison, again, the averages for the electrical
reaction time of subjects _A_, _B_, _C_, _D_, _E_ and _F_, and the
same for the tactual reaction time of subjects 1, 2, 3, 4, 5 and 6 are
herewith given together. All averages are for twenty reactions, except
for _D_ and 5, for which there are ten.

Besides the usual determination for the tactual reaction-time work on
the six subjects named, there is given in Table XVI. the electrical
reaction time of these animals to a two-cell current. Comparison of
the electrical and tactual results are of interest in this case
because the mean variation for each is about 34[sigma], being
34.3[sigma], for the electrical and 33.8[sigma], for the tactual.

TABLE XV.

Average of 20 Electrical Average of 20 Tactual
Frog. Reactions. Frog. Reactions.
_A_ 149.5[sigma] 1 188.3[sigma]
_B_ 158.3 2 199.1
_C_ 191.0 3 212.1
_D_ 167.0 4 213.0
_E_ 182.4 5 199.8
_F_ 176.3 6 221.9
Gen. Avs. 167.9 205.7

TABLE XVI.

REACTION TIME FOR TACTUAL AND ELECTRICAL STIMULI.

Tactual Reaction Time. Electrical Reaction Time.

Frog. Average. Mean Variation. Average. Mean Variation.

1 188.3[sigma] 167.3[sigma]
2 199.1 180.1
3 212.1
4 213.0 210.3
5¹ 199.8 138.5
6 221.9 164.4
Gen. Avs. 205.7 33.8 172.1 34.3

¹For 5 the average of ten instead of twenty is given.

VIII. EQUAL VARIABILITY AS A CRITERION OF COMPARABILITY OF REACTION
TIME FOR DIFFERENT KINDS OF STIMULI.

Since variability as indicated in the study of the influence of
different strengths of electrical stimulus becomes less as the
stimulus increases, parity in variability for different stimuli offers
a basis for the comparison of reaction times. Certain it is that there
is no use in comparing the reaction times for different senses or
different qualities of stimuli unless the relative values of the
stimuli are taken into consideration; but how are these values to be
determined unless some such index as variability is available? If the
reaction time to tactual stimuli as here presented is to be studied in
its relation to the electrical reaction time, it will mean little
simply to say that the former is longer than the latter, because the
electrical reaction time for a one-cell stimulus happens to be
somewhat less than that for the particular tactual stimulus used. For
it is clear that this tactual reaction time is really shorter than the
reaction time to a weak current. In making variability a basis of
comparison it must be assumed that the strength of stimulus is the
important factor, and that all other variable conditions are, so far
as possible, excluded. If, now, on the basis of parity in variability
we compare the tactual and electrical reaction times, it is apparent
that the tactual is considerably longer. The tactual average of Table
XV. is 205.7[sigma], while the electrical reaction time which has
approximately the same variability is 172.1[sigma]. It may well be
objected that I have no right to make variability the basis of my
comparison in these experiments, because the work for the various
kinds of stimuli was done under different conditions. Admitting the
force of this objection, and at the same time calling attention to the
fact that I do not wish to lay any stress on the results of the
comparisons here made, I take this opportunity to call attention to
the possibility of this criterion.

The use of variability as a basis of comparison would involve the
assumptions (1) that a certain intensity of every stimulus which is to
be considered is capable of producing the shortest possible, or reflex
reaction, and that this reaction is at the same time the least
variable; (2) that as the strength of a stimulus decreases the
variability increases until the threshold is reached.

Suppose, now, it is our desire to compare the results of reactions to
different intensities of electrical and tactual stimuli; let the
figures be as follows:

Reaction Time. Variability.
Stimulus Strength. Elect. Touch. Elect. Touch.
8 50[sigma] 50[sigma] 10[sigma] 10[sigma].
4 130 155 25 30
2 175 220 40 40
1 300 320 50 60

In the double columns the results for electrical stimuli are given
first, and in the second column are the tactual. Stimulus 8 is assumed
to be of sufficient strength to induce what may be designated as
forced movement, and whatever the quality of the stimulus this
reaction time is constant. I make this statement theoretically,
although all the evidence which this work furnishes is in support of
it. So, likewise, is the variability of this type of reaction time
small and nearly constant. At the other extreme, stimulus 1 is so weak
as to be just sufficient to call forth a response; it is the so-called
threshold stimulus. Whether all qualities of stimulus will give the
same result here is a question to be settled by experimentation. Wundt
contends that such is the case, but the observations I have made on
the electrical and tactual reactions of the frog cause me to doubt
this assumption. It seems probable that the 'just perceptible stimulus
reaction time' is by no means the same thing for different qualities
of stimulus. Those modifications of the vital processes which alone
enable organisms to survive, make their appearance even in the
response to the minimal stimulus. In one case the just perceptible
stimulus may cause nothing more than slight local changes in
circulation, excretion, muscular action; in another it may produce,
just because of the particular significance of the stimulus to the
life of the organism, a violent and sudden motor reaction. But grant,
if you will, that the threshold reaction time is the same for all
kinds of stimuli, and suppose that the variability is fairly constant,
then, between the two extremes of stimuli, there are gradations in
strength which give reaction times of widely differing variabilities.
If, now, at some point in the series, as, for instance, to stimulus 2,
the variability for different kinds of stimuli is the same either with
reference to the reaction time (ratio) or absolutely, what
interpretation is to be put upon the fact? Is it to be regarded as
merely a matter of chance, and unworthy of any special attention, or
should it be studied with a view to finding out precisely what
variability itself signifies? It is obvious that any discussion of
this subject, even of the possible or probable value of variability as
a criterion for the comparative study of stimuli, can be of little
value so long as we do not know what are the determining factors of
variations of this sort. The only suggestion as to the meaning of such
a condition (_i.e._, equal variability at some point)--and our studies
seem to show it for touch and electrical stimulation--which I feel
justified in offering at present, is that parity in variability
indicates equality in strength of stimuli, that is, the electrical
stimulus which has a reaction time of the same variability as a
tactual stimulus has the same effect upon the peripheral nervous
system as the tactual, it produces the same amplitude and perhaps the
same form of wave, but the reaction times for the two stimuli differ
because of the biological significance of the stimuli. The chances are
that this is wholly dependent upon the central nervous system.

IX. SUMMARY.

1. This paper gives the results of some experiments on the frog to
determine its electrical and tactual reaction time. It is the
beginning of comparative reaction-time studies by which it is hoped
important information may be gained concerning the significance and
modes of action of the nervous system. Comparative physiology has
already made clear that the time relations of neural processes deserve
careful study.

2. According to the strength of the stimulus, electric stimulation of
the frog causes three types of reaction: (1) A very weak or threshold
stimulus results in a deliberate or delayed reaction, the time of
which may be anywhere from 300[sigma] (thousandths of a second) to
2,000[sigma]. (2) A very strong stimulus causes a spinal reflex, whose
time is from 50 to 80[sigma]; and (3) a stimulus of intermediate
strength causes a quick instinctive reaction of from 150 to 170[sigma]
in duration.

3. The reaction time for electric stimuli whose relative values were
1, 2 and 4 were found to be 300.9[sigma], 231.5[sigma] and
103.1[sigma].

4. The reaction time of the frog to a tactual stimulus (contact of a
rubber point) is about 200[sigma].

5. The variability of reaction times of the frog is great, and
increases as the strength of the stimulus decreases.

6. When two kinds of stimuli (_e.g._, electrical and tactual) give
reaction times of equal variability, I consider them directly
comparable.

7. According to this criterion of comparability the reaction time to
electric stimulation which is comparable with that to tactual is
172.1[sigma]; and it is to be compared with 205.7[sigma]. Both of
these have a variability of approximately 34[sigma]. On this basis one
may say that the tactual reaction time is considerably longer than the
electrical.

PART III. AUDITORY REACTIONS OF FROGS.

X. HEARING IN THE FROG.

A. Influences of Sounds in the Laboratory.

After determining the simple reaction time of the green frog to
tactual and electrical stimulation, I attempted to do the same in case
of auditory stimuli. In this I was unsuccessful because of failure to
get the animal to give a motor response which could be recorded. The
animal was placed in an experimenting box with a string attached to
one hind leg as in the experiments described in Part II., and after it
had become accustomed to the situation a sound was made. A wide range
of sounds were tried, but to none except the croak of another frog was
a motor reaction frequently given. Even a loud noise, such as the
explosion of a large pistol cap, caused a visible motor reaction only
in rare cases. In fifty trials with this stimulus I succeeded in
getting three reactions, and since all of them measured between 230
and 240[sigma] it is perhaps worth while to record the result as
indicative of the auditory reaction time. As these were the only
measurements obtained, I have no satisfactory basis for the comparison
of auditory with other reaction times.

The remarkable inhibition of movement shown by the frog in the
presence of strong auditory stimulation, at least what is for the
human being a strong stimulus, led me to inquire concerning the limits
and delicacy of the sense of hearing in frogs. In the vast quantity of
literature on the structure and functions of the sense organs of the
animal I have been able to find only a few casual remarks concerning
hearing.

In approaching the problem of frog audition we may first examine the
structure of the ear for the purpose of ascertaining what sounds are
likely to affect the organ. There is no outer ear, but the membrana
tympani, or ear drum, covered with skin, appears as a flat disc from 5
to 10 mm. in diameter on the side of the head just back of the eye and
a little below it. In the middle ear there is but one bone, the
columella, forming the connecting link between the tympanum and the
internal ear. The inner ear, which contains the sense organs,
consists of a membranous bag, the chief parts of which are the
utriculus, the sacculus, the lagena, and the three semicircular
canals. The cavity of this membranous labyrinth is filled with a
fluid, the endolymph; and within the utriculus, sacculus and lagena
are masses of inorganic matter called the otoliths. The auditory nerve
terminates in eight sense organs, which contain hair cells. There is
no cochlea as in the mammalian ear. The assumption commonly made is
that vibrations in the water or air by direct contact cause the
tympanic membrane to vibrate; this in turn causes a movement of the
columella, which is transmitted to the perilymphatic fluid of the
inner ear. The sensory hair cells are disturbed by the movements of
the otoliths in the endolymph, and thus an impulse is originated in
the auditory nerve which results in a sensation more or less
resembling our auditory sensation. It is quite probable that the
frog's sense of hearing is very different from ours, and that it is
affected only by gross air vibrations. This conclusion the anatomy of
the ear supports.

Although there does not seem to be a structural basis for a delicate
sense of hearing, one must examine the physiological facts at hand
before concluding that frogs do not possess a sense of hearing similar
to our own. First, the fact that frogs make vocal sounds is evidence
in favor of the hearing of such sounds at least, since it is difficult
to explain the origin of the ability to make a sound except through
its utility to the species. Granting, however, that a frog is able to
hear the croaks or pain-screams of its own species, the range of the
sense still remains very small, for although the race of frogs makes a
great variety of sounds, any one species croaks within a narrow range.

Having satisfied myself that motor reactions for reaction-time
measurements could not be gotten to any ordinary sounds in the
laboratory, I tried the effect of the reflex croaking of another frog
of the same species. In attempting to get frogs to croak regularly, I
tested the effect of removing the brain. The animals are said to croak
reflexly after this operation whenever the back is stroked; but for
some reason I have never been successful in getting the reaction
uniformly. In many cases I was able to make normal animals croak by
rubbing the back or flanks, and to this sound the animals under
observation occasionally responded by taking what looked like an
attitude of attention. They straightened up and raised the head as if
listening. In no case have other motor responses been noticed; and the
above response was so rare that no reaction-time measurements could be
made.

Again, while working with the green frog on habit formation, I one day
placed two animals in a labyrinth from which they could escape by
jumping into a tank of water. Several times when one frog jumped into
the water I noticed the other one straighten up and hold the
'listening' or 'attentive' attitude for some seconds. As the animals
could not see one another this is good evidence of their ability to
hear the splash made by a frog when it strikes the water.

B. Influence of Sounds in Nature.

In order to learn how far fear and artificial conditions were causes
of the inhibition of response to sounds in the laboratory, and how far
the phenomenon was indicative of the animal's inability to perceive
sounds, I observed frogs in their native haunts.

By approaching a pond quietly, it is easy to get within a few yards of
frogs sitting on the banks. In most cases they will not jump until
they have evidence of being noticed. Repeatedly I have noted that it
is never possible to get near to any frogs in the same region after
one has jumped in. In this we have additional proof that they hear the
splash-sound. To make sure that sight was not responsible for this
on-guard condition in which one finds the frogs after one of their
number has jumped into the water, I made observations on animals that
were hidden from one another. The results were the same. I therefore
conclude that the splash of a frog jumping into the water is not only
perceived by other frogs in the vicinity, but that it is a peculiarly
significant sound for them, since it is indicative of danger, and
serves to put them 'on watch.'

A great variety of sounds, ranging in pitch from a low tone in
imitation of the bull frog's croak to a shrill whistle, and in
loudness from the fall of a pebble to the report of a pistol, were
tried for the purpose of testing their effects upon the animals in
their natural environment. To no sound have I ever seen a motor
response given. One can approach to within a few feet of a green frog
or bull frog and make all sorts of noises without causing it to give
any signs of uneasiness. Just as soon, however, as a quick movement is
made by the observer the animal jumps. I have repeatedly crept up very
close to frogs, keeping myself screened from them by bushes or trees,
and made various sounds, but have never succeeded in scaring an animal
into a motor response so long as I was invisible. Apparently they
depend almost entirely upon vision for the avoidance of dangers.
Sounds like the splash of a plunging frog or the croak or pain-scream
of another member of the species serve as warnings, but the animals do
not jump into the water until they see some sign of an unusual or
dangerous object. On one occasion I was able to walk to a spot where a
large bull frog was sitting by the edge of the water, after the frogs
about it had plunged in. This individual, although it seemed to be on
the alert, let me approach close to it. I then saw that the eye turned
toward me was injured. The animal sat still, despite the noise I made,
simply because it was unable to see me; as soon as I brought myself
within the field of vision of the functional eye the frog was off like
a flash.

Many observers have told me that frogs could hear the human voice and
that slight sounds made by a passer-by would cause them to stop
croaking. In no case, however, have such observers been able to assert
that the animals were unaffected by visual stimuli at the same time. I
have myself many times noticed the croaking stop as I approached a
pond, but could never be certain that none of the frogs had seen me.
It is a noteworthy fact that when one frog in a pond begins to croak
the others soon join it. Likewise, when one member of such a chorus is
frightened and stops the others become silent. This indicates that the
cessation of croaking is a sign of danger and is imitated just as is
the croaking. There is in this fact conclusive evidence that the
animals hear one another, and the probability is very great that they
hear a wide range of sounds to which they give no motor reactions,
since they do not depend upon sound for escaping their enemies.

The phenomenon of inhibition of movement in response to sounds which
we have good reason to think the frogs hear, and to which such an
animal as a turtle or bird would react by trying to escape, is thus
shown to be common for frogs in nature as well as in the laboratory.
This inhibition is in itself not surprising, since many animals
habitually escape certain of their enemies by remaining motionless,
but it is an interesting phenomenon for the physiologist. We have to
inquire, for instance, what effects sounds which stimulate the
auditory organs and cause the animal to become alert, watchful, yet
make it remain rigidly motionless, have on the primary organic rhythms
of the organism, such as the heart-beat, respiration, and peristalsis.
It is also directly in the line of our investigation to inquire how
they affect reflex movements, or the reaction time for any other
stimulus--what happens to the reaction time for an electrical
stimulus, for example, if a loud noise precede or accompany the
electrical stimulus.

For the purpose of determining the range of hearing in the frog, I was
driven to study the influence of sounds upon respiration. Although the
animals did not make any detectable movement, not even of an eyelid,
in response to noises, it seemed not improbable that if the sounds
acted as auditory stimuli at all, they would in some degree modify the
form or rate of the respiratory movement.

C. Influence of Sounds on Respiration.[16]

[16] For full discussion of the normal respiratory movements of
the frog see Martin, _Journal of Physiology,_ Vol. 1., 1878,
pp. 131-170.

The method of recording the respiration was the direct transference of
the movement of the throat by means of a pivoted lever, one end of
which rested against the throat, while the other served as a marker on
a revolving drum carrying smoked paper. The frog was put into a small
box, visual stimuli were, so far as possible, excluded and the lever
was adjusted carefully; a record was then taken for at least half a
minute to determine the normal rate of respiration in the absence of
the stimulus whose effect it was the chief purpose of the experiment
to discover. Then, as soon as everything was running smoothly, the
auditory stimulus was given. The following records indicate the
effects of a few stimuli upon the rate of breathing:

1. Stimulus, 100 V. tuning fork.

Number of respirations for 10 cm. _before_ stimulus 18.0, 17.0; number
of respirations for 10 cm. _after_ stimulus 19.0, 17.3.

The records indicate very little change, and contradict one another.
For the same stimulus the experiment was tried of taking the normal
respiration record for a complete revolution of the drum, and then at
once taking the record for the same length of time (about two minutes)
with the tuning-fork vibrating close to the frog. The following result
is typical and proves that the sound has little effect.

Number of respirations in a revolution _before_ stimulus: First rev.
88; second rev. 88. Number of respirations in a revolution _during_
stimulus: First rev. 87; second rev. 88.

Concerning the influence of tuning-fork stimuli more will be said
later in a consideration of the effects of auditory stimuli upon
reactions to visual stimuli.

2. The influence of falling water as an auditory stimulus. Water was
allowed to fall about two feet in imitation, first, of a plunging
frog, and second, of water falling over rocks. In representing the
effect of the stimulus on the rate of respiration, I have given the
distance on the drum covered by the ten complete respirations just
preceding the stimulus and the ten following it.

10 Respirations. 10 Respirations.
_Before_ Stimulus. _After_ Stimulus.
1st Stim. 13.0 cm. 11.8 cm.
2d Stim. 12.7 cm. 12.7 cm.

With a smaller animal.

1st Stim. 5.4 cm. 4.8 cm.
2d Stim. 4.9 cm. 4.7 cm.
Average for 5 5.00 cm. 4.86 cm.

_These records show a marked increase in the rate of respiration just
after the auditory stimulus is given for the first time._ The stimulus
has less effect when repeated after an interval of one or two minutes,
and if repeated several times it finally causes no noticeable change.
On the whole, the sound of falling water seems to arouse the animals
to fuller life. The stimulus appears to interest them, and it
certainly accelerates respiration. This is precisely what one would
expect from a sound which is of special significance in the life of
the animal.

3. In case of a loud shrill whistle inhibition of respiration
resulted. This probably means that the frogs were frightened by the
sound. Falling water served rather to excite their natural-habitat
associations, whereas, the whistle, being an uncommon and unassociated
sound, caused fear. It is evident to the casual observer that the frog
sometimes inhibits and sometimes increases its respiratory movements
when frightened, so the result in this experiment is in no way
surprising. I am by no means certain, however, that a longer series of
observations on several individuals would give constant inhibitory
results. My immediate purpose in the work was to get evidence of
hearing; the respiratory changes were of secondary importance,
although of such great interest that I have planned a more thorough
special study of them for the future.

A few sample results showing the influence of the whistle upon a small
bull-frog follow:

Length of 10 Resps. Length of 10 Resps.
_Before_ Stimulus in cm. _After_ Stimulus in cm.
1st Stim. 6.0 6.7
2d " 5.4 6.0
3d " 5.9 5.8
1st " 4.7 5.4
2d " 4.4 4.6

As a test-check observation for comparison, the influence of a visual
stimulus upon respiration was noted under the same conditions as for
the auditory. Effect of turning on electric light over box.

Length in cm. of 10 Resps. Length in cm. of 10 Resps.
_Before_ Stimulus. _After_ Stimulus.
4.8 4.4
5.3 4.6
4.5 4.0