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

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

These results indicate an increase in the respiration rate due to the

visual stimulus.

4. Of the other auditory stimuli used, the pistol-cap explosion gave

very irregular results. For one animal it caused acceleration, for

another inhibition. There is, however, good evidence that the sounds

were heard.

5. The ringing of a bell gave results similer to those for a whistle,

and the sound of a 500 S.V. tuning fork usually caused a slight

increase in the rate of breathing. In these experiments I therefore

have evidence, through their effects upon respiration, of the frog’s

ability to hear sounds ranging from 50 V. to at least 1,000 V.

The croak of the green frog ranges from 100 to 200 V., so far as I

have been able to determine. That of the bull frog is lower, from 50

to 75; and in the leopard frog the range is from 80 to 125. The latter

is very different from the green frog in its croaking, in that it

croaks whenever disturbed, whereas, the green frog rarely responds in

that way to a stimulus.

We are now in a position to say that the failure of frogs to give

motor reactions to strong auditory stimuli is not due to their

inability to be affected by the stimuli, but is a genuine inhibition

phenomenon.

XI. THE EFFECTS OF AUDITORY STIMULI ON VISUAL REACTIONS.

Further experimental evidence of hearing was gotten from some work

done to test the influence of sounds upon motor reactions to visual

stimuli. Frogs, like most other amphibians, reptiles and fishes, are

attracted by any small moving object and usually attempt to seize it.

They never, so far as I have noticed, feed upon motionless objects,

but, on the other hand, will take almost anything which moves.

Apparently the visual stimulus of movement excites a reflex. A very

surprising thing to those who are unfamiliar with frog habits is the

fear which small frogs have of large ones. Put some green frogs or

small bull frogs into a tank with large bull frogs, and the little

ones will at once show signs of extreme fear; they jump about in the

most excited manner and try hard to escape. The cause of their fear

soon appears, since it is usually only a few minutes until the little

ones are swallowed by their wide-mouthed, cannibalistic fellows.

It is, moreover, well known that a bit of red flannel fastened to a

hook attracts frogs and is an excellent method of capturing them. Red

seems to be the color which they most readily notice.

This tendency of the frog to attempt to seize any moving object I made

use of to test the value of sounds. By placing a frog in a glass

aquarium which was surrounded by a screen, back of which I could work

and through a small hole in which I was able to watch the animal

without being noticed by it, and then moving a bit of red cardboard

along one side of the aquarium, I could get the frog to jump at it

repeatedly. In each attempt to get the moving object, the animal

struck its head forcibly against the glass side of the aquarium. There

was, therefore, reason to think that a few trials would lead to the

inhibition of the reaction. Experiment discovered the fact that a

hungry frog would usually jump at the card as many as twenty times in

rapid succession.

In this reaction to a visual stimulus there appeared good material for

testing audition. I therefore arranged a 500 S.V. tuning fork over the

aquarium and compared the reactions of animals to the visual stimulus

alone, with that to the visual stimulus when accompanied by an

auditory stimulus. The tuning-fork sound was chosen because it seemed

most likely to be significant to the frog. It is similar to the sounds

made by the insects upon which frogs feed. For this reason one would

expect that the sight of a moving object and the sound of a

tuning-fork would tend to reënforce one another.

The experiments were begun with observations on the effects of moving

objects on the respiration. In case of a normal rate of 54

respirations per minute sight of the red object caused an increase to

58. Then the same determination was made for the auditory stimulus.

The tuning-fork usually caused an increase in rate. In a typical

experiment it was from 65 per minute to 76. The observations prove

conclusively that the 500 S.V. sound is heard. My attention was turned

to the difference of the environment of the ear in its relation to

hearing. Apparently frogs hear better when the tympanum is partially

under water than when it is fully exposed to the air.

Having discovered by repeated trials about how vigorously and

frequently a frog would react to the moving red card, I tried the

effect of setting the fork in vibration a half minute before showing

the card. It was at once evident that the sound put the frog on the

alert, and, when the object came into view, it jumped at it more

quickly and a greater number of times than when the visual stimulus

was given without the auditory. This statement is based on the study

of only two animals, since I was unable to get any other frogs that

were in the laboratory at the time to take notice of the red

cardboard. This was probably because of the season being winter. I

venture to report the results simply because they were so definite as

to point clearly to the phenomenon of the reënforcement of the

visual-stimulus reaction by an auditory stimulus.

Concerning the influence of this combining of stimuli on the reaction

time, I am only able to say that the reaction to the moving object

occurred quicker in the presence of the auditory stimulus. When the

red card was shown it was often several seconds before the frog would

notice it and attempt to get it, but when the sound also was given the

animal usually noticed and jumped toward the moving card almost

immediately.

Unfortunately I have thus far been unable to get chronoscopic

measurements of the reaction times in this reënforcement phenomenon. I

hope later to be able to follow out the interesting suggestions of

these few experiments in the study