Harvard Psychological Studies Part 75

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 s.e.x 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.

[Ill.u.s.tration: FIG 8: Distribution of 277 reactions.]

The column of figures at the left indicates the number of reactions at any point. Below the base line are the cla.s.ses. For convenience of plotting the reactions have been grouped into cla.s.ses which are separated by 25[sigma]. Cla.s.s 1 includes all reactions between 1[sigma] and 25[sigma], cla.s.s 2 all from 25[sigma] to 50[sigma], and so on to 400[sigma], thereafter the cla.s.ses 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 cla.s.sification 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 pa.s.ses 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 pa.s.s 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 repet.i.tion. 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 pus.h.i.+ng 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.

[Ill.u.s.tration: Fig. 9. Ground Plan of Reaction Box for Electrical Stimuli (Group 2). _IC_, interrupted circuits; _CC_, chronoscope circuit; _X_, key for making stimulus circuit and breaking chronoscope circuit; _B_, stimulus battery; _S_, string from reaction key to animal. Scale 1/2.]

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 s.e.x were as follows:

Length. Weight. s.e.x.

Number 1 7.5 cm. 35 grams. Male.

Number 2 7.3 " 37 " Male.

Number 4 8.2 " 50.4 " Female?