STUDIES IN ANIMAL PSYCHOLOGY. — часть 3

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

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.

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:

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.

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 of reënforcement and inhibition as caused by simultaneously given stimuli.

A few observations made in connection with these experiments are of general interest. The frog, when it first sees a moving object, usually draws the nictitating membrane over the eye two or three times as if to clear the surface for clearer vision. Frequently this action is the only evidence available that the animal has noticed an object. This movement of the eye-lids I have noticed in other amphibians and in reptiles under similar conditions, and since it always occurs when the animals have need of the clearest possible vision, I think the above interpretation of the action is probably correct.

Secondly, the frog after getting a glimpse of an object orients itself by turning its head towards the object, and then waits for a favorable chance to spring. The aiming is accurate, and as previously stated the animal is persistent in its attempts to seize an object.

XII. THE PAIN-SCREAM OF FROGS.

While making measurements of the frog's reaction time to electrical stimulation, I noticed that after a few repetitions of a 2-volt, .0001-ampère stimulus an animal would frequently make a very peculiar noise. The sound is a prolonged scream, like that of a child, made by opening the mouth widely. The ordinary croak and grunt are made with closed or but slightly opened mouth. The cry at once reminds one of the sounds made by many animals when they are frightened. The rabbit, for example, screams in much the same way when it is caught, as do also pigs, dogs, rats, mice and many other animals. The question arises, is this scream indicative of pain? While studying reaction time I was able to make some observations on the relation of the scream to the stimulus.

First, the scream is not given to weak stimuli, even upon many repetitions. Second, it is given to such strengths of an electrical stimulus as are undoubtedly harmful to the animal. Third, after a frog has been stimulated with a strong current (two volts), until the scream is given with almost every repetition, it will scream in the same way when even a weak stimulus is applied. If, for instance, after a two-volt stimulus has been given a few times, the animal be merely touched with a stick, it will scream. It thus appears as if the strong stimulus increases the irritability of the center for the scream-reflex to such an extent that even weak stimuli are sufficient to cause the reaction. Are we to say that the weak stimulus is painful because of the increased irritability, or may it be concluded that the reflex is in this case, like winking or leg-jerk or the head-lowering and puffing, simply a forced movement, which is to be explained as an hereditary protective action, but not as necessarily indicative of any sort of feeling. Clearly if we take this stand it may at once be said that there is no reason to believe the scream indicative of pain at any time. And it seems not improbable that this is nearer the truth than one who hears the scream for the first time is likely to think.

The pain-scream is of interest in this consideration of auditory reactions because it increases the range of sounds which we should expect frogs to hear if we grant the probability of them hearing their own voices.

It may be worth while to recall at this point the fact that a whistle from the human lips—the nearest approach to the pain-scream among the sounds which were used as stimuli in the experiments on respiration—caused marked inhibition of respiration. Perhaps this fact may be interpreted in the light of the pain-scream reaction. I may add that I have never seen a frog give a motor reaction to the pain-scream. Thinking it would certainly alarm the animals and cause them to make some movement which would serve for reaction-time measurements, I made repeated trials of its effects, but could never detect anything except respiratory changes.

FOOTNOTES.

1 Thorndike, Edward: 'A Note on the Psychology of Fishes,' American Naturalist. 1899, Vol. XXXIII., pp. 923-925.

2 Triplett, Norman: 'The Educability of the Perch,' Amer. Jour. Psy., 1901, Vol. XII., pp. 354-360.

3 Yerkes, Robert Mearns: 'The Formation of Habits in the Turtle,' Popular Science Monthly, 1901, Vol. LVIII., pp. 519-535.

4 Helmholtz, H.: 'Vorläufiger Bericht über die Portpflanzungsgeschwindigkeit der Nervenreizung.' Arch. f. Anal. u. Physiol., 1850, S. 71-73.

5 Exner, S.: 'Experimentelle Untersuchung der einfachsten psychischen Processe.' Pflüger's Arch., Bd. 8. 1874, S. 526-537.

6 Wundt, W.: 'Untersuchungen zur Mechanik der Nerven und Nervencentren.' Stuttgart, 1876.

7 Krawzoff, L., und Langendorff, O.: 'Zur elektrischen Reizung des Froschgehirns.' Arch. f. Anal. u. Physiol., Physiol. Abth., 1879, S. 90-94.

8 Wilson, W.H.: 'Note on the Time Relations of Stimulation of the Optic Lobes of the Frog.'Jour. of Physiol., Vol. XI., 1890, pp. 504-508.

9 Sanders-Ezn: 'Vorarbeit für die Erforschung des Reflexmechanismus in Lendentmark des Frosches.' Berichte über die Verhandlungen der Kgl. sächs. Gesellsch. d. Wissensch. zu Leipzig, 1867, S. 3.

10 Goltz, F.: 'Beiträge zur Lehre von den Functionen der Nervencentren des Frosches.' Berlin, 1869, 130 S.

11 Steiner, J.: 'Untersuchungen über die Physiologie des Froschhirns.' Braunschweig, 1885, 127 S.

12 Schrader, M.G.: 'Zur Physiologie des Froschgehirns.' Pflüger's Arch., Bd. 41, 1887, S. 75-90.

13 Merzbacher, L.: 'Ueber die Beziebungen der Sinnesorgane zu den Reflexbewegungen des Frosches.' Pflüger's Arch., Bd. 81, 1900, S. 223-262.

14 Merzbacher, L.: 'Untersuchungen über die Regulation der Bewegungen der Wirbelthiere. I. Beobachtungen an Fröschen.' Pflüger's Arch., Bd. 88, 1901, S. 453-474, 11 Text-figuren.

15 This apparatus was essentially the same as Scripture's device for the giving of tactual stimulation.

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