Chapter XXVIII: Section IV: , 1. The experiments of Table IX, C, repeat those of A with (8)

_Electric and visual stimuli_ (moving object). For the purpose of determining the effect upon reaction-time to an electric stimulus of stimulation of the eye by a rapidly moving object, experiments were made in which, as in the case of electric and photic stimuli, reactions to electric stimulus alone and to the visual and electric were observed alternately. Thus in the case of each pair of reactions it was possible to note whether the visual stimulus shortened or lengthened the reaction-time. The visual stimulus was given by quickly moving a finger before a window in the reaction-box.

Two series of twenty pairs of reactions each were taken with each of two frogs. In the first series the finger was suddenly moved across the window and the electric stimulus was given either simultaneously or a small fraction of a second later. It was impossible to arrange for accurate measurement of the temporal relations of the two stimuli in the case of these tests. In the second series the finger was moved back and forth before the opening in the reaction-box for an interval of at least a second before the electric stimulus was given.

These experiments, which were in the nature of preliminary tests, yielded the following results. When the stimuli were given almost simultaneously the visual reënforced the electric as was indicated by a shortening of the reaction-time. As appears in the upper part of Table 2, the average time of forty reactions, twenty for each frog, to the electric stimulus was 148^{σ},[152] and to the same stimulus when it followed the visual 128^{σ}. Furthermore, examination of the several pairs of reactions shows, as is indicated in the table, that there were twenty-seven cases in which the visual stimulus caused shortening of the reaction-time (reënforcement of the electric stimulus) to thirteen in which it caused lengthening (inhibition). When the visual stimulus preceded the electric by at least a second, the reaction-time to the electric stimulus was greatly lengthened. The averages are 150^{σ} for the electric stimulus alone, 178^{σ} when it was preceded by the visual. In this series there are twenty-five cases of inhibition to fourteen of reënforcement.

_Electric and visual stimuli_ (moving red disc). The indications of the importance of the temporal relations of stimuli, so far as reaction-time results are concerned, furnished by these crude preliminary observations led to a more accurate study of the subject. A revolving disc, which moved at the rate of one revolution per minute, was so arranged that at a certain point it closed an electric circuit in which a magnet had been placed. This magnet attracted a steel arm at the end of which a disc of red cardboard 12 mm. in diameter was suspended. With the making of the circuit the steel arm was drawn downward suddenly and the red disc, by reason of the vibrations of the arm moved rapidly back and forth in front of a window in the reaction-box. In this way the moving object was exposed to view about ten cm. to the right and three cm. in front of the right eye of the frog. The revolving disc, a fraction of a second later, completed the electric stimulus circuit. Thus both stimuli were given automatically, at such an interval apart as the experimenter desired. In the two series of results now to be described the intervals were 0.1 and 0.5 second respectively.

TABLE 2

Reaction-time to Electric Stimulation Alone, and to the Same when preceded for 0.1, 0.5, or 1.0 Second by Visual Stimulus.

Key:
1 = Frog.
2 = Electric Alone.
3 = Visual 0.1" before elect.
4 = Number Inhibited.
5 = Number Reënforced.
6 = Number Equal.
7 = Visual 1.0" before elect.

1 2 3 4 5 6 2 7 4 5 6

Preliminary Series. Visual Stimulus Moving Finger.
Averages for 20 reactions.
No. 5. 179ˢ 158ˢ 6 14 0 163ˢ 206ˢ 14 6 0
No. 6. 116 98 7 13 0 136 150 11 8 1
Gen.
Aver. 148 128 13 27 0 150 178 25 14 1

Visual Stimulus Moving Red Disc.
Visual 0.1" before electric. Visual 0.5" before electric.

Series I. Averages for 25 reactions.

No. 5. 177 163 10 15 0 170 255 15 9 1
No. 6. 148 112 6 19 0 115 178 18 7 0

Series II. Averages for 25 reactions.

No. 5. 135 120 7 18 0 155 259 24 1 0
No. 6. 128 111 6 19 0 132 227 17 7 1
Gen.
Aver. 147 126 29 71 0 143 230 74 24 2

These series consisted of twenty-five pairs of reactions each, with two animals. The results of the series are presented separately, in the lower half of Table 2, because the experiments which constitute them were separated by a period of three weeks. It is to be noted that these results agree fully with those of the preliminary series. The visual stimulus of a moving red disc, given 0.1 second before a 2 cell electric stimulus, reënforces the electric reaction, _i. e._, it shortens the time of reaction. The same visual stimulus given 0.5 second before tends to inhibit the electric reaction, _i. e._, it lengthens the time of reaction.

_Tactual and auditory stimuli._ Since in the frog auditory stimuli under experimental conditions seldom if ever cause visible motor reactions, the study of the influence of this mode of stimulation upon the reactions to other simultaneous or succeeding stimuli is of special interest. In the investigation of the relations of auditory stimulation to other forms of reaction _amount_ of reaction instead of _reaction-time_ was taken as a measure of the influence of the stimulus. By a method the details of which may be most easily understood by reference to the plan of the apparatus in Figure 1, the influence of auditory stimuli on the leg-movement induced by tactual stimulation was observed.

In these experiments the frog sat astride a wooden support, held in position by linen bands over the back and a wire screen cap over the head. The hind legs hung free, and any movement of one of them in response to a stimulus could be read in millimetres by reference to a scale on the wooden support. This method of measuring the value of a stimulus in terms of leg-reflex has been used by several investigators--most recently by Merzbacher.[153] I have found it desirable, as did Merzbacher, to observe the movements of a shadow of the leg on the scale and thus read the amount of movement, rather than to watch the leg itself and attempt to project it upon the scale.

As is indicated in Fig. 1, the auditory and tactual stimuli were given automatically by means of a swinging pendulum, _P_, which was held in position by the magnet _a_ until released by the experimenter. Early in its swing the pendulum turned the key, _m_, thus completing a circuit which caused the auditory stimulus to be given; later in the swing the key, _n_, was turned, and the tactual stimulus thus given through the magnetic release of the lever, _l_. The interval between the auditory and the tactual stimuli could be varied from 0 to 2" by changing the position of the key, _n_. For intervals over 1" it was necessary to arrange this key so that the tactual stimulus was given at some time during the return swing of the pendulum.

The auditory stimulus used was either the sound of a quick hammer blow (momentary stimulus of Series I), or the ringing of an electric bell for a certain length of time (prolonged stimulus of Series II). In Fig. 1 the bell is shown. It was placed eighty cm. from the frog, and in order that the influence of vibration of the experiment table might be avoided it was suspended from the pendulum frame. When the hammer was used it was placed sixty cm. from the frog, on the pendulum table. The holder for the frog and the tactual apparatus occupied a separate table which was not disturbed by the jars of the pendulum table.

The tactual stimulus was given by a rubber cone, _T_, two mm. in diameter at its apex. This rubber point, after the electric release of the lever to which it was attached, struck the frog at the middle point of a line drawn between the posterior margins of the tympana. The intensity of the stimulus could be varied by weighting the lever, _l_, at _w_.

All experiments were made with the green frog, _Rana clamata_ Daudin. The reactions were taken regularly at half-minute intervals in pairs: first, a tactual stimulus reaction, then an auditory-tactual reaction. Ten, fifty, or one hundred pairs constituted a series. So far as the condition of the frog is concerned there seems to be nothing undesirable in long series, for fatigue does not appear, and so long as the animal is kept moist and in an unconstrained position, it continues to react normally, and without frequent struggles to escape. The advantage for the purposes of this investigation of taking the reactions in pairs, rather than taking separate series of reactions for each stimulus or combination of stimuli, is obvious. It enables us to compare directly the reactions of each pair, in other words those reactions which took place under most nearly identical conditions, and to note at once whether the auditory stimulus reënforced or inhibited the tactual reaction.

During a series the intensity of the tactual stimulus was changed as conditions demanded, but for any one pair of reactions it was always the same. It not infrequently happened that an intensity which at first caused merely a slight movement of the leg, later in the series uniformly brought about a maximal contraction, or the reverse might be true, and inasmuch as a maximal reaction to the tactual stimulus alone left no opportunity for judging of the influence of the auditory stimulus, when it was given in addition to the tactual, it was always necessary in such cases so to alter the intensity of the tactual stimulus that a medium reaction resulted.

The frogs, after being placed in the saddle-like holder and held firmly for a few seconds, seldom struggled very much, but if bound tightly they became irresponsive to the stimuli.[154] It was, therefore, necessary after they had quieted down to loosen the bands which held them in position. For the purpose of excluding the influence of visual stimuli a wire screen cap covered with black cloth was put over the head; this served to keep the animal in position as well as to exclude visual stimulation.

_a. Momentary auditory stimulation._ Four frogs were used for a study of the influence of the momentary sound produced by a hammer blow, and for each of these animals fifty pairs of reactions were recorded in series each day. The temporal relation of the stimuli was changed daily during a week of experimentation: the results therefore consist of fifty pairs of reactions with each frog for each of the following seven intervals: (1) Auditory and tactual stimuli simultaneous, (2) auditory .25" before tactual, (3) auditory .45" before, (4) auditory .15" before, (5) auditory .65" before, (6) auditory .35" before, (7) auditory .90" before. The intervals were used in the experiments in the above order to avoid the formation of the definite habits of reaction which regular increase in the interval would have favored.

Typical of the results with all the animals are the following (Table 3) which were obtained with No. 1, a male. The figures in each case indicate the average of fifty reactions. Reënforcement and inhibition are expressed in terms of the tactual reaction, _i. e._, the auditory-tactual reaction is so many per cent greater (reënforcement) or less (inhibition) than the tactual. In the tables reënforcement is indicated by the + sign; inhibition by the - sign. In the last column of the table is given the number of reactions that were reënforced or inhibited. This was determined by comparing directly the reactions of each pair. Cases in which the two reactions were the same were distributed equally between the two classes: tactual reactions reënforced by auditory stimulus, and tactual reactions inhibited by auditory stimulus. Assuming that the auditory stimulus was without effect upon the tactual reaction, the number of reactions in these two classes would be approximately the same, hence all auditory-tactual reactions over half in a series, _i. e._, over twenty-five, which are greater than the corresponding tactual reactions, are reënforced reactions, and can be taken as a measure of the reënforcing influence of the auditory stimulus. In the same manner all reactions over half which show inhibition can be taken as a measure of the inhibitory value of the auditory stimulus.

As preliminary tests described in an earlier paper[155] furnished evidence of sex-differences, it is worth while to compare the results given by the males and females in these experiments with momentary auditory stimulation. For purposes of comparison I have presented in Table 4 the reënforcement-inhibition values given by the males and females for each interval. Column one contains the value of the auditory-tactual reaction in terms of the tactual reaction; column two, the number of reactions in excess of half which were reënforced or inhibited.

TABLE 3. FROG NO. 1. MOMENTARY AUDITORY STIMULUS, HAMMER BLOW. WEIGHT USUALLY 5 OR 10 GRAMS

Reaction Reaction Amount of Number of
to to Auditory Reënforc'm't reactions
Tactual and Tactual or Reënforced
Interval. Stim. Stim. Inhibition. or Inhibited.

0" 6.84mm. 11.08mm. +62.0% +17.0
.15 22.22 28.96 +30.3 +17.0
.25 16.30 21.72 +33.3 +13.0
.35 24.90 25.32 + 1.7 + 0.5
.45 17.56 13.64 -22.3 -10.0
.65 17.46 15.72 -10.0 - 6.0
.90 31.26 31.48 + 0.7 + 0.5

TABLE 4. MOMENTARY AUDITORY STIMULUS, HAMMER BLOW

_Males_ _Females_
Nos. 1 and 3. Nos. 2 and 4.
Per centum No. of Per centum No. of
Interval Diff. Reacts. Diff. Reacts.
0" +82.5% (Reënf't) +17.5 +58.0% +12.7
.15 +58.1 +17.0 +25.4 + 8.5
.25 +32.3 +12.7 +39.8 +12.7
.35 + 4.0 + 1.2 - 9.7 - 3.2
.45 -13.5 (Inhibition) - 7.2 -13.9 - 7.2
.65 -12.5 - 6.2 -11.8 - 7.2
.90 - 0.7 - 1.5 - 2.6 - 0.5

In these results two striking differences between the males and females appear: first, the reënforcement is not so great for the females as for the males; second, inhibition appears earlier and continues longer with the females than with the males. The average reënforcement with simultaneous stimuli is 82.5% for the males against 58.0% for the females. Inhibition begins to appear in case of the females when the interval between the stimuli is .25" to .35"; in case of the males it appears between .35" and .45". Finally at .90" interval inhibition is slightly greater for the females.

Although the exact significance of these facts is unknown, it is not improbable that they are indicative of fundamentally important sex-differences in reaction to sound. The males among frogs are usually the vocalists, although in some species the females also croak. Moreover, in case of the green frog the tympanum of the male is much larger than that of the female. The results presented would seem to indicate that certain sounds stimulate the males to activity, whereas they inhibit activity in the females.

Graphically represented, the results of the momentary auditory stimulus experiments with frogs Nos. 1, 2, 3, and 4 are as follows:

The curves are all plotted by the method which will now be described in connection with Fig. 2. This figure presents the reënforcement-inhibition curves for the males No. 1 (solid line in the figure) and No. 3 (broken line). If in this figure we let the zero-point on the ordinates represent the value of the reaction to the tactual stimulus when given alone, then the value of the reaction to the auditory-tactual stimuli would be represented at some point above the zero-point if this reaction was greater than the tactual reaction (reënforcement), and below the zero-point if the reaction was less than the tactual (inhibition). Since one of our chosen measures of reënforcement and inhibition is the amount, in per cent of tactual reaction, by which the auditory-tactual reaction exceeds or falls short of the tactual reaction, such a curve of reënforcement-inhibition as that of Fig. 2 (solid line) can be constructed at once from the data given in column four of Table 3. Here the auditory stimulus, when simultaneous with the tactual, caused 62% reënforcement, as is indicated in the figure. The figures in the left-hand margin of the curves indicate amount of reënforcement or inhibition in per cent of tactual reaction; those at the bottom of the curves mark the intervals. On the curves dots indicate the intervals used in the experiments. Each of the curves is plotted on the basis of 700 reactions.

In every way comparable with the curves for the males No. 1 and No. 3 in Fig. 2 are those for the females No. 2 and No. 4 of Fig. 3. The similarity of the two curves in each figure is noteworthy. Inasmuch as the conditions of experimentation were the same for all the animals this would seem to indicate sex-differences which are worthy of further investigation. The curves show clearly the greater reënforcement in the males, and the greater inhibition in the females.

Figures 4 and 5 are the reënforcement-inhibition curves for the same series of experiments plotted on the basis of the _number_ of reactions in excess of half that were reënforced or inhibited. As there were fifty pairs of reactions with each frog for each interval, uniform reënforcement would be represented by twenty-five reactions above the base-line; uniform inhibition by twenty-five reactions below the base-line. The number of reactions is indicated by the figures in the left margin; the intervals, by those below the base-line. As an illustration of the application of the method of plotting, the curve for male No. 1 (solid line) of Fig. 4 is constructed from the data of column five of Table 3. With simultaneous stimuli 17 reactions in excess of half, _i. e._, 17 + 25, or 42, were reënforced; at .35" interval .5 of a reaction was the average amount of reënforcement; at .45" interval 10 reactions in excess of half, _i. e._, 35, were inhibited, therefore the curve falls to 10 below the base-line.

Just as Figures 2 and 3 permit of direct comparison of the results of the measurement of the _amount_ of reënforcement and inhibition for males and females, so Figures 4 and 5 make possible comparison in similar fashion of the _number_ of reënforced and inhibited reactions for the sexes. It is to be noted that the two sets of curves, plotted on the bases of _amount_ and _number_ of reaction, agree in all important respects.

Figure 6 is the composite curve of amount of reënforcement-inhibition for the four animals; Figure 7 is the composite curve of the number of reactions reënforced and inhibited.

Summarily stated, the results of the experiments thus far described are: (1) The auditory stimulus of a quick hammer blow produces the maximum amount of reënforcement of tactual reaction when it is given simultaneously with the tactual stimulus; (2) as the interval between the auditory and the tactual stimulus approaches .35″ the amount of reënforcement gradually decreases; (3) when given .35″ before the tactual stimulus the auditory is practically without effect upon the tactual reaction; (4) as the interval increases above .35″ inhibition begins to appear; (5) the inhibitory influence of the auditory stimulus is greatest when the interval is about .45″; (6) when the interval is as long as .90″ the auditory stimulus is again ineffective. It thus appears that the reënforcement-inhibition curve of this particular stimulus under the conditions described is representative of a neural process which completes itself, in passing through two phases, a positive phase (reënforcement) and a negative phase (inhibition), in about one second.

_b. Prolonged auditory stimulation._ The experiments previously described have proved that a momentary auditory stimulus, which when given alone never produces a visible motor reaction, either reënforces or inhibits the reaction to a tactual stimulus which it accompanies or precedes. The experiments now to be described were made for the purpose of ascertaining whether reënforcement and inhibition occur in the same way if the auditory stimulus is prolonged, instead of momentary.

In a trial series of experiments with frog No. 1, one hundred pairs of reactions were recorded for each of six intervals of auditory stimulation. The auditory stimulus was given by the ringing of an electric bell. For all intervals the ringing of the bell continued until the tactual stimulus was given. When the two stimuli were given simultaneously the auditory stimulus was necessarily momentary, as in the foregoing experiments, but for all other relationships of the stimuli the bell rang for a certain length of time before the tactual stimulus was given. The six relations of the stimuli were: (1) simultaneous, (2) bell .2″ before and until tactual, (3) bell .6″ before, (4) bell 1.05″ before, (5) bell 1.5″ before, and (6) bell 2.0″ before. The other conditions of these experiments were the same as those previously described, except that the auditory stimulus was here given by the opening of the key which released the pendulum, instead of being given by the turning of a key in the course of the pendulum swing. This method of giving the auditory stimulus as the pendulum was released was found unsatisfactory because of the irregularity of the magnetic release; at one time the pendulum would start immediately, at another time there would be a delay of as much as .1″.

The reënforcement-inhibition curve plotted on the basis of the 1200 reactions in this series is presented in Fig. 8. Before stopping to consider the important features of this curve we should note the results of certain more accurate experiments with prolonged auditory stimulation.

With two animals, No. 2, a female, and No. 3, a male, fifty pairs of reactions were taken for nine different intervals (see Table 5) of auditory stimulation. Each of the curves of Figures 9 and 10 is therefore based upon 900 reactions. The conditions for these experiments were the same as those for the momentary stimulation series, save that the electric bell took the place of the electrically actuated hammer, as the mechanism for auditory stimulation.

The important facts exhibited by the results of these prolonged auditory stimulation experiments in contrast with those with momentary auditory stimulation are: (1) That whereas for the momentary auditory stimulus of a hammer blow the reënforcement is greatest for simultaneous stimuli, in case of the prolonged stimulation with the electric bell, reënforcement increases during an interval of .25″ of auditory stimulation. Hence, the two conditions of stimulation give us different types of reënforcement-inhibition curve. For the momentary stimulus the maximum reënforcement appears at simultaneity, and for the prolonged stimulus at .25″; (2) that the transition from reënforcement to inhibition occurs at 1.2″ in the prolonged stimulation curves, while in the momentary stimulation curves it occurs at .35″; (3) that the maximum inhibition which appears in the curves under discussion at about 1.5″ is less in comparison with the amount of reënforcement than that of the momentary stimulation curves; (4) that the auditory stimulus becomes ineffective when the interval during which it continues before tactual stimulation is 2.0″. The curves of Figures 8, 9, and 10 are then representations of a neural process which passes through a positive and a negative phase in about 2″. The effect of prolongation of the auditory stimulation interval is to lengthen the period of reënforcement; the period of inhibition shows little modification.

For the purpose of showing in greater detail the nature of the results of this work the data from which the curves of Figures 9 and 10 were constructed are presented in the accompanying Table 5.

Having now presented the results of my own investigation I wish to call attention to certain of their relationships to the work of other investigators, and to discuss briefly their significance.

TABLE 5. PROLONGED AUDITORY STIMULATION (ELECTRIC BELL)

_Frog No. 2._ _Female._ _Weight usually 25 grams._

Auditory Number of
and Amount of Reactions,
Tactual Tactual Reënforcement Reënforced
Interval. Stimulation. Stimulation. or Inhibition. or Inhibited.

0″ 9.20 mm. 12.12 mm. + 31.7% +10.0
.25 4.56 11.88 +160.5 +22.5
.45 8.94 16.94 + 89.5 +18.5
.65 17.18 22.50 + 31.0 +15.5
.90 9.42 13.32 + 41.4 +14.5
1.20 10.54 9.64 - 8.5 - 2.5
1.40 24.00 20.64 - 14.0 - 6.0
1.58 19.16 17.80 - 7.1 - 7.0
1.95 14.50 15.40 + 6.2 + 5.5

_Frog No. 3._ _Male._ _Weight usually 5 or 10 grams._

0″ 14.92 mm. 25.20 mm. + 68.9% +11.0
.25 15.88 38.54 +142.7 +24.0
.45 13.48 26.02 + 92.9 +13.5
.65 18.30 27.94 + 52.6 +13.0
.90 20.94 29.06 + 38.8 + 9.5
1.20 21.90 30.58 + 39.6 + 1.0
1.40 19.18 18.34 - 4.4 - 5.5
1.58 32.24 26.30 - 18.1 - 3.0
1.95 13.86 14.14 + 2.0 + 2.0

VI. DISCUSSION OF LITERATURE AND RESULTS

The literature on reënforcement and inhibition is large, and even that portion of it which deals especially with the importance of the temporal relations of stimuli in connection with reënforcement and inhibition is so extensive that it does not seem worth while to attempt to give a systematic résumé of it for the purposes of this paper. I shall therefore call attention merely to those investigations which have contributed directly to the solution of the problems with which we are now concerned.

Bowditch and Warren[156] discovered that knee-jerk in the human subject is reënforced when an auditory, a visual, or a tactual stimulus precedes the tendon blow by .1″ to .5″, whereas the same stimuli have an inhibitory influence when they are given from .5″ to 1.0″ before the tendon blow.

At the suggestion of Bowditch, Cleghorn[157] undertook to investigate the influence of complication of stimuli upon voluntary movements. In this research graphic records taken in connection with an ergograph indicated (1) that "a sensory stimulus" applied just as the muscle was beginning to contract (voluntarily) caused an increase in the height of the contraction, and (2) that the relaxation following a contraction with intercalated sensory stimulus is quicker and more complete than when no stimulus is given (p. 344). Cleghorn did not give special attention to the significance of the temporal relations of the stimuli which he employed, and his work was limited to the phenomenon of reënforcement of voluntary action by reason of the appearance, during the progress of his research, of an excellent paper on the interference of stimuli by Hofbauer.[158]

Hofbauer covered thoroughly the ground which Cleghorn had planned to work over. The ergographic method was employed also by Hofbauer in his very careful study of the interference of impulses in the central nervous system of man. It was noticed that while the subject was rhythmically contracting a certain group of muscles in response to some prearranged signal (_e.g._, the sound of a metronome) the report of a pistol caused the contraction which immediately followed it to be much greater than the average of the rhythmic series, while the next contraction was correspondingly less than the average. It thus appeared that the sudden sound caused, first, reënforcement of the voluntary movement, then, inhibition. The reënforcement is greatest, according to Hofbauer, when the voluntary movement occurs immediately after the pistol report. When the report precedes the metronome signal by .2″ reënforcement is still marked, but thereafter it decreases rapidly in amount, until finally at .5″ inhibition appears. When the interval between the two stimuli is 1.0″ the first stimulus has practically no effect upon the voluntary movement in response to the second. (Hofbauer, p. 558.)

What Bowditch and Warren, not to mention other students of the subject, have described for reflex action in man, Hofbauer, Cleghorn, and others have shown to hold true also of voluntary movements. Unfortunately my own investigation was completed up to the point of the writing of this paper before I read Hofbauer's work, so I have not followed methods of dealing with my data which would make our results directly and easily comparable. But, whatever may be the relations of our results in detail, there can be no doubt that what he has demonstrated for man is true in its important aspect of the reënforcement-inhibition phenomena for the frog.

Important in their bearings upon the phenomena of reënforcement and inhibition which we are now considering, are the various studies of refractory period and rhythm of nerve cell and fibre. The existence of a refractory period in neural substance, similar to that demonstrated for certain kinds of muscle by Marey,[159] Englemann,[160] Kaiser,[161] Cushny and Matthews,[162] Woodworth,[163] and many others, has been proved by Broca and Richet.[164]

Broca and Richet found that in the normal dog the refractory period of the nerve substance is too short to be easily detectable, they therefore experimented with animals which were lightly chloralized and kept at a temperature of 30 to 34° (the mean normal temperature of the dog is about 39.5°). Under these conditions a dog, when two identical stimuli (quality and intensity the same) were applied to the cerebral cortex successively, exhibited the following reactions: (1) When the stimuli were separated by .01″ they reënforced one another (addition); (2) when the interval was .1″ they inhibited the reaction partially (subtraction).

Concerning this phenomenon Richet writes in his dictionary of physiology (p.5): "Marey showed, in 1890, that the heart of the frog, at certain moments of systole, was inexcitable. Now our experiments prove that the cerebral apparatus, a certain time after the excitation, also ceases to be excitable: it then has a refractory phase, and this refractory phase is much more prolonged than that of the cardiac muscle." In a later publication Richet[165] makes the somewhat startling statement that a refractory period is not exhibited by the nerves of cold-blooded animals. In the tortoise, according to his results, reënforcement occurs so long as the interval between the two stimuli is not greater than 2″, while for longer intervals each stimulus to all appearances works independently. Richet seems to have generalized from a study of the tortoise. That his generalization is unwarranted seems to me highly probable in the light of the results of this paper, for there are many reasons for supposing that the reënforcement-inhibition phenomena with which we have been dealing in case of the frog are manifestations of the existence of the same process in the nervous system which under somewhat different conditions of experimentation exhibits itself in the so-called refractory period.

The researches of Richet and his students indicate that the time of the process which conditions the phenomena of reënforcement-inhibition is about .1″. Stimuli given at .1″ intervals do not interfere with one another. That the process underlying the refractory period and the reënforcement-inhibition phenomena of our experiments is a rhythmic double-phase process is made still more probable by the following results. Horsley and Schäfer[166] found that the rate of response of the monkey to cortical stimulation was 12 per second, and Schäfer[167] discovered that the maximum rate of volitional impulses in man is 10 to 12 per second.

It was shown by Exner that certain movements of the foot of a rabbit could be produced by stimulating either the cortex or the skin of the foot. Simultaneous stimulation of both regions gives reënforcement. Stimulation of the cortex, if given not more than 3″ before subliminal stimulation of the skin, renders the latter effective. When both stimuli are subliminal each makes the subsequent one effective if the interval between them is not over 1/8″ (Schäfer[168]). Similarly for the dog Exner[169] proved that cortical and cutaneous stimuli reënforced one another, when both were subliminal, if the interval between them was not greater than .6″. Cortical and auditory stimuli, and auditory and cutaneous (of the skin of foot) gave similar results.

Physiologists have long been familiar with several aspects of the phenomena of reënforcement and inhibition in the frog, but I know of no detailed study of the significance of the temporal relations of stimuli in this connection. Goltz[170] called attention to the inhibition of the croaking reflex by peripheral stimulation, as well as to several similar phenomena. Nothnagel,[171] Lewisson,[172] and Wydensky[173] further contributed to our knowledge of the interference effects of stimuli in the frog. Wydensky proved that the application of an induced current to a nerve-muscle preparation may result in either contraction or relaxation of the muscle, according to the frequency of stimulation.

More recently Merzbacher[174] has dealt with the influences of complication of stimuli in the frog with the purpose of ascertaining the relations of the sense-organs to the reflex movements of the animal. His first paper is concerned especially with the functional importance of the eye in connection with reflexes. Unfortunately for the demands of this research, he did not attend particularly to the temporal relations of his stimuli. That a visual and a cutaneous stimulus were given either "at the same time or within a short interval of one another" (p. 250) is not the sort of information our problems demand.

According to Merzbacher's very interesting results a visual stimulus reënforces the reaction to a cutaneous stimulus. As the results of this paper show, this is only half a truth, for the two stimuli may either reënforce or inhibit one another's reactions. As Merzbacher observed no evidences of reaction to auditory stimulation he presumably did not attempt to study the influences of the ear in connection with reflexes.

There can be no doubt that the words reënforcement and inhibition as at present used in connection with the functions of the nervous system cover a multitude of widely differing phenomena. We can at once distinguish at least two important kinds of reënforcement or inhibition: first, that which is due to the functioning of special augmentary or inhibitory portions of the nervous system; second, that which is the result of the complication of stimuli. Any and every process in the nervous system may have either a reënforcing or an inhibiting influence upon simultaneous or succeeding processes; doubtless most processes or impulses at various times have both effects. The nervous system is constantly being modified by impulses from many sources, which suppress or strengthen one another according to their relative intensity, their temporal relations, and the motor relations of the portions of the organism which they affect.

The existence of the so-called refractory period in brain cortex and nerve indicates that every stimulus causes certain fundamentally important changes in the condition of the neural substance. These changes we may for convenience of illustration describe as modification of excitability, or of the functional capacity of central or peripheral tissues. Every stimulus causes a portion of the neural substance to pass from its normal state through a condition of increased excitability, which we may designate the positive phase, to a condition of diminished excitability, the negative phase. There is first an increase in the functional capacity of the tissues, then a decrease. If during the course of the change produced by a given stimulus a second stimulus becomes effective its result in reaction is determined by the particular phase of the tissues upon which it intrudes. If the nervous system is in the condition of increased excitability, and the two stimuli act upon sensory regions whose motor connections are not antagonistic, the reaction will be reënforced, as we say, by the previous stimulus; if, however, the second stimulus falls upon the negative phase of the nerve substance, the reaction will be partially or totally inhibited.

The facts which are most prominent as the result of this investigation are, first, that the temporal relation of stimuli is an important condition of certain forms of reënforcement and inhibition; second, that the interference effects of two stimuli cannot be studied to advantage without attention to the relations of the forms of reaction which are appropriate to each stimulus.

V. SUMMARY

1. Motor reactions of the green frog to electric stimuli are inhibited either partially or wholly by photic stimuli. The visual stimulus of a moving object has a like effect. It has been found, furthermore, that the same visual stimulus may either inhibit or reënforce the motor reaction in response to electric stimulation. When the two stimuli are given simultaneously reënforcement occurs, when the visual stimulus precedes the electric by half a second or more inhibition appears.

2. An auditory stimulus, which does not produce any visible reaction when given alone, modifies respiration and the reactions to other stimuli when given in connection with them.

3. The momentary auditory stimulus of a quick hammer blow when simultaneous with tactual stimulation reënforces the reaction to the latter stimulus. This reënforcement, or increase in the amount of reaction, ranges from 50 to 100% of the average reaction to the tactual stimulus alone. When the auditory stimulus is given before the tactual reënforcement occurs in gradually decreasing amount until the interval between the two stimuli reaches .35″; at this point the auditory stimulus has no apparent effect upon the tactual reaction. As the interval is still further increased inhibition appears and continues for intervals between .35″ and .9″. Reënforcement is greatest when the two stimuli are simultaneous; inhibition is greatest when the momentary auditory stimulus precedes the tactual by .4″ to .6″. When the interval reaches .9″ the first stimulus does not affect the reaction to the second.

4. Reënforcement is greater for the males than for the females; inhibition appears sooner and lasts longer in case of the females. This apparently indicates that the males are stimulated to activity by certain auditory stimuli, whereas the females are rendered passive by similar sounds.

5. Prolonged auditory stimulation by means of an electric bell causes reënforcement and inhibition, according to the temporal relations of the stimuli, as does momentary auditory stimulation, with the following differences: The maximum reënforcement occurs when the tactual stimulus is given about .25″ after auditory stimulation has begun; reënforcement continues for a period of 1.2″, _i. e._, when the electric bell continues to ring until the tactual stimulus is given, it reënforces the tactual reaction from simultaneity to 1.2″. Inhibition then appears, and continues until 1.8″. Both momentary and prolonged auditory stimulation cause first reënforcement, then inhibition of the appropriate reaction to a tactual stimulus.

6. The reënforcement-inhibition curves for the frog are very similar to those for man.

7. In case of the several pairs of stimuli whose interference effects have been studied reënforcement-inhibition appears. The first stimulus reënforces reaction to the second so long as the interval between them is not more than about .4″, while it inhibits the reaction when the interval is longer. Whether this reënforcement-inhibition curve as given in the experiments described may similarly be obtained for any and every pair of stimuli, no matter what their relation to reactions, remains to be determined.

8. In connection with the study of the mutual relations of stimuli of which this paper gives an account certain facts concerning the sense of hearing have been discovered. A summary statement of the results on hearing may be found on page 551.

FOOTNOTES:

[Footnote 139: The results brought together in this paper have been published in part in connection with other work in the following papers: Inhibition and Reënforcement of Reaction in the Frog, Jour. of Comp. Neurol. and Psychol., vol. 14, p. 124, 1904. Bahnung und Hemmung der Reactionen auf tactile Reize durch akustische Reize beim Frosche, Arch. f. d. ges. Physiol., vol. 107, p. 207, 1905. The Sense of Hearing in Frogs, Jour. of Comp. Neurol. and Psychol., vol. 15, p. 279, 1905.]

[Footnote 140: The Organ and the Sense of Taste in Fishes, Bulletin U.S. Fish Commission for 1902, pp. 237-272.]

[Footnote 141: Animal Behavior, Woods Hole Lecture Series, p. 300, 1899.]

[Footnote 142: The Response of the Frog to Light, American Journal of Physiology, vol. 9, p. 476, 1903.]

[Footnote 143: The Instincts, Habits and Reactions of the Frog, Harvard Psychological Studies, vol. 1, p. 590, 1903.]

[Footnote 144: The Skin and the Eyes as Receptive Organs in the Reactions of Frogs to Light, American Journal of Physiology, vol. 10, p. 31, 1903.]

[Footnote 145: Ueber die electromotorischen Erscheinungen an Hautsinnesnerven bei adaequater Reizung, Archiv für d. ges. Physiologie, vol. 63, p. 503, 1896.]

[Footnote 146: Harvard Psychological Studies, vol. 1, p. 592, 1903.]

[Footnote 147: Abstract of paper read before Section F of American Association for the Advancement of Science in Philadelphia, 1904. Science, vol. 21, p. 265, 1905. See also Bulletin of the U. S. Fish Commission for 1902, pp. 45-64, and the same for 1904, pp. 183-207.]

[Footnote 148: Anatomie des Frosches, VI, Lehre von Integument und von den Sinnesorganen, pp. 751, 752, 1904.]

[Footnote 149: Journal of Comparative Neurology and Psychology, vol. 15, pp. 279-304, 1905.]

[Footnote 150: Ueber die Beziehungen der Sinnesorgane zur den Reflexbewegungen des Frosches, Arch. f. d. ges. Physiol., vol. 81, pp. 222-262, 1900.]

[Footnote 151: "Blendung oder blosse Lichtentziehung erhoht die Erregbarkeit für mechanische Reize" (p. 253).]

[Footnote 152: Thousandths of a second.]

[Footnote 153: Arch. f. d. ges. Physiol., vol. 81, p. 227, 1900.]

[Footnote 154: A case of inhibition.]

[Footnote 155: Arch. f. d. ges. Physiol., vol. 107, p. 213, 1905.]

[Footnote 156: Journal of Physiology, vol. 9, pp. 60, 61, 1890.]

[Footnote 157: American Journal of Physiology, vol. 1, p. 336, 1898.]

[Footnote 158: Arch. f. d. ges. Physiol., vol. 68, p. 546, 1897.]

[Footnote 159: Travaux du Laboratoire de Marey, 1876.]

[Footnote 160: Arch. f. d. ges. Physiol., vol 59, p. 309, 1894.]

[Footnote 161: Zeitschr. f. Biol., vol. 32, p. 1, 1895.]

[Footnote 162: Journal of Physiology, vol. 21, p. 213, 1897.]

[Footnote 163: American Journal of Physiology, vol. 8, p. 213, 1902.]

[Footnote 164: Comptes rendus, vol. 124, p. 573, 1897.]

[Footnote 165: Nature, vol. 60, p. 629, 1899.]

[Footnote 166: Journal of Physiology, vol. 7, p. 101, 1886.]

[Footnote 167: Journal of Physiology, vol. 7, p. 111, 1886.]

[Footnote 168: Text-book of Physiology, p. 841, London, 1900.]

[Footnote 169: Arch. f. d. ges. Physiol., vol. 28, p. 495, 1882.]

[Footnote 170: Beiträge zur Lehre von den Functionen der Nervencentren des Frosches, p. 41, Berlin, 1869.]

[Footnote 171: Centralb. f. d. med. Wissensch., vol. 7, p. 211, 1869.]

[Footnote 172: Arch. f. Anat., u. Physiol., p. 259, 1869.]

[Footnote 173: Arch. d. Physiol. norm. et pathol., vol. 4, p. 690, 1892.]

[Footnote 174: Arch. f. d. ges. Physiol., vol. 81, p. 222, 1900.]

THE TEMPORAL RELATIONS OF NEURAL PROCESSES

BY ROBERT M. YERKES

Muscle contraction-time, according to the determinations of several investigators, varies about .0035".[175] Sanderson states that the time for direct stimulation of the muscle is approximately .0035" and for indirect stimulation, by means of the nerve, .007". The rate of nerve-transmission in the frog ranges from 25 to 35 metres per second.

Reflex reaction-time, as might be expected, varies widely with the nature of the reaction elicited by a stimulus, the condition of the animal, and the quality and strength of the stimulus. For many of the simple motor reactions of the frog it ranges between 20 and 60^{σ}.[176] Whether reflex reaction-time is to be sharply contrasted with instinctive and voluntary reaction-times, or whether they indistinguishably merge into one another is a question of considerable interest and importance for the student of the evolution of activity.

Voluntary reaction-time may be as short as 150^{σ} or as long as life, in an animal capable of profiting by experience as does the frog. It is preëminently the delayed type of reaction-time.

So much concerning the temporal relations of neural processes in the frog being well established, the purpose of the present paper is to call attention to some experimental results which indicate the existence of clearly defined types of reaction, and suggest possible values of reaction-time as a sign of mind.

The specific problems to be considered are: (1) Do reaction-times, in any given animal, range with equal frequency of occurrence from short to long, or are there certain modes (most frequented classes) which indicate definite types of reaction, such, for example, as the reflex, instinctive, etc.? (2) If there is distribution of the reaction-times about one or more modes, what are the types of reaction indicated thereby? (3) Finally, is reaction-time of service as a sign or measure of consciousness?

I wish especially to call attention to the fact that this paper deals with the reactions of the frog, not with animal reactions in general.

REACTIONS TO ELECTRICAL STIMULATION AND TYPES OF REACTION

Two years ago in connection with a discussion of the reaction-time of the green frog to electrical and tactual stimuli,[177] I presented a curve showing the distribution of 277 reaction-times to an electrical stimulus. The curve exhibited two clearly defined modes: one at between 60 and 70^{σ} and the other at about 160^{σ}. There was further a group of delayed reactions ranging about 500^{σ}. This form of distribution was interpreted, at the time, as indicative of three types of reaction, called, respectively, the reflex, the instinctive, and the delayed.

I have since obtained and examined with reference to form of distribution the further data which are presented in this paper. The reactions are all those of the green frog to electrical stimulation. The stimulus was applied by means of wires on the reaction-board on which the frog rested during the experiments. When reaction occurred in response to the electrical stimulus a circuit through the time-measuring apparatus was broken by the release of a delicate spring which had been held in place up to the instant of reaction by the weight of the frog. A Hipp chronoscope, controlled by a Cattell falling screen, served as a time-measuring mechanism. Three intensities of stimulus were used: (1) A current from one Mesco dry cell, (2) from two cells, and (3) from four cells.

Of the reactions whose time was measured there are three series. Series I is constituted by the recorded reaction-times in response to a one-cell stimulus, Series II, those in response to a two-cell stimulus, and Series III, those in response to a four-cell stimulus. The number of reactions, range and mode of each series are as follows:

Number of reactions Range Mode
Series I 193 161-798^{σ} 235^{σ}
Series II 288 41-647 235
Series III 256 61-178 105

The distribution of the 481 reaction-times of Series I and II is shown by Figure 1; that of the 256 reaction-times of Series III, by Figure 2. For both of these distribution polygons the reaction-times were arranged in ten^{σ} classes, beginning with the class 41-50^{σ}[178] in the case of the combined Series I and II and with the class 61-70^{σ} in the case of Series III.

Series I exhibits a primary mode at 235^{σ}. There are no reflex reactions in this series, unless it be maintained that the reflex reactions of the frog may have a reaction-time of over 160^{σ}, but there are a number of delayed reactions, some of which have reaction-times as long as 798^{σ}. This intensity of stimulation (one cell) may be said to call forth prompt reactions, which we may provisionally call instinctive, and delayed reactions, which have all the appearances of voluntary acts. There are no reactions which come within the range commonly considered as the reflex range of the frog (20-60^{σ}), and there are relatively few delayed reactions: almost all centre about the mode 235^{σ}.

Series II, in contrast with Series I, exhibits a secondary mode at 65^{σ} in addition to the primary mode at 235^{σ}. The stimulus-intensity of this series (roughly twice as great as that for Series I) induces a variety of short reaction, which did not appear in the case of the one-cell stimulus, and at the same time fewer delayed reactions. The range of the reaction-times for the two series is about the same, but the lower limits are markedly different.

Observation of the subjects during the experiments revealed two methods of reaction to the two-cell stimulus: a locomotor reaction (jump) which at once removed the animal from the source of stimulation, and a twitch of the hind legs which was instantly followed by the above-mentioned locomotor reaction. The leg reactions constitute the reflex group of Fig. 1, the usual prompt locomotor reactions, the instinctive group, and the slow locomotor reactions, the delayed or voluntary group.

It is to be noted that the instinctive reaction-time mode is the same for the two intensities of stimulation. This apparently indicates that change in intensity of stimulation causes a change in the type of reaction, not merely a gradual change in the position of the mode. For example, the modal reaction-time of 235^{σ} given by a one-cell stimulus did not shift to 200^{σ} or lower, as might have been expected, but instead there appeared a new type of reaction. The average reaction-times for the two series indicate a decrease in time with increase in intensity of stimulation, but they give no indication of the really important difference in the two series of reactions. The great importance of the distribution of the data, in addition to the common statistical quantities, is manifest.

Series III, whose reactions occurred in response to a very strong stimulus, differs in several important respects from the other series. Its range is much narrower, only 117^{σ}. Delayed reactions are lacking, and so also, curiously enough, are the reflex reactions of Series II. Instead of either or both of the modes of Series II, there appears in Series III an intermediate mode at 105^{σ}.

Our interpretation of these facts is facilitated by results of observation of the reacting subject. The leg reflex which frequently occurred in response to the two-cell stimulus never appeared in response to the four-cell stimulus. This in part explains the lack of the short reaction-time mode of Series II; it does not, however, account for the lack of delayed reactions. The latter fact may be referred to the intensity of the stimulus. Another difficulty in interpretation appears in connection with the intermediate mode, 105^{σ}. Is this to be considered an instinctive mode, as were those at 235^{σ}, or a reflex mode? Where is the line between reflex and instinctive action to be drawn? These results very clearly indicate that no line can be drawn, except quite arbitrarily. Reflex reaction-time, in the case of the frog, is continuous with instinctive, yet for any given situation the reflex, instinctive, and delayed (voluntary?) modes are likely to appear, as, for example, in the case of the data of this paper. Our conclusion must be, therefore, that although types of reaction are indicated by reaction-time results, the mode for a given type varies too much in position with different conditions to make it possible to say that a particular reaction-time is that of a certain type.

We may safely say, then, that for any given subject, the muscle contraction-time, nerve transmission-time, and simple sensory reaction-time to the constant stimulus in question being known, we should be able safely to interpret reaction-time records in terms of reaction types. For reflex, instinctive, and voluntary are terms which designate modes of reaction, albeit not isolated classes, for they intergrade.

Whether there are more types of reaction than are indicated by the data of this report does not concern us at present, for the practical as well as the theoretical bearings of our conclusions depend upon the existence of types, and not upon their number.

REACTION-TIME AS AN INDICATION OF CONSCIOUSNESS

Hesitation in reaction is commonly accepted as an important sign of volitional consciousness in man; consequently delayed reactions in lower animals are supposed to be indicative of psychic processes. Granting this much, reaction-time may be used as a sign of consciousness. It cannot be denied that the longer the reaction-time of a given animal the greater the probability that the reaction is conditioned by mental processes. Such a statement, it is true, has a basis neither better nor worse than that of most of our inferences concerning the nature of the actions of our fellow beings. As I have already attempted to show in a discussion of criteria of consciousness in animal psychology,[179] there is no one criterion of consciousness which can be used alone satisfactorily, but instead there are numerous signs of mind each of which has value according to the number and variety of our observations concerning its occurrence in connection with states of consciousness. The more of such signs we discover and learn to evaluate properly in relation to consciousness in its different grades and to one another, the safer will be our inferences concerning the existence of mental processes in animals.

Reaction-time is presented in this paper as an additional sign of mind. Like all other signs it is of value only if used as one of a series of indications of mental life. For if we attempt to judge of consciousness by reference to reaction-time alone, we may be seriously misled, whereas if we use it in connection with docility, variability, neural specialization, and other recognizedly valuable signs, we may be greatly aided in our inference. As in juristic procedure judgment is not based upon one bit of evidence nor even upon the evidence of a single witness, but upon evidence accumulated from all available sources, so in our attempts to judge of the existence of consciousness, it matters not whether the being be human or infra-human, we should make use of all phenomena which are recognized as signs of mind. The chief task of comparative psychology at present is the discovery and evaluation of signs of mind.