The same observer states that the cat readily learns to respond to articulate sounds. A 7-months-old animal not hitherto called by any name was placed in a cage. The experimenter stood at a distance of 1 meter from the front of the cage and called the name given to the animal — '' Pet." Ten seconds were allowed the animal, if neces- sary, in which to give a proper motor response. Whether or not a response was obtained the animal was fed. In con- junction with the name given, other words were called, viz., " no feed," and when these latter words were called the cat was not fed. The response on the animal 's part was to rear up on its legs at the front of the cage. On the third day this animal began to show indications of forming the habit. The animal continued to improve and on the ninth day, or after 150 trials in all had been given, it re- EESPONSE TO ARTICULATE SOUNDS 383 sponcled to the name 19 times in 20 trials. But it had not learned wholly to inhibit on the no feed call. On the thir- teenth day, or after 250 trials, the cat had perfected the association. The words were called in varying tones of voice, — quite loud tones and very low ones. As a third test the words were called by different persons. Another and much older animal showed no clear indication of forming the habit until the tenth day of experimentation. On that day it responded to '' Mary " 10 times and to " no feed " 6 times. After this, progress was slow; so slow that Shep- herd attributed its slowness to the marked inactivity of the older animal. He noticed, however, that when the cat's name was called it would look up to the top of the box or at the food lying at a distance of about 1 meter from the front of the cage. The type of response re- quired of this animal was then changed. It was counted sufficient indication of a habit if the animal looked up towards the food Avhen its name was called. Improvement was rapid. On the second day thereafter there were defi- nite signs of the appearance of the desired habit. On the twenty -fifth day it responded to the name '' Mary " 19 times and to "no feed " 4 times. The animal never succeeded entirely in inhibiting the tendency to respond to the '' no feed " call. Shepherd, in summarizing, says that the younger cat formed the habit in 250 trials and the older in 490.
(c) Raccoons: reactions of raccoons to tones and articulate sounds. — Cole tested the ability of raccoons to react positively to a high tone and to inhibit response at a low tone. The response expected was that of mounting a high box on hearing the high tone. Pure tones were not available, so the highest possible note, Al, on an ordi- nary A French harp, and the lowest, A", were used as stimuli. For the first few trials the hand was extended towards the high box when the food signal was given and the animal fed when it mounted the box. When this aid was withdrawn it was found that No. 1 was practically perfect in its response to the high tone, refusing to move at the low tone. No. 2 did not form the habit. There 384 AUDITORY AND RELATED FUNCTIONS seems to have been no effort to control the experiments. It is possible that the observer's movements might have been the cue to the reaction.
Shepherd states that raccoons can learn to respond to articulate sounds. Each raccoon was placed in a separate cage. Four cages were arranged in different parts of the room. The observer sat from 4 to 8 feet from the cages. The names of the raccoons were called in irregular order and the observer noted whether each animal responded to its own name or to all the names. Each animal was fed when it responded to its own name and was not fed when the other names were called. The names of the animals were Jack, Jim, Tom, and Dolly. The experiments were continued for 18 days, at the end of which time all the animals appeared to know their names perfectly. After the names had been learned, the observer called such words as '' box," " floor " after the name, i.e., he called " Jack, box, floor " in succession and not alternately. No substan- tial difference in the percentage of proper responses was noted. Again, the sound of the voice was varied. The number of trials required to perfect the association varied with the different animals: Jack, 270 trials; Tom, 375; Tests upon mice and rats. — Yerkes has made some in- teresting observations upon the dancing mouse. His re- sults are in agreement with those of Zoth. Both deny auditory sensitivity of any kind in the adult dancer. This animal is insensitive to such noises as are made by clapping the hands, shouting, whistling, exploding pistol caps, strik- ing on steel bars, and even the squealing of other mice. They are likewise insensitive to tones, such as the Galton whistle throughout its entire range, the Appunn whistle, and to the Konig forks, giving tones from 1024 to 16,382 d.v. Indirect methods were attempted, as with the frog. Yerkes first tested to see if sounds interfered with their whirling. It had no effect. He then tested by the dis- crimination method, using punishment and food as mo- tives. The animal had to enter one of two boxes which could be interchanged in position. If it attempted to TESTS UPON THE DANCING MOUSE 385 enter the wrong box, it was warned by a bell. The mouse should then have sought the other box. Punishment en- sued if it persisted in entering the box before which it was warned. No positive results came from these experi- ments. A different condition obtained in the case of young mice. When they are tested at or before the age of three weeks, evidence of auditory sensitivity is found; it ap- pears in the form of starts, or trembling of the entire body. When the young are tested with the Konig forks, steel bars, etc., the results are as follows: During the first two weeks of life there is no evidence of hearing; during the third week, certain individuals respond vigorously to sudden high tones and loud noises. Not all of the young are sensitive even at this period. After the third week, no reaction is obtained. No careful experiments have been made upon the white rat, but all agree that from about the twelfth day after birth they respond by starts, cessation of activity, etc., to sounds. Some experiments made in the Hopkins laboratory in a control cage similar to that described on p. 87 show that the rats can learn to go to the right when a shrill rattling noise is made the instant before the animal is released and to go to the left when no sound is made before the animal is released. Sensitiveness to ordinary environmental sounds, such as feeding of companions, voice of mates, danger calls, etc., has been observed in a number of mammals, such as the guinea pig, porcupine, etc.
Incidental tests upon other mammals: {a) monkeys. — Although observations apparently show that the monkeys are extremely sensitive to noises in their environment, to calls and cries of companions, etc., there is little reliable experimental data on the subject. Shepherd, with the noise-producing device described on p. 382, finds that the two rhesus monkeys tested required respectively 80 and 110 trials to perfect a habit of responding to the louder of two noises. Their sensitivity to pitch was tested with the German mouth organ. When A-3 was sounded the monkey under observation had to climb to a platform. To other tones the animal had to inhibit action. One of the 386 AUDITORY AND RELATED FUNCTIONS animals formed the habit in 60 trials, the other in 80 trials.
(b) Horses. — Tests made by Pfungst on cavalry horses do not confirm the view that they have the ability to re- spond appropriately to the various bugle calls when not di- rected (involuntarily by the rider). At the bugle call the horses, if standing, would start to walk. If the commands were given while the horses were trotting or walking no effect of the order was noted. Likewise tests on ability to respond unequivocally to their own names failed: any simi- lar word spoken with the same inflection produced the same effect. Pfungst finds that few habits are developed around auditory stimuli.
(c) Bats. — Hahn finds that bats are very sensitive to vibrations of high frequency. A sharp whistle, sucking noise with the lips, tearing a sheet of paper, etc., caused them to start violently, but lower pitched noises had no effect. It has been stated that the sound of the hairy-armed bat's voice has a frequency of 17,000 d.v. The pitch of the voice of American species has not been determined. It is not known whether bats hunt prey (insects, etc., usually at twilight) on the basis of sound.
Auditory Response in Birds In pigeons and parrots. — It has been shown by Rouse that mechanical jars and sounds have marked effect upon the breathing rate of pigeons. It has also been shown that pigeons, chickens, etc., hurry through a labyrinth when they can hear other birds pecking at food. The sensitivity of parrots to sound has often been remarked. On p. 295 we described the experiments made by Lashley on the imi- tation of sounds by parrots.
Auditory Response in Amphibia Frogs. — It is an interesting fact that the frog does not respond visibly to sound when tested under experimental conditions. In nature the animal seems to utilize sound HEARING IN FROGS 387 stimuli as warning data. We quote Yerkes' observations on the behavior of frogs in their natural habitat.
" In order to learn how far fear and artificial conditions were causes of the inhibition of responses 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 the 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 re- sponsible for this on-guard condition in which one finds the frogs after one of their number has jumped into the water, I made ob- servations 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, was 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 signs 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 towards 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 func- tional eye the frog was off like a fiash.
" 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 in. Likewise, when 388 AUDITORY AND RELATED FUNCTIONS 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 w^hich 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 mo- tionless, but it is an interesting phenomenon for the physiologist. We have to inquire, for instance, what effects sounds which stimu- late 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 of time for any other stimulus — what happens to the reaction time for an electrical stimulus, for example, if a loud noise precede or accom- pany the electrical stimulus." {Harvard Psychological Studies, I, When certain physiological processes are being recorded, e.g., breathing, it can be shown that auditory stimulation is effective. If the animal is being stimulated while breath- ing is being observed, marked changes in the rate and form of the curve are noticeable. The easiest way to observe the indirect effect of auditory stimulation is to mount the frog in a saddle in such a way that its legs hang free (p. 89). When the frog has ceased to struggle it is pos- sible to stimulate it tactually and to measure the distance to which the leg is jerked up. It is found that sounds given shortly before the tactual stimulus influence the height of the leg movement. If the sound (an electric bell) precedes the tactual stimulus by 1" it has no effect. If the interval is not longer than.35" it usually causes re- inforcement. When the interval is.4" to.9" there is partial inhibition of the leg reaction. The green frog was tested under three conditions: (1) When the tympanum was exposed fully to the air, although the body was submerged up to the level of the ear drum; (2) when the tympanum was half under water, the head and nares being in the HEARING IN FISHES 389 air; and (3) when the head of the frog was submerged to a depth of 4 cm. A bell electro-magnetically driven hung in the air and excluded from vision w^as used as the audi- tory stimulus. The effect of the sound on the leg reaction noted above was obtained under these several conditions. The range to which the frog's ear is responsive is from 50 to 10,000 vibrations per second. The response to sound may still be obtained after the tympana and columellae are removed. Sectioning of the eighth nerve, however, causes complete loss of sensitivity. Attention is called to the fact that the influence of sounds is more marked in the spring months than in the winter months.
Auditory Response in Fishes Some investigators who report lack of auditory sensi- tivity in fishes. — Bateson's early observations on the effect of blasting on fish are interesting. It caused the congers to draw back a few inches, flat fish to bury themselves, and pouting to scatter momentarily in all directions. Cer- tain other fish are not affected by the sound. Similar re- sults were obtained by striking with a heavy stick upon an aquarium containing soles. Bateson concludes that the fish are sensitive to the sound of sudden shocks when severe but not to the sound of bodies moving in the water when the latter are unseen by them. E.g., striking a glass with a stone under water and out of sight of the fish does not produce a response.
Kreidl reached similar conclusions with goldfish. These fish never responded to sound produced either in the water or outside in the air, but they did respond to heavy me- chanical jars. Fish made sensitive by strychnine w^ould respond to jars produced by tapping the aquarium and even to clapping the hands in the air, but not to tuning forks or vibrating rods, even when these were in contact with the water. Kreidl removed the auditory nerves and the attached ear sacs and then gave the animals strychnine. In all cases they responded as did the strychnized animals whose ears were intact. His conclusion was that the 390 AUDITORY AND RELATED FUNCTIONS cutaneous receptors are stimulated by the sound waves (those produced by jars, etc.).
Lee obtained no evidence that fish hear. He used the clapping of the hands, striking stones together in the air and in the water, the human voice, etc. He, with Kreidl, found that they are extremely sensitive to jars.
Bernoulli, who has recently repeated the experiments of Zenneck cited below, fails to find any response to sound. A bell with Cg for its fundamental, with a basal diameter of 94 mm. and a height of 62 -mm., was fastened to a firm support with the dome immersed. The bell was electro- magnetically driven. The key for closing the circuit was placed on land several meters away, behind a stone wall. The fish C Forellen," also eels and individuals belonging to " Zander," Lucio perca Sandra Cuv.) were tested in an open stream. He was never able to get the slightest re- sponse. Further tests showed that certain species of fish {Salmo fario L., and Thymallus vulgaris Nilos) were in- sensitive to shrill pipes, the sound waves from which were conducted to the water by the aid of a metal tube 4 m. long and 30 mm. in diameter. Other observations showed that fish C^ Zander ") were totally insensitive to pistol shots fired at a distance of 2 km. He believes that when the fish respond at all they do so because they are either tactually or visually stimulated.* While the above summaries do not exhaust the literature on the negative side, they are the most important ones.
Investigators reporting sensitivity to auditory stimu- lation.— Parker ^ has been the champion of hearing in fish for many years. His experiments, though, are not con- ducive to complete trust. He argues that most of the work upon fish has failed to note that the auditory reflexes may be very inconspicuous and that a closer scrutiny of their behavior might reveal changes in them due to the effect of sound stimuli. He began his experiments upon * Parker and his students seem never to have considered the fact that the fish might be stimulated by the wave motion in the water.
^ It will be recalled, however, that Parker himself was unable to get any response to auditory stimuli in the dogfish (Mustelus canis) by the method about to be described.
HEARING IN FISHES 391 killifish, Fundulus hetererocUtus. There are three possi- bilities of obtaining response to such vibrations: (1) Stimu- lation through the skin; (2) stimulation through the lateral line organs; and (3) stimulation through the eighth nerve (auditory response?). In response to the vibrations made by the string (see p. 393 for method) he noted in normal fish four kinds of movement: (1) Vibratory movements of the pectoral fins; (2) change in the rate of respiratory move- ment, usually increased; (3) if the sound was at all intense there was a slight movement of the caudal fin; (4) finally, the fish under strong stimulation would make a quick spurt or spring forward. In fish whose auditory nerves had been cut, he failed to obtain the movements of the pectoral fins, which was the most characteristic response and the one easiest to observe. Ten fish were observed and 10 observations were made upon each animal. In 82 ob- servations he obtained no response from the pectoral fin. In 18 there was a slight movement. He next made the skin insensitive by cutting the sensory nerve supply innervating the skin area — viz., the fifth, seventh, and part of the tenth cranial nerves and transecting the cord between the fourth and fifth vertebraB. The auditory organs, after such an operation, were presumably still completely func- tional. Parker states that the auditory responses in such animals were normal.
Recently, Bigelow, under the guidance of Parker, re- tested goldfish (Carassius auratus L.). His results are wholly different from those obtained by Kreidl (p. 389). His method of testing the fish was as follows: An aquarium was made with one end of wood. An electric tuning fork of 100 vibrations per second was used as the stimulus. The fork rested upon a table separate from the one which supported the tank. After the fork was started it was moved until its base came in contact with the wooden end of the aquarium. " This could easily be ac- complished without observable jar to the water in the aquarium, and certainly in itself had no effect on the fishes. For when I made the fork, not in vibration, touch the aquarium in the usual way, the fishes gave no reaction, 392 AUDITORY AND RELATED FUNCTIONS although to the vibrating fork they were very responsive; I tried this many times. ' ' When tested under these condi- tions normal fish respond to the stimulus usually in one of several different ways, such as tail jerks followed by forward swimming movement; tail jerks without locomo- tion; tail jerks and trunk jerks followed by a turn to one side, etc. Of 193 observations 150 gave positive results. Of the 43 failures 12 were observations on albino fish. The other 31 were due, Bigelow states, to the difficulty of observing certain individuals which were in continual rapid motion. The skin was next made insensitive by cutting the cord just posterior to the pectoral fins, and the lateral branches of the tenth, fifth, and seventh nerves on both sides of the body. When such fish are tested (they lie quietly on their sides on the bottom of the tank unless stimulated) they are normal essentially in their reactions to the fork. When, however, the eighth nerves were cut, reaction to the fork disappeared. Similarly, Zenneck has obtained evidence of response to sound in three fresh-water fish (Leuciscus rutilus, L. dohula, and Alburnus lucidus). He used a bell electro-magnetically driven. Occasionally he put a piece of leather over the place where the clapper struck. The fish responded by swimming away when the stroke of the bell was given, but did not respond when the leather damped the sound. In exact contradiction to this work stands that of Bernoulli (cited above, p. 390). The work of Bernoulli is more recent and seems to have been carried out much more carefully.
Intensity of sound stimuli in water. — Parker criticizes all work earlier than his own on account of the fact that the sound fish were supposed to respond to was always generated, not in the water where the fish reside, but in the air. He cites an experiment where a dinner bell was rung in the air by a person standing breast deep in the water. The listener remained a few feet away with head under water. The sound seemed to cease when the diver got his head under water. In like manner a bell rung or hit with a stone under water is heard at best faintly by a person standing in the water unless his head is also im- EXPERIMENTS ON SOUND INTENSITIES 393 mersed. In a recent paper he states that the noise even of a motor boat is extremely faint under water. The author, with the cooperation of Dr. Alfred G. Mayer and Dr. A. J. Goldfarb, at the Marine Biological Laboratory of the Car- negie Institution, Tortugas, Florida, has made several tests upon sounds heard under water. The experimenter tapped two small pieces of coral together under water. The ob- servers would swim farther and farther away, diving at intervals to listen for the sound. It was found that this faint noise was heard clearly for a distance of at least 200 feet. The same sound made in the air and heard by the observer with his head in the air was audible for a much greater distance than in the test just described. Parker is unquestionably right in his statement that sounds made in the air are all but inaudible to an observer whose head is under water. In one experiment carried out by the author at Tortugas with Dr. Goldfarb, it was found that the noise of a 38-caliber revolver w^hen fired in the air directly over the surface of the water could not be heard by an observer who had dived to a depth of 4 feet. The fact that sounds made in the air offer very faint stimula- tion to the ear under water has led Parker to adopt a very crude apparatus to increase the sound intensity, viz., to insert a wooden board in place of one of the glass sides of one of his tanks, and to attach to it a string, vibrating at 40 d.v. per second.