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.
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 classes. For convenience of plotting the reactions have been grouped into classes which are separated by 25σ. Class 1 includes all reactions between 1σ and 25σ, class 2 all from 25σ to 50σ, and so on to 400σ, thereafter the classes are separated by 100σ. It is noticeable that there is one well-marked mode at 75σ. A second mode occurs at 175σ. 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 500σ there is a third mode; but as such this has little meaning, since the reactions are usually pretty evenly distributed from 300σ on to 2000σ; if there is any grouping, however, it appears to be about 500σ and 800σ.
The first mode has already been called the reflex mode. The short reactions referred to usually lie between 40σ and 80σ, and since experiment has shown conclusively that the spinal reflex occupies about 50σ, 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 ampère. This was used as the stimulus for series 1. 2 cells, 0.5 to 1.0 volt, 0.00003 to 0.00006 ampère. This was used for series 2. 4 cells, 1.2 to 1.8 volt, 0.00007 to 0.0001 ampère. This was used for series 3.
