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.