Sixteen Experimental Investigations from the Harvard Psychological Laboratory by Hugo Münsterberg (100 books to read .txt) 📕

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