TABLE VI.

ASSOCIATION OF ELECTRICAL AND VISUAL STIMULI. FROG No. 1a, 2a, 3a, 4a, 5a, A and Z.

Frog. Total No. Days. Result. Trials.

No. 1a 180 18 Increase in number of long reaction toward end. No evidence of association.

No. 2a 180 17 Increase in number of short reactions toward end. No evidence of association.

No. 3a 180 17 Marked increase in the number of short reactions toward end. No other evidence of association.

No. 4a 200 19 Slight increase in the short reactions. There were a few responses to the light on the third day.

No. 5a 200 20 No increase in the number of short reactions. Few possible responses to light on second and third days.

Frog A 250 20 No evidence of association.

Frog Z 450 28 No evidence of association.

To all appearances this is the same kind of an association that was formed, in the case of the labyrinth experiments, between the tactual and the electrical stimuli. Why it should not have been formed in this case is uncertain, but it seems not improbable that the light was too strong an excitement and thus inhibited action. There is also the probability that the frog was constrained by being placed in a small box and having the experimenter near.

III. SUMMARY.

1. The green frog is very timid and does not respond normally to most stimuli when in the presence of any strange object. Fright tends to inhibit movement.

2. That it is able to profit by experience has been proved by testing it in simple labyrinths. A few experiences suffice for the formation of simple associations; but in case of a series of associations from fifty to a hundred experiences are needed for the formation of a perfect habit.

3. Experiment shows that the frog is able to associate two kinds of stimuli, e.g., the peculiar tactual stimulus given by a wire and a painful electric stimulus which in the experiments followed the tactual. In this case the animal learns to jump away, upon receiving the tactual stimulus, before the experimenter gives the electric stimulus.

4. Vision, touch and the organic sensations (dependent upon direction of turning) are the chief sensory factors in the associations. The animals discriminate colors to some extent.

5. Perfectly formed habits are hard to change.

6. Fear interferes with the formation of associations.

7. Associations persist for at least a month.

PART II. REACTION TIME OF THE GREEN FROG TO ELECTRICAL AND TACTUAL STIMULI.

IV. THE PROBLEMS AND POSSIBILITIES OF COMPARATIVE REACTION-TIME STUDIES.

Animal reaction time is at present a new field of research of evident importance and full of promise. A great deal of time and energy has been devoted to the investigation of various aspects of the time relations of human neural processes; a multitude of interesting facts have been discovered and a few laws established, but the results seem disproportionate to the amount of patient labor expended. Physiologists have determined the rate of transmission of the neural impulse for a few animals, and rough estimates of the time required for certain changes in the nervous system have been made, but this is all we have to represent comparative study. Just the path of approach which would seem most direct, in case of the time of neural changes, has been avoided. Something is known of the ontogenetic aspect of the subject, practically nothing of the phylogenetic; yet, in the study of function the comparative point of view is certainly as important as it is in the study of structure. In calling attention to the importance of the study of animal reaction time I would not detract from or minimize the significance of human investigations. They are all of value, but they need to be supplemented by comparative studies.

It is almost impossible to take up a discussion of the time relations of neural processes without having to read of physiological and psychological time. The time of nerve transmission, we are told, is pure physiological time and has nothing whatever to do with psychic processes; the time occupied by the changes in brain centers is, on the contrary, psychological time. At the very beginning of my discussion of this subject I wish to have it clearly understood that I make no such distinction. If one phase of the neural process be called physiological time, with as good reason may all be so named. I prefer, therefore, to speak of the time relations of the neural process.

Of the value of reaction-time studies, one may well believe that it lies chiefly in the way of approach which they open to the understanding of the biological significance of the nervous system. Certainly they are not important as giving us knowledge of the time of perception, cognition, or association, except in so far as we discover the relations of these various processes and the conditions under which they occur most satisfactorily. To determine how this or that factor in the environment influences the activities of the nervous system, and in what way system may be adjusted to system or part-process to whole, is the task of the reaction-time investigator.

The problems of reaction time naturally fall within three classes: Those which deal with (1) nerve transmission rates; (2) the time relations of the spinal center activities, and (3) brain processes. Within each of these groups there are innumerable special problems for the comparative physiologist or psychologist. Under class 1, for instance, there is the determining of the rates of impulse transmission in the sensory and the motor nerves, (a) for a variety of stimuli, (b) for different strengths of each stimulus, (c) for different conditions of temperature, moisture, nourishment, fatigue, etc., in case of each stimulus, (d) and all this for hundreds of representative animals. From this it is clear that lines of work are not lacking.

Closely related to these problems of rate of transmission are certain fundamental problems concerning the nature of the nerve impulse or wave. Whether there is a nerve wave, the reaction-time worker has as favorable an opportunity to determine as anyone, and we have a right to expect him to do something along this line. The relations of the form of the nerve impulse to the rhythm of vital action, to fatigue and to inhibition are awaiting investigation. Some of the most important unsettled points of psychology depend upon those aspects of neural activities which we ordinarily refer to as phenomena of inhibition, and which the psychologist is helpless to explain so long as the physiological basis and conditions are not known.

Then, too, in the study of animals the relation of reaction time to instincts, habits, and the surroundings of the subject are to be noted. Variability and adaptability offer chances for extended biological inquiries; and it is from just such investigations as these that biology has reason to expect much. The development of activity, the relation of reflex action to instinctive, of impulsive to volitional, and the value of all to the organism, should be made clear by reaction-time study. Such are a few of the broad lines of inquiry which are before the comparative student of animal reaction time. It is useless to dwell upon the possibilities and difficulties of the work, they will be recognized by all who are familiar with the results of human studies.

In the study of the time relations of neural processes Helmholtz was the pioneer. By him, in 1850, the rate of transmission of the nerve impulse in the sciatic nerve of the frog was found to be about 27 meters per second[4]. Later Exner[5] studied the time occupied by various processes in the nervous system of the frog by stimulating the exposed brain in different regions and noting the time which intervened before a contraction of the gastrocnemius in each case. Further investigation of the frog's reflex reaction time has been made by Wundt[6], Krawzoff and Langendorff[7], Wilson[8] and others, but in no case has the method of study been that of the psychologist. Most of the work has been done by physiologists who relied upon vivisectional methods. The general physiology of the nervous system of the frog has been very thoroughly worked up and the papers of Sanders-Ezn[9], Goltz[10] Steiner[11] Schrader[12] and Merzbacher[13],[14] furnish an excellent basis for the interpretation of the results of the reaction-time studies.

[4] Helmholtz, H.: 'Vorlaeufiger Bericht ueber die Portpflanzungsgeschwindigkeit der Nervenreizung.' Arch. f. Anal. u. Physiol., 1850, S. 71-73.

[5] Exner, S.: 'Experimentelle Untersuchung der einfachsten psychischen Processe.' Pflueger'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 fuer die Erforschung des Reflexmechanismus in Lendentmark des Frosches.' Berichte ueber die Verhandlungen der Kgl. saechs. Gesellsch. d. Wissensch. zu Leipzig, 1867, S. 3.

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

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

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

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

[14] Merzbacher, L.: 'Untersuchungen ueber die Regulation der Bewegungen der Wirbelthiere. I. Beobachtungen an Froeschen.' Pflueger's Arch., Bd. 88, 1901, S. 453-474, 11 Text-figuren.

In the present investigation it has been my purpose to study the reactions of the normal frog by the reaction-time methods of the psychologist. Hitherto the amount of work done, the extent of movements or some other change has been taken as a measure of the influence of a stimulus. My problem is, What are the time relations of all these reactions? With this problem in mind I enter upon the following program: (1) Determination of reaction time to electrical stimuli: (a) qualitative, (b) quantitative, (c) for different strengths of current; (2) Determination of reaction time to tactual stimuli (with the same variations); (3) Auditory: (a) qualitative, (b) quantitative, with studies on the sense of hearing; (4) Visual: (a) qualitative, (b) quantitative, with observations concerning the importance of this sense in the life of the frog, and (5) Olfactory: (a) qualitative, (b) quantitative.

The present paper presents in rather bare form the results thus far obtained on electrical, tactual, and auditory reaction time; discussion of them will be deferred until a comparison of the results for the five kinds of stimuli can be given.

V. METHOD OF STUDY.

The measurements of reaction time herein considered were made with the Hipp Chronoscope. Cattell's 'Falling Screen' or 'Gravity Chronoscope' was used as a control for the Hipp. The Gravity Chronoscope consists of a heavy metal plate which slides easily between two vertical posts, with electrical connections so arranged that the plate, when released from the magnet at the top of the apparatus, in its fall, at a certain point breaks an electric circuit and at another point further down makes the same circuit. The rate of fall of the plate is so nearly constant that this instrument furnishes an accurate standard time with which Hipp readings may be compared, and in accordance with which the Hipp may be regulated. For, since the rate of a chronoscope varies with the strength of the current in use, with the variations in temperature and with the positions of the springs on the magnetic bar, it is always necessary to have some standard for corrections. In these experiments the time of fall of the gravity chronoscope plate, as determined by the graphic method with a 500 S.V. electric tuning fork, was 125[sigma] (i.e., thousandths of a second).

This period, 125[sigma], was taken as a standard, and each hour, before the beginning of reaction-time experiments, the time of the plate's fall was measured ten times with the Hipp, and for any variation of the average thus obtained from 125[sigma], the standard, the necessary corrections were made by changing the position of the chronoscope springs or the strength of the current.

The standard of comparison, 125[sigma], is shorter than most of the reaction times recorded, but since the time measured was always that from the breaking to the making of the circuit passing through the chronoscope it cannot be urged that there were errors resulting from the difference of magnetization which was caused by variations in the reaction time. But it is evident that the danger from differences in magnetization, if such exists, is not avoided in this way; instead, it is transferred from the reaction time proper to the period of preparation immediately preceding the reaction; for, from the moment the chronoscope is started until the stimulus is given a current is necessarily passing through the instrument. At a verbal signal from the operator the assistant started the chronoscope; the stimulus was then given by the operator, and the instrument recorded the time from the breaking of the circuit, effected by the stimulating apparatus, to the making of the circuit by the reaction of the animal. Despite precautions to prevent it, the period from the starting of the chronoscope to the giving of the stimulus was variable, and errors were anticipated, but a number of the tests proved that variations of even a second did not cause any considerable error.

A fairly constant current for the chronoscope was supplied by a six-cell 'gravity battery' in connection with two storage cells, GB (Fig. 6). This current could be used for two hours at a time without any objectionable diminution in its strength. The introduction of resistance by means of the rheostat, R, was frequently a convenient method of correcting the chronoscope.



Fig. 6 represents the general plan of the apparatus used in these experiments.

The general method of experimentation is in outline as follows:

1. At a 'ready' signal from the operator the assistant makes the chronoscope circuit by closing a key, K (Fig. 6), and then immediately starts the chronoscope.

2. Stimulus is given by the operator as soon as the chronoscope is started, and by this act the chronoscope circuit is broken and the record begun.

3. Animal reacts and by its movements turns a key, RK (Fig. 6), thus making the chronoscope circuit and stopping the record.

4. Assistant stops chronoscope and takes reading.



The steps of this process and the parts of the apparatus concerned in each may be clearly conceived by reference to the diagram given in Fig. 6. The various forms of stimulating apparatus used and the modification of the method will be described in the sections dealing with results. The same reaction key was used throughout (see Fig. 7). Its essential features are a lever l, pivoted in the middle and bearing a post at either end, p, p. From the middle of this lever there projected upward a small metal bar, b, through the upper part of which a string to the animal ran freely except when it was clamped by the spring, s. This string, which was attached to the subject's leg by means of a light elastic band, after passing through the bar ran over a wheel, w, and hung tense by reason of a five-gram weight attached to the end. Until everything was in readiness for an experiment the string was left free to move through the bar so that movement of the animal was not hindered, but the instant before the ready-signal was given it was clamped by pressure on s. The diagram shows the apparatus arranged for a reaction. The current is broken, since 1 and 2 are not in contact, but a slight movement of the animal turns the lever enough to bring 1 against 2, thus making the circuit and stopping the chronoscope. When the motor reaction of the subject was violent the string pulled out of the clamp so that the animal was free from resistance, except such as the string and weight offered. The five-gram weight served to give a constant tension and thus avoided the danger of error from this source. Between experiments the weight was placed on the table in order that there might be no strain upon the subject.

That the subject might be brought into a favorable position for an experiment without being touched by the operator a special reaction box was devised.

The animals used in these studies were specimens of Rana clamitans which were kept in a tank in the laboratory throughout the year.

VI. ELECTRIC REACTION TIME.

The reaction time to electrical stimuli was determined first because it seemed probable that this form of the pain reaction would be most useful for comparison with the auditory, visual, olfactory and tactual reactions. In this paper only the electrical and the tactual reaction times will be considered. The former will be divided into two groups: (1) Those resulting from a stimulus given by touching electrodes to the leg of the frog, and (2) those gotten by having the frog resting upon wires through which a current could be passed at any time.

Group 1 of the electrical reactions were taken under the following conditions. A reaction box about 40 cm. in diameter was used. The mean temperature of the experimenting room was about 20 deg. C. In all cases the string was attached to the left hind leg of the frog, and the stimulus applied to the middle of the gastrocnemius muscle of the right hind leg. Reaction times were taken in series of ten, excluding those which were imperfect. As the moistness of the skin affects the strength of the electric stimulus received, it was necessary to moisten the animal occasionally, but as it did not seem advisable to disturb it after each experiment this was done at intervals of five minutes throughout the series. Were it not for this precaution it might be said that lengthening of the reaction times toward the end of a series simply indicated the weakening of the stimulus which resulted from the gradual drying of the skin. The stimulus in this group was applied by means of the stimulating apparatus of Fig. 6. It is merely two wire electrodes which could be placed upon the animal, with the additional device of a key for the breaking of the chronoscope circuit the instant the stimulus was given. The most serious objection to this method of stimulating is that there is a tactual as well as an electrical stimulus.

Before presenting averages, two representative series of reactions may be considered.

SERIES I. FROG B. APRIL 9, 1900. 10 A.M.

Temperature 19 deg. C. String to left hind leg. Stimulus to right hind leg.

Strength of stimulating current 1.0 volt, .0001 ampere.

Number of Experiment. Hour. Reaction Time. Remarks.

1 10.25 No reaction. 2 10.27 No reaction. 3 10.30 139[sigma] 4 10.34 164 5 10.35 102 6 10.37 169 7 10.39 151 8 10.40 152 9 10.42 144 10 10.43 152 11 10.45 122 12 10.51 179 13 10.54 No reaction.

Average of 10, 147.4[sigma]

SERIES 2. FROG F. ELECTRICAL STIMULUS.

No. Hour. Reaction Time. Remarks. Deviation from Mean.

1 10.19 35[sigma] Probable reaction to visual stim. 2 10.22 173 4.7 3 10.24 161 - 7.3 4 10.25 133 -35.3 5 10.26 199 30.7 6 10.28 130 -38.3 7 10.32 179 10.7 8 10.34 187 18.7 9 10.35 60 Probable reflex. 10 10.37 183 14.7 11 10.38 166 - 2.3 12 10.39 172 3.7

Average of 10, 168.3[sigma] Average of first 5, 159.2[sigma] Average Variation, 16.64[sigma] Average of second 5, 177.4[sigma]

Both are fairly representative series. They show the extremely large variations, in the case of series 1, from 102 to 179[sigma]. In all these experiments such variation is unavoidable because it is impossible to have the conditions uniform. A very slight difference in the frog's position, which could not be detected by the operator, might cause considerable difference in the time recorded. Efforts were made to get uniform conditions, but the results seem to show that there is still much to be desired in this direction.

Tables VII. contains the results of four series of ten reactions each for frog A. It will be noticed that the time for the first five in each series is much shorter than that for the last five; this is probably indicative of fatigue.

TABLE VII.

REACTION TIME OF FROG A TO ELECTRICAL STIMULI.

Series of Averages Averages of Averages of ten reactions. of series. first five. second five. 1 163.1[sigma] 134.6[sigma] 191.6[sigma] 2 186.2 176.2 196.2 3 161.1 125.2 197.0 4 158.3 101.6 215.0 General averages 167.2[sigma] 134.4[sigma] 199.9[sigma]

TABLE VIII.

REACTION TIME OF FROG B TO ELECTRICAL STIMULI.

1 132.7[sigma] 118.2[sigma] 147.4[sigma] 2 196.6 167.8 225.4 3 147.4 145.5 149.8 4 157.5 152.0 163.0 General averages 158.6[sigma] 145.9[sigma] 171.4[sigma]

TABLE IX.

NORMAL AND REFLEX REACTION TIME OF SIX ANIMALS TO ELECTRICAL STIMULUS.

Normal. Reflex. Average for 20 Average for 20 Frog. reactions. Mean Var. reactions. Mean Var. A 149.5[sigma] 24.0[sigma] B 158.3 16.0 51.5[sigma] 8.0[sigma] C 191.0 24.3 D 167.0 10.1 E 182.4 28.0 45.1 5.5 F 176.3 10.2 46.0 4.5 General Average. 167.9[sigma] 18.8[sigma] 47.5[sigma] 6.0[sigma]

For D the average is for ten reactions.

B and E were males, F a female; the sex of the others was not determined by dissection and is uncertain.

Early in the experiments it became evident that there were three clearly defined types of reactions: there were a number of reactions whose time was shorter than that of the ordinary quick voluntary pain reaction, and there were also many whose time was considerably longer. The first type it was thought might represent the spinal reflex reaction time. For the purpose of determining whether the supposition was true, at the end of the series of experiments three of the frogs were killed and their reflex reaction time noted. This was done by cutting the spinal cord just back of the medulla, placing the animal on an experimenting board close to the reaction key with the thread from the key fastened to the left leg as in case of the previous work and stimulating the gastrocnemius with an induced current by the application of wire electrodes.

In Table IX. the reflex reaction times for the three animals are given.

The following results obtained with frog E show that the time of reaction increases with the increase in the time after death. The average of 20 reactions by E taken an hour after the cord had been cut was 45.5[sigma]; the average of 20 taken twenty hours later was 55.85[sigma].

As a rule the reflex reactions were but slightly variable in time as is indicated by the accompanying series.

SERIES OF REFLEX REACTIONS OF FROG F. Taken at rate of one per minute.

1 50[sigma] 2 58 3 55 4 59 5 48 6 46 7 45 8 51 9 42 10 44

Throughout these experiments it was noticed that any stimulus might cause (1) a twitch in the limb stimulated, or (2) a twitch followed by a jump, or (3) a sudden jump previous to which no twitch could be detected. And it soon appeared that these types of reaction, as it seems proper to call them, would have to be considered in any determination of the mean reaction time. As proof of the type theory there is given (Fig. 8) a graphic representation of 277 reactions to the electrical stimulus.



The column of figures at the left indicates the number of reactions at any point. Below the base line are the classes. For convenience of plotting the reactions have been grouped into classes which are separated by 25[sigma]. Class 1 includes all reactions between 1[sigma] and 25[sigma], class 2 all from 25[sigma] to 50[sigma], and so on to 400[sigma], thereafter the classes are separated by 100[sigma]. It is noticeable that there is one well-marked mode at 75[sigma]. A second mode occurs at 175[sigma]. This is the primary and in our present work the chiefly significant mode, since it is that of the quick instinctive reaction to a stimulus. At 500[sigma] there is a third mode; but as such this has little meaning, since the reactions are usually pretty evenly distributed from 300[sigma] on to 2000[sigma]; if there is any grouping, however, it appears to be about 500[sigma] and 800[sigma].

The first mode has already been called the reflex mode. The short reactions referred to usually lie between 40[sigma] and 80[sigma], and since experiment has shown conclusively that the spinal reflex occupies about 50[sigma], there can be little doubt that the first mode is that of the reflex reaction time.

The second mode represents those reactions which are the result of central activity and control. I should be inclined to argue that they are what we usually call the instinctive and impulsive actions. And the remaining reactions represent such as are either purely voluntary, if any frog action can be so described, or, in other words, depend upon such a balancing of forces in the brain as leads to delay and gives the appearance of deliberate choice.

Everything points to some such classification of the types as follows: (1) Stimuli strong enough to be injurious cause the shortest possible reaction by calling the spinal centers into action, or if not spinal centers some other reflex centers; (2) slightly weaker stimuli are not sufficient to affect the reflex mechanism, but their impulse passes on to the brain and quickly discharges the primary center. There is no hesitation, but an immediate and only slightly variable reaction; just the kind that is described as instinctive. As would be expected, the majority of the frog's responses are either of the reflex or of this instinctive type. (3) There is that strength of stimulus which is not sufficient to discharge the primary center, but may pass to centers of higher tension and thus cause a response. This increase in the complexity of the process means a slower reaction, and it is such we call a deliberate response. Precisely this kind of change in neural action and in reaction time is at the basis of voluntary action. And (4) finally, the stimulus may be so weak that it will not induce a reaction except by repetition. Just above this point lies the threshold of sensibility, the determination of which is of considerable interest and importance.

Group 2 of the electrical reactions consists of three series taken to determine the relation of strength of stimulus to reaction time. The conditions of experimentation differed from those for group 1 in the following points: (1) The stimulus was applied directly by the making of a circuit through wires upon which the subject rested (Fig. 9); (2) the thread was attached to the right hind leg; (3) the thread, instead of being kept at the tension given by the 5-gram weight as in the former reactions, was slackened by pushing the upright lever of the reaction key one eighth of an inch toward the animal. This was done in order to avoid the records given by the slight twitches of the legs which precede the motor reaction proper. For this reason the reactions of group 2 are not directly comparable with those of group 1. Fig. 9 is the plan of the bottom of a reaction box 15 cm. at one end, 30 cm. at the other, 60 cm. long and 45 cm. deep. On the bottom of this, at one end, a series of interrupted circuits were arranged as shown in the figure. The wires were 1.2 cm. apart, and an animal sitting anywhere on the series necessarily touched two or more, so that when the stimulus key, X, was closed the circuit was completed by the animal's body; hence, a stimulus resulted. The stimulus key, X, was a simple device by which the chronoscope circuit, c, c, was broken at the instant the stimulus circuit, s, c, was made.

Cells of 'The 1900 Dry Battery' furnished the current used as a stimulus. Three different strengths of stimulus whose relative values were 1, 2 and 4, were employed in the series 1, 2 and 3. Careful measurement by means of one of Weston's direct-reading voltmeters gave the following values: 1 cell, 0.2 to 0.5 volt, 0.00001 to 0.00003 ampere. This was used as the stimulus for series 1. 2 cells, 0.5 to 1.0 volt, 0.00003 to 0.00006 ampere. This was used for series 2. 4 cells, 1.2 to 1.8 volt, 0.00007 to 0.0001 ampere. This was used for series 3.



The reactions now under consideration were taken in sets of 24 in order to furnish evidence on the problem of fatigue. The stimulus was given at intervals of one minute, and the subject was moistened at intervals of ten minutes. To obtain 24 satisfactory reactions it was usually necessary to give from thirty to forty stimulations. Five animals, numbers 1, 2, 4, 5, and 6, served as subjects. They were green frogs whose size and sex were as follows:

Length. Weight. Sex. Number 1 7.5 cm. 35 grams. Male. Number 2 7.3 " 37 " Male. Number 4 8.2 " 50.4 " Female? Number 5 7.1 " 25 " Female. Number 6 7.8 " 42 " Male.

For most of these frogs a one-cell stimulus was near the threshold, and consequently the reaction time is extremely variable. In Table X. an analysis of the reactions according to the number of repetitions of the stimulus requisite for a motor reaction has been made. Numbers 1 and 5 it will be noticed reacted most frequently to the first stimulus, and for them 48 satisfactory records were obtained; but in case of the others there were fewer responses to the first stimulus, and in the tabulation of series 1 (Table XI.) averages are given for less than the regular sets of 24 reactions each.

TABLE X.

ANALYSIS OF REACTIONS TO ONE-CELL STIMULUS.

Frog. Reactions to To 2d. To 3d. To 4th. To 5th. More. Total No. first Stimulus. of Reactions. 1 53 2 1 0 0 1 57 2 20 12 5 5 4 12 58 4 31 15 1 0 2 8 57 5 51 11 1 2 0 1 66 6 45 15 6 3 1 5 75 Totals, 200 55 14 10 7 27 313

Table XI. is self-explanatory. In addition to the usual averages, there is given the average for each half of the sets, in order that the effect of fatigue may be noted. In general, for this series, the second half is in its average about one third longer than the first half. There is, therefore, marked evidence of tiring. The mean reaction time for this strength of stimulus is difficult to determine because of the extremely great variations. At one time a subject may react immediately, with a time of not over a fifth of a second, and at another it may hesitate for as much as a second or two before reacting, thus giving a time of unusual length. Just how many and which of these delayed responses should be included in a series for the obtaining of the mean reaction time to this particular stimulus is an extremely troublesome question. It is evident that the mode should be considered in this case rather than the mean, or at least that the mean should be gotten by reference to the mode. For example, although the reaction times for the one-cell stimulus vary all the way from 150[sigma] to 1000[sigma] or more, the great majority of them lie between 200[sigma] and 400[sigma]. The question is, how much deviation from the mode should be allowed? Frequently the inclusion of a single long reaction will lengthen the mean by 10[sigma] or even 20[sigma]. What is meant by the modal condition and the deviation therefrom is illustrated by the accompanying curve of a series of reaction times for the electric stimulus of group I.

_____________ _8_ ____________ _7_ _______ ______ _6_ _______ ______ _5_ _______ ______ _4_ ______ _ _ _____ _3_ __ ____ _ _ _____ _2_ __ _ _ __ _ _ _ _ ____ _1_ _ _ _ _ __ _ _ _ _ _ _ _ _ _ 100 110 120 130 140 150 160 170 180 190 200 210 220 230

The column of figures at the left indicates the number of reactions; that below the base line gives the reaction times in classes separated by 10[sigma]. Of thirty-one reactions, seven are here in the class 170[sigma]. This is the model class, and the mean gotten by taking the average of 31 reactions is 162[sigma]. If the mode had been taken to represent the usual reaction time in this case, there would have been no considerable error. But suppose now that in the series there had occurred a reaction of 800[sigma]. Should it have been used in the determination of the mean? If so, it would have made it almost 30[sigma] greater, thus removing it considerably from the mode. If not, on what grounds should it be discarded? The fact that widely varying results are gotten in any series of reactions, points, it would seem, not so much to the normal variability as to accidental differences in conditions; and the best explanation for isolated reactions available is that they are due to such disturbing factors as would decrease the strength of the stimulus or temporarily inhibit the response. During experimentation it was possible to detect many reactions which were unsatisfactory because of some defect in the method, but occasionally when everything appeared to be all right an exceptional result was gotten. There is the possibility of any or all such results being due to internal factors whose influence it should be one of the objects of reaction-time work to determine; but in view of the fact that there were very few of these questionable cases, and that in series I, for instance, the inclusion of two or three reactions which stood isolated by several tenths of a second from the mode would have given a mean so far from the modal condition that the results would not have been in any wise comparable with those of other series, those reactions which were entirely isolated from the mode and removed therefrom by 200[sigma] have been omitted. In series I alone was this needful, for in the other series there was comparatively little irregularity.

The results of studies of the reaction time for the one-cell electric stimulus appear in Table XI. The first column of this table contains the average reaction time or mean for each subject. Nos. 2 and 4 appeared to be much less sensitive to the current than the others, and few responses to the first stimulus could be obtained. Their time is longer than that of the others, and their variability on the whole greater. Individual differences are very prominent in the studies thus far made on the frog. The one-cell stimulus is so near the threshold that it is no easy matter to get a mean which is significant. Could the conditions be as fully controlled as in human reaction time it would not be difficult, but in animal work that is impossible. No attempt has thus far been made to get the reaction time in case of summation effects except in occasional instances, and in so far as those are available they indicate no great difference between the normal threshold reaction and the summation reaction, but on this problem more work is planned.

There are large mean variations in Table XI., as would be anticipated. Since the reactions were taken in sets of 24, the means of each set as well as that of the total are given, and also, in columns 4 and 5, the means of the first half and the last half of each set.

A comparison of Tables XI., XII. and XIII. makes clear the differences in reaction time correlated with differences in the strength of an electric stimulus. For Table XI., series I, the relative value of the stimulus was I; for Table XII., series 2, it was 2, and for Table XIII., series 3, it was 4. Throughout the series from I to 3 there is a rapid decrease in the reaction time and in the variability of the same. The reaction time for stimulus I, the so-called threshold, is given as 300.9[sigma]; but of the three it is probably the least valuable, for reasons already mentioned. The mean of the second series, stimulus 2, is 231.5[sigma] while that of the third, stimulus 4, is only 103.1[sigma]. This great reduction in reaction time for the four-cell stimulus apparently shows the gradual transition from the deliberate motor reaction, which occurs only after complex and varied central neural activities, and the purely reflex reaction, which takes place as soon as the efferent impulse can cause changes in the spinal centers and be transmitted as an afferent impulse to the muscular system.

TABLE XI.

ELECTRICAL STIMULUS REACTION TIME. SERIES 1.

Average Average of Average Average Mean Var Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.

1 238.5* 33.3* 216.0* 205.6* 226.7* 33.2* 261.0 248.0 274.1 33.3 2 458.0 219.0 458.0 270.4 643.8 219.0 4 273.4 59.9 273.4 245.7 301.1 59.9 5 263.9 50.5 268.6 244.7 292.5 44.9 259.2 236.0 282.4 56.1 6 271.1 65.1 322.6 273.2 372.0 87.9 219.6 208.5 230.6 42.3 Gen Av. 300.9 85.5 300.9 244.8 356.8 85.5

Totals. For No. 1 the averages are for 2 sets of 24 reactions each, 48 " 2 " " one set of 12 " " 12 " 4 " " one set of 24 " " 24 " 5 " " two sets of 24 " " 48 " 6 " " two sets of 24 and 12 reactions, respectively, 36

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

TABLE XII.

ELECTRICAL STIMULUS REACTION TIME. SERIES 2.

Average Average of Average Average Mean Var Frog. of all. Mean Var. Sets. of 1st h. of 2d h. of Sets.

1 227.3* 33.7* 229.4* 209.1* 249.6* 25.5* 225.2 207.3 243.0 42.1 2 240.1 30.9 239.0 222.3 255.1 29.0 241.3 220.2 262.4 32.8 4 270.3 56.5 298.5 285.3 311.4 62.8 242.2 206.0 278.4 50.2 198.5 26.2 195.0 197.5 193.0 33.5 202.0 195.2 209.0 18.8 6 224.4 24.4 221.6 209.7 233.7 23.6 227.2 213.5 241.0 25.1 Gen. Av. 231.5 34.3 231.0 216.6 246.6 34.3

For No. 5 the averages are for two sets of 18 each; for all the others there are 24 in each set.

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

TABLE XIII.

ELECTRICAL STIMULUS REACTION TIME. SERIES 3.

Average Average Average Average Mean Var. Frog. of all. Mean Var. of all. of 1st h. of 2d h. of Sets. 1 93.6* 13.5* 91.8* 93.2* 90.4* 13.5* 95.4 91.8 99.0 13.5 2 99.9 12.8 92.2 89.4 95.0 17.4 107.5 105.9 109.0 8.2 4 125.2 16.3 113.5 106.5 120.5 13.6 136.0 135.7 138.2 19.0 5 94.4 8.0 88.6 90.5 88.6 8.2 100.2 97.8 102.7 7.9 6 102.5 12.2 104.2 98.6 109.9 12.8 100.9 101.0 108.3 11.6 Gen. Avs. 103.1 12.5 103.1 101.0 105.9 12.5

For each animal there are two sets of 24 reactions each.

*Transcriber's Note: All values in [sigma], 1/1000ths of a second.

The spinal reflex for a decapitated frog, as results previously discussed show, is approximately 50[sigma]; and every time the four-cell stimulus is given this kind of a reaction results. There is a slight twitch of the legs, immediately after which the animal jumps. Now for all these series the thread was slackened by one eighth of an inch, but the reflex time was determined without this slack. Calculation of the lengthening of the reaction time due to the slack indicated it to be between 20 and 30[sigma], so if allowance be made in case of the reactions to the four-cell stimulus, the mean becomes about 70[sigma], or, in other words, nearly the same as the spinal reflex. The conclusion seems forced, therefore, that when a stimulus reaches a certain intensity it produces the cord response, while until that particular point is reached it calls forth central activities which result in much longer and more variable reaction times. It was said above that the series under consideration gave evidence of the gradual transition from the reflex to the volitional in reaction time. Is this true, or do we find that there are well-marked types, between which reactions are comparatively rare? Examination of the tables VII., VIII., IX., XI., XII. and XIII. will show that between 70[sigma] and 150[sigma] there is a break. (In tables XI., XII. and XIII., allowance must always be made for the slack in the thread, by subtracting 30[sigma].) All the evidence furnished on this problem by the electrical reaction-time studies is in favor of the type theory, and it appears fairly clear that there is a jump in the reaction time from the reflex time of 50-80[sigma], to 140 or 150[sigma], which may perhaps be taken as the typical instinctive reaction time. From 150[sigma] up there appears to be a gradual lengthening of the time as the strength of the stimulus is decreased, until finally the threshold is reached, and only by summation effect can a response be obtained.

The most important averages for the three series have been arranged in Table XIV. for the comparison of the different subjects. Usually the reaction time for series 3 is about one half as long as that for series 2, and its variability is also not more than half as large. In the small variability of series 3 we have additional reason for thinking that it represents reflexes, for Table IX. gives the mean variation of the reflex as not more than 8[sigma], and the fact that the means of this series are in certain cases much larger is fully explained by the greater opportunity for variation afforded by the slack in the thread.

TABLE XIV.

MEANS, ETC., FOR EACH SUBJECT FOR THE THREE SERIES. (TIME IN [sigma])

Mean First Second Mean Frog. Half. Half. Variation. Series 1 238.5 226.8 259.4 33.3 Series 2 227.3 208.2 246.3 33.7 No. 1 Series 3 93.6 92.5 94.7 13.5

Series 1 458.0 270.4 643.8 219.0 Series 2 240.1 221.2 258.8 30.9 No. 2 Series 3 99.9 97.6 102.0 12.8

Series 1 273.4 245.7 301.1 59.9 Series 2 270.3 245.6 294.9 56.5 No. 4 Series 3 125.2 121.1 129.3 16.3

Series 1 263.9 240.4 287.4 50.5 Series 2 198.5 196.4 201.0 26.2 No. 5 Series 3 94.4 94.2 94.7 8.0

Series 1 271.1 240.8 301.3 65.1 Series 2 224.4 211.6 237.3 24.4 No. 6 Series 3 102.5 99.8 109.1 12.2

A striking fact is that the averages for the first and last half of sets of reactions differ more for the weak than for the strong stimulus. One would naturally expect, if the increase were a fatigue phenomenon purely, that it would be greatest for the strongest stimulus; but the results force us to look for some other conditions than fatigue. A stimulus that is sufficiently strong to be painful and injurious to an animal forces an immediate response so long as the muscular system is not exhausted; but where, as in series 1 and 2 of the electrical stimulus, the stimulus is not harmful, the reason for a sudden reaction is lacking unless fear enters as an additional cause. Just as long as an animal is fresh and unfamiliar with the stimulus there is a quick reaction to any stimulus above the threshold, and as soon as a few experiences have destroyed this freshness and taught the subject that there is no immediate danger the response becomes deliberate. In other words, there is a gradual transition from the flash-like instinctive reaction, which is of vast importance in the life of such an animal as the frog, to the volitional and summation responses. The threshold electrical stimulus does not force reactions; it is a request for action rather than a demand, and the subject, although startled at first, soon becomes accustomed to the experience and responds, if at all, in a very leisurely fashion. The reaction time to tactual stimuli, soon to be considered, was determined by giving a subject only three or four stimulations a day; if more were given the responses failed except on repetition or pressure; for this reason the data on fatigue, or lengthening of reaction time toward the end of a series, are wanting in touch. A few tests for the purpose of discovering whether the time would lengthen in a series were made with results very similar to those of the threshold electrical stimulus; the chief difference lies in the fact that the responses to touch fail altogether much sooner than do those to the electrical stimulus. This, however, is explicable on the ground that the latter is a stimulus to which the animal would not be likely to become accustomed so soon as to the tactual.

First Half. Second Half. Second % Greater. Series 1 244.8[sigma] 356.8[sigma] 46 per cent Series 2 216.6 246.6 14 " Series 3 101.0 105.9 5 "

If pure fatigue, that is, the exhaustion of the nervous or muscular system, appears anywhere in this work, it is doubtless in series 3, for there we have a stimulus which is so strong as to force response on penalty of death; the reaction is necessarily the shortest possible, and, as a matter of fact, the motor reaction (jump forward) here occupies little more time than the leg-jerk of a decapitated frog. This probably indicates that the reaction is a reflex, and that the slight increase in its length over that of the spinal reflex is due to occasional cerebellar origin; but of this there can be no certainly from the evidence herewith presented. At any rate, there is no possibility of a voluntary reaction to the strong current, and any changes in the general character of the reaction time in a series will have to be attributed to fatigue of the nervous or muscular systems. The second halves of the sets of series 3 are 5 per cent. longer than the first, and unless this is due to the partial exhaustion of the nervous system it is hard to find an explanation of the fact. Fatigue of the muscles concerned seems out of the question because the reactions occur at the rate of only one per minute, and during the rest interval any healthy and well-nourished muscle would so far recover from the effect of contraction that it would be able to continue the rhythmic action for long periods.

To the inquiry, Does fatigue in the experiments mean tiring by the exhaustion of nerve energy, or is the lengthening in reaction time which would naturally be attributed to tiring due to the fact that experience has shown quick reaction to be unnecessary? we shall have to reply that there is evidence in favor of both as factors. There can be little doubt that in case of the strong stimuli there is genuine fatigue which makes quick reaction impossible; but at the same time it is certain that the 40 to 50 per cent. increase of the second half of sets in series 1 over the first half can not be due to fatigue, for the strain is here evidently much less than for series 3. Rather, it would seem that habituation instead of exhaustion is the all-important cause of the difference in series 1 and 2. It becomes clear from these considerations that the repetition of a stimulus can never mean the repetition of an effect.

VII. TACTUAL REACTION TIME.

In the following work on the reactions to tactual stimulation the subject was placed in a large reaction box with a thread attached to one of its legs and passing to a reaction key, as in the experiments already described. The box in which the subject was confined was surrounded by movable cloth curtains to prevent the animal's escape and at the same time permit the experimenter to work without being seen by the frog.

Tactual stimulation was given by means of a hand key[15] similar to that used for electrical stimulation which is represented in Fig. 6. The touch key ended in a hard-rubber knob which could be brought in contact with the skin of the subject. This key was fixed to a handle of sufficient length to enable the operator to reach the animal wherever it chanced to be sitting in the reaction box. Stimulation was given by allowing the rubber point of the touch key to come in contact with the skin in the middle region of the subject's back. As soon as the point touched the animal the chronoscope circuit was broken by the raising of the upper arm of the key.

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

As a precaution against reactions to visual stimuli, which it might well be supposed would appear since the subject could not in every case be prevented from seeing the approaching apparatus, the frog was always placed with its head away from the experimenter so that the eyes could not readily be directed toward the touch apparatus. Notwithstanding care in this matter, a reaction occasionally appeared which was evidently due to some disturbance preceding the tactual stimulus which served as a warning or preparation for the latter. All such responses were at once marked as questionable visual reactions and were not included in the series of touch reactions proper.

As has been mentioned in connection with the discussion of fatigue, it was found absolutely necessary to have the subjects perfectly fresh and active, and for this purpose it was advisable to give not more than three or four stimulations at any one time. The subject was usually kept in the reaction box from 30 to 45 minutes, dependent upon the success of the experiments. As the work progressed it became evident that the responses to the stimulus were becoming less and less certain and slower, that the subjects were becoming accustomed to the novel experience and no longer suffered the surprise which had been the cause of the prompt reactions at first. It seemed best for this reason not to continue the work longer than two weeks, and as a consequence it was impossible to base the averages on more than twenty reactions for each subject.

So far as the tension of the thread is concerned, the condition for the tactual reaction time was the same as that for the first group of electrical reaction-time experiments. In comparing the tactual with the electrical of series 1, 2 and 3, allowance must be made for the slack in the latter cases.

Selection of the tactual reaction times upon which the mean is based, has been made with reference to the mode for each set of experiments. Inspection of the curves given by the reactions of each subject indicated that the great majority of the responses lay between 100 and 300[sigma], and that those which were beyond these limits were isolated and, in all probability, exceptional reactions due to some undetected variation in conditions which should throw them out of the regular series. On this account it was thought best to use only reactions between 100 and 300[sigma].

For convenience of comparison, again, the averages for the electrical reaction time of subjects A, B, C, D, E and F, and the same for the tactual reaction time of subjects 1, 2, 3, 4, 5 and 6 are herewith given together. All averages are for twenty reactions, except for D and 5, for which there are ten.

Besides the usual determination for the tactual reaction-time work on the six subjects named, there is given in Table XVI. the electrical reaction time of these animals to a two-cell current. Comparison of the electrical and tactual results are of interest in this case because the mean variation for each is about 34[sigma], being 34.3[sigma], for the electrical and 33.8[sigma], for the tactual.

TABLE XV.

Average of 20 Electrical Average of 20 Tactual Frog. Reactions. Frog. Reactions. A 149.5[sigma] 1 188.3[sigma] B 158.3 2 199.1 C 191.0 3 212.1 D 167.0 4 213.0 E 182.4 5 199.8 F 176.3 6 221.9 Gen. Avs. 167.9 205.7

TABLE XVI.

REACTION TIME FOR TACTUAL AND ELECTRICAL STIMULI.

Tactual Reaction Time. Electrical Reaction Time.

Frog. Average. Mean Variation. Average. Mean Variation.

1 188.3[sigma] 167.3[sigma] 2 199.1 180.1 3 212.1 4 213.0 210.3 5 199.8 138.5 6 221.9 164.4 Gen. Avs. 205.7 33.8 172.1 34.3

For 5 the average of ten instead of twenty is given.

VIII. EQUAL VARIABILITY AS A CRITERION OF COMPARABILITY OF REACTION TIME FOR DIFFERENT KINDS OF STIMULI.

Since variability as indicated in the study of the influence of different strengths of electrical stimulus becomes less as the stimulus increases, parity in variability for different stimuli offers a basis for the comparison of reaction times. Certain it is that there is no use in comparing the reaction times for different senses or different qualities of stimuli unless the relative values of the stimuli are taken into consideration; but how are these values to be determined unless some such index as variability is available? If the reaction time to tactual stimuli as here presented is to be studied in its relation to the electrical reaction time, it will mean little simply to say that the former is longer than the latter, because the electrical reaction time for a one-cell stimulus happens to be somewhat less than that for the particular tactual stimulus used. For it is clear that this tactual reaction time is really shorter than the reaction time to a weak current. In making variability a basis of comparison it must be assumed that the strength of stimulus is the important factor, and that all other variable conditions are, so far as possible, excluded. If, now, on the basis of parity in variability we compare the tactual and electrical reaction times, it is apparent that the tactual is considerably longer. The tactual average of Table XV. is 205.7[sigma], while the electrical reaction time which has approximately the same variability is 172.1[sigma]. It may well be objected that I have no right to make variability the basis of my comparison in these experiments, because the work for the various kinds of stimuli was done under different conditions. Admitting the force of this objection, and at the same time calling attention to the fact that I do not wish to lay any stress on the results of the comparisons here made, I take this opportunity to call attention to the possibility of this criterion.

The use of variability as a basis of comparison would involve the assumptions (1) that a certain intensity of every stimulus which is to be considered is capable of producing the shortest possible, or reflex reaction, and that this reaction is at the same time the least variable; (2) that as the strength of a stimulus decreases the variability increases until the threshold is reached.

Suppose, now, it is our desire to compare the results of reactions to different intensities of electrical and tactual stimuli; let the figures be as follows:

Reaction Time. Variability. Stimulus Strength. Elect. Touch. Elect. Touch. 8 50[sigma] 50[sigma] 10[sigma] 10[sigma]. 4 130 155 25 30 2 175 220 40 40 1 300 320 50 60

In the double columns the results for electrical stimuli are given first, and in the second column are the tactual. Stimulus 8 is assumed to be of sufficient strength to induce what may be designated as forced movement, and whatever the quality of the stimulus this reaction time is constant. I make this statement theoretically, although all the evidence which this work furnishes is in support of it. So, likewise, is the variability of this type of reaction time small and nearly constant. At the other extreme, stimulus 1 is so weak as to be just sufficient to call forth a response; it is the so-called threshold stimulus. Whether all qualities of stimulus will give the same result here is a question to be settled by experimentation. Wundt contends that such is the case, but the observations I have made on the electrical and tactual reactions of the frog cause me to doubt this assumption. It seems probable that the 'just perceptible stimulus reaction time' is by no means the same thing for different qualities of stimulus. Those modifications of the vital processes which alone enable organisms to survive, make their appearance even in the response to the minimal stimulus. In one case the just perceptible stimulus may cause nothing more than slight local changes in circulation, excretion, muscular action; in another it may produce, just because of the particular significance of the stimulus to the life of the organism, a violent and sudden motor reaction. But grant, if you will, that the threshold reaction time is the same for all kinds of stimuli, and suppose that the variability is fairly constant, then, between the two extremes of stimuli, there are gradations in strength which give reaction times of widely differing variabilities. If, now, at some point in the series, as, for instance, to stimulus 2, the variability for different kinds of stimuli is the same either with reference to the reaction time (ratio) or absolutely, what interpretation is to be put upon the fact? Is it to be regarded as merely a matter of chance, and unworthy of any special attention, or should it be studied with a view to finding out precisely what variability itself signifies? It is obvious that any discussion of this subject, even of the possible or probable value of variability as a criterion for the comparative study of stimuli, can be of little value so long as we do not know what are the determining factors of variations of this sort. The only suggestion as to the meaning of such a condition (i.e., equal variability at some point)—and our studies seem to show it for touch and electrical stimulation—which I feel justified in offering at present, is that parity in variability indicates equality in strength of stimuli, that is, the electrical stimulus which has a reaction time of the same variability as a tactual stimulus has the same effect upon the peripheral nervous system as the tactual, it produces the same amplitude and perhaps the same form of wave, but the reaction times for the two stimuli differ because of the biological significance of the stimuli. The chances are that this is wholly dependent upon the central nervous system.

IX. SUMMARY.

1. This paper gives the results of some experiments on the frog to determine its electrical and tactual reaction time. It is the beginning of comparative reaction-time studies by which it is hoped important information may be gained concerning the significance and modes of action of the nervous system. Comparative physiology has already made clear that the time relations of neural processes deserve careful study.

2. According to the strength of the stimulus, electric stimulation of the frog causes three types of reaction: (1) A very weak or threshold stimulus results in a deliberate or delayed reaction, the time of which may be anywhere from 300[sigma] (thousandths of a second) to 2,000[sigma]. (2) A very strong stimulus causes a spinal reflex, whose time is from 50 to 80[sigma]; and (3) a stimulus of intermediate strength causes a quick instinctive reaction of from 150 to 170[sigma] in duration.

3. The reaction time for electric stimuli whose relative values were 1, 2 and 4 were found to be 300.9[sigma], 231.5[sigma] and 103.1[sigma].

4. The reaction time of the frog to a tactual stimulus (contact of a rubber point) is about 200[sigma].

5. The variability of reaction times of the frog is great, and increases as the strength of the stimulus decreases.

6. When two kinds of stimuli (e.g., electrical and tactual) give reaction times of equal variability, I consider them directly comparable.

7. According to this criterion of comparability the reaction time to electric stimulation which is comparable with that to tactual is 172.1[sigma]; and it is to be compared with 205.7[sigma]. Both of these have a variability of approximately 34[sigma]. On this basis one may say that the tactual reaction time is considerably longer than the electrical.

PART III. AUDITORY REACTIONS OF FROGS.

X. HEARING IN THE FROG.

A. Influences of Sounds in the Laboratory.

After determining the simple reaction time of the green frog to tactual and electrical stimulation, I attempted to do the same in case of auditory stimuli. In this I was unsuccessful because of failure to get the animal to give a motor response which could be recorded. The animal was placed in an experimenting box with a string attached to one hind leg as in the experiments described in Part II., and after it had become accustomed to the situation a sound was made. A wide range of sounds were tried, but to none except the croak of another frog was a motor reaction frequently given. Even a loud noise, such as the explosion of a large pistol cap, caused a visible motor reaction only in rare cases. In fifty trials with this stimulus I succeeded in getting three reactions, and since all of them measured between 230 and 240[sigma] it is perhaps worth while to record the result as indicative of the auditory reaction time. As these were the only measurements obtained, I have no satisfactory basis for the comparison of auditory with other reaction times.

The remarkable inhibition of movement shown by the frog in the presence of strong auditory stimulation, at least what is for the human being a strong stimulus, led me to inquire concerning the limits and delicacy of the sense of hearing in frogs. In the vast quantity of literature on the structure and functions of the sense organs of the animal I have been able to find only a few casual remarks concerning hearing.

In approaching the problem of frog audition we may first examine the structure of the ear for the purpose of ascertaining what sounds are likely to affect the organ. There is no outer ear, but the membrana tympani, or ear drum, covered with skin, appears as a flat disc from 5 to 10 mm. in diameter on the side of the head just back of the eye and a little below it. In the middle ear there is but one bone, the columella, forming the connecting link between the tympanum and the internal ear. The inner ear, which contains the sense organs, consists of a membranous bag, the chief parts of which are the utriculus, the sacculus, the lagena, and the three semicircular canals. The cavity of this membranous labyrinth is filled with a fluid, the endolymph; and within the utriculus, sacculus and lagena are masses of inorganic matter called the otoliths. The auditory nerve terminates in eight sense organs, which contain hair cells. There is no cochlea as in the mammalian ear. The assumption commonly made is that vibrations in the water or air by direct contact cause the tympanic membrane to vibrate; this in turn causes a movement of the columella, which is transmitted to the perilymphatic fluid of the inner ear. The sensory hair cells are disturbed by the movements of the otoliths in the endolymph, and thus an impulse is originated in the auditory nerve which results in a sensation more or less resembling our auditory sensation. It is quite probable that the frog's sense of hearing is very different from ours, and that it is affected only by gross air vibrations. This conclusion the anatomy of the ear supports.

Although there does not seem to be a structural basis for a delicate sense of hearing, one must examine the physiological facts at hand before concluding that frogs do not possess a sense of hearing similar to our own. First, the fact that frogs make vocal sounds is evidence in favor of the hearing of such sounds at least, since it is difficult to explain the origin of the ability to make a sound except through its utility to the species. Granting, however, that a frog is able to hear the croaks or pain-screams of its own species, the range of the sense still remains very small, for although the race of frogs makes a great variety of sounds, any one species croaks within a narrow range.

Having satisfied myself that motor reactions for reaction-time measurements could not be gotten to any ordinary sounds in the laboratory, I tried the effect of the reflex croaking of another frog of the same species. In attempting to get frogs to croak regularly, I tested the effect of removing the brain. The animals are said to croak reflexly after this operation whenever the back is stroked; but for some reason I have never been successful in getting the reaction uniformly. In many cases I was able to make normal animals croak by rubbing the back or flanks, and to this sound the animals under observation occasionally responded by taking what looked like an attitude of attention. They straightened up and raised the head as if listening. In no case have other motor responses been noticed; and the above response was so rare that no reaction-time measurements could be made.

Again, while working with the green frog on habit formation, I one day placed two animals in a labyrinth from which they could escape by jumping into a tank of water. Several times when one frog jumped into the water I noticed the other one straighten up and hold the 'listening' or 'attentive' attitude for some seconds. As the animals could not see one another this is good evidence of their ability to hear the splash made by a frog when it strikes the water.

B. Influence of Sounds in Nature.

In order to learn how far fear and artificial conditions were causes of the inhibition of response 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 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 responsible for this on-guard condition in which one finds the frogs after one of their number has jumped into the water, I made observations 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, 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]

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

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.

10 Respirations. 10 Respirations. Before Stimulus. After Stimulus. 1st Stim. 13.0 cm. 11.8 cm. 2d Stim. 12.7 cm. 12.7 cm.

With a smaller animal.

1st Stim. 5.4 cm. 4.8 cm. 2d Stim. 4.9 cm. 4.7 cm. Average for 5 5.00 cm. 4.86 cm.

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:

Length of 10 Resps. Length of 10 Resps. Before Stimulus in cm. After Stimulus in cm. 1st Stim. 6.0 6.7 2d " 5.4 6.0 3d " 5.9 5.8 1st " 4.7 5.4 2d " 4.4 4.6

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.

Length in cm. of 10 Resps. Length in cm. of 10 Resps. Before Stimulus. After Stimulus. 4.8 4.4 5.3 4.6 4.5 4.0

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 reenforce 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 reenforcement 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 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.