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 148σ,[152] and to the same stimulus when it followed the visual 128σ. 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 150σ for the electric stimulus alone, 178σ 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.
| Frog. | Electric Alone. | Visual 0.1˝ before elect. | Number Inhibited. | Number Reënforced. | Number Equal. | Electric Alone. | Visual 1.0˝ before elect. | Number Inhibited. | Number Reënforced. | Number Equal. |
| Preliminary Series. | Visual Stimulus Moving Finger. | Averages for 20 reactions. | ||||||||
| No. 5. | 179σ | 158σ | 6 | 14 | 0 | 163σ | 206σ | 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.