Zoth (31 p. 170) has pointed out with reason and force that testing the sensitiveness of the mice is especially difficult because of their restlessness. They are almost constantly executing quick, jerky movements, starting, stopping, or changing the direction of movement, and it is therefore extremely difficult to tell with even a fair degree of certainty whether a given movement which occurs simultaneously with a sound is a response to the sound or merely coincident with it. With great care in the exclusion of the influence of extraneous stimuli, Zoth tried a large number of experiments to test the hearing of both young and adult dancers. Not once did he observe an indubitable auditory reaction. As he says, "I have performed numerous experiments with the Galton whistle, with a squeaking glass stopper, with caps and cartridges, without being able to come to any certain conclusion. With reference to the Galton whistle and particularly to the tone which was said to have been heard extremely well by Cyon's mice, I believe I am rather safe in asserting that my mice, young (12-13 days) as well as old, do not react to the König Galton whistle (7210 Vs.). They could not be awakened out of sleep by repetitions of the sound, nor enticed out of their nests, and their dancing could not be interrupted" (31 p. 170). Zoth's experiments appear to be the most careful and critical of those thus far considered.
Last to be mentioned, but in many respects of greatest interest and value, is the work of Kishi (21 p. 482) on the problem of hearing. To this acute observer belongs the credit of calling attention emphatically to the ear movements which are exhibited by the dancer. Frequently, as he remarks, the ears move as if the animal were listening or trying to determine the direction whence comes a sound, yet usually the mouse gives no other sign of hearing. That the absence of ordinary reactions to sounds is due to deafness, Kishi, like Panse, is led to doubt because his anatomical studies have not revealed any defects in the organs of hearing which would seem to indicate the lack of this sense.
This historical survey of the problem of hearing has brought out a few important facts. No one of the several investigators of the subject, with the exception of Cyon, is certain that the dancer can hear, and no one of them, with the exception of Rawitz, is certain that it cannot hear! Cyon almost certainly observed two kinds of dancing mice. Those of his dancers which exhibited exceptional ability to climb in the vertical direction and which also gave good evidence of hearing certain sounds may have been hybrids resulting from the crossing of the dancer with a common mouse, or they may have been exceptional specimens of the true dancer variety. A third possibility is suggested by Rawitz's belief in the ability of the young dancer to hear. Cyon's positive results may have been obtained with immature individuals. I am strongly inclined to believe that Cyon did observe two types of dancer, and to accept his statement that some of the mice could hear, whereas others could not. It is evident, in the light of our examination of the experimental results thus far obtained by other investigators, that neither the total lack of sensitiveness to sounds in the adult nor the presence of such sensitiveness in the young dancer has been satisfactorily proved.
I shall now report in detail the results of my own study of the sense of hearing in the dancer. As the behavior of the young differs greatly from that of the adult, by which is meant the sexually mature animal, I shall present first the results of my experiments with adults and later, in contrast, the results obtained with mice from one to twenty-eight days old.
My preliminary tests were made with noises. While carefully guarding against the interference of visual, tactual, temperature, and olfactory stimuli, I produced noises of varying degrees of loudness by clapping the hands together suddenly, by shouting, whistling, exploding pistol caps, striking steel bars, ringing an electric bell, and causing another mouse to squeak. To these sounds a common mouse usually responds either by starting violently, or by trembling and remaining perfectly quiet for a few seconds, as if frightened. The adult dancers which I have tested, and I have repeated the experiment scores of times during the last three years with more than a hundred different individuals, have never given unmistakable evidence of hearing. Either they are totally deaf or there is a most surprising lack of motor reactions.
Precisely the same results were obtained in tests made with the Galton whistle throughout its range of pitches, and with Appuun whistles which, according to their markings, ranged from 2000 Vs. (C_4) to 48,000 (G_9), but which undoubtedly did not correspond at all exactly to this range, and with a series of König tuning forks which gave tones varying in pitch from 1024 to 16,382 complete vibrations.
I am willing to trust these experimental results the more fully because during all the time I have had adult dancers under observation I have never once seen a reaction which could with any fair degree of certainty be referred to an auditory stimulus. Never once, although I have tried repeatedly, have I succeeded in arousing a dancer from sleep by producing noises or tones, nor have I ever been able to observe any influence of sounds on the dance movements. All of Cyon's signs have failed with my mice. Occasionally what looked like a response to some sound appeared, but critical observation invariably proved it to be due to some other cause than the auditory stimulus. A sound produced above the animal is very likely to bring about a motor reaction, as Cyon claims; but I have always found it to be the result of the currents of air or odors, which usually influence the animal when the experimenter is holding any object above it. I do not wish to maintain that Cyon's conclusions are false; I merely emphasize the necessity for care in the exclusion of other stimuli. The mice are extremely sensitive to changes in temperature, such, for example, as are produced by the breath of the experimenter, and one must constantly guard against the misinterpretation of behavior.
In a single experiment with mice over a month old, I observed what might possibly indicate sensitiveness to sound. While holding a mouse, thirty- five days old, in my hand I pursed my lips and made a very shrill sound by drawing in air; the mouse seemed to start perceptibly according to the indications given by my sense of touch. I repeated the stimulus several times and each time I could see and feel the animal start slightly. With two other individuals which I tested the reaction was less certain, and with several others I failed to get any indication of response. This would seem to prove that the three individuals which responded happened to be sensitive to that particular tone at the age of five weeks. The test is unsatisfactory because the vibrations from my own body may have brought about the reaction instead of the air vibrations produced by my lips, and I therefore merely mention it in the enumeration of the various experimental tests which I have made.
If we should conclude from all the negative evidence that is available, or that could be obtained, that the dancer is totally deaf, it might fairly be objected that the conclusion is unsafe, since an animal does not necessarily respond to stimuli by a visible change in the position or relations of its body. Death feigning may fairly be considered a response to a stimulus or stimulus complex, yet there may be no sign of movement. The green frog when observed in the laboratory usually gives no indication whatever, by movements that are readily observable, that it hears sounds which occur about it, but I have been able to show by means of indirect methods of study that it is stimulated by these same sounds.[1] Its rate of respiration is changed by the sounds, and although a sound does not bring about a bodily movement, it does very noticeably influence movements in response to other stimuli which occur simultaneously with the sound. I discovered that under certain rather simple experimental conditions the green frog would regularly respond to a touch on the back by drawing its hind leg up toward the body. Under the same conditions the sound of an electric bell caused no visible movement of the leg, but if at the instant the back was touched the bell was rung, the leg movement was much greater than that brought about by the touch alone. This suggests at once the desirability of studying the sense of hearing in the dancer by some indirect method. The animal may be stimulated, and yet it may not give any visible sign of the influence of the auditory stimulus.
[Footnote 1: "The Sense of Hearing in Frogs." Journal of Comparative
Neurology and Psychology, Vol. XV, p. 288, 1905.]