Unfortunately I have thus far been unable to get chronoscopic measurements of the reaction times in this reenforcement phenomenon.  I hope later to be able to follow out the interesting suggestions of these few experiments in the study of reenforcement 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-ampere 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.