In studying the various influences of complication of stimuli in the frog, I have used two methods: the measurement of reaction-time and of the amount of reaction. The reaction-time results will be presented first.
Reaction-time to electric stimulation of the skin was studied with special attention to the influence of other stimuli which were given in definite temporal relation to the electric stimulus. A Hipp chronoscope, controlled by a Cattell's falling screen, served as a time-measuring apparatus. The other essentials of the apparatus were a reaction-box, and devices for giving the stimuli and indicating the reaction. On the bottom of the reaction-box a series of wires were so placed that an electric stimulus could be given to the frog resting upon them by the closing of a key in the hands of the experimenter. In preparation for each experiment the frog was placed upon these open circuit wires in such a position that the weight of its body pressed upon a delicate spring in the floor of the box, thus causing the chronoscope circuit to be completed. The forward jump of the frog in response to stimulation caused the breaking of this circuit by the release of the spring upon which the animal rested. When all was in readiness for an experiment the chronoscope was started, and a key closed which simultaneously gave an electric stimulus to the frog and completed a circuit which caused the chronoscope record to begin. The stimulus consisted of a current from one or more "Mesco" dry cells. The motor reaction of the frog broke the chronoscope circuit, thus causing the chronoscope record to stop. It was then possible for the experimenter to read from the dials of the chronoscope the time, in thousandths of seconds, intervening between stimulus and reaction (reaction-time). In case of additional stimuli in connection with the electric, various simple devices were introduced to meet the demands of the experiments. These will be described in connection with the statement of results in each case.
Electric and photic stimuli. A photic stimulus was given from one to two seconds before the electric stimulus by the turning on of a sixteen-candle-power incandescent light, which was placed thirty cm. in front of the frog in the case of one series of experiments and fifteen cm. above it in another. The light uniformly inhibited reaction to the electric stimulus, as is shown by the results of Table 1.
TABLE 1
Title of investigation,—Electric-Visual (Red Light).
Experimented on,—Green Frog No. 4.
Harvard Psychological Laboratory,—9.40 A.M., Feb. 28, 1902.
Chronoscope control average, 189σ,—Electric stimulus, 1 Cell.
| NO LIGHT. | |
| Number of Experiment. | Reaction-time. |
| 1 | 152σ |
| 2 | 145 |
| 3 | 221 |
| 4 | 327 |
| 5 | 263 |
| 6 | 271 |
| 7 | 329 |
| 8 | 215 |
| 9 | 225 |
| 10 | 216 |
| LIGHT BEFORE ELECTRIC STIM. | |
| 11 | No reaction. |
| 12 | No reaction. |
| 13 | No reaction. |
| 14 | No reaction. |
| 15 | No reaction. |
| NO LIGHT. | |
| 16 | 216 |
The inhibitory influence of light depends upon the intensity of the electric stimulus. Even a very strong light will not cause much retardation of reaction to a three or four cell current. As the strength of the electric stimulus decreases the delay of reaction increases, until finally there is complete inhibition. At this point, an electric stimulus, to which the frog would react almost invariably when there is no disturbing condition, will fail to cause reaction in the presence of a sudden increase in light intensity.
Merzbacher[150] states that the leg reflex of a frog, so placed that its legs hang free in the air, is greater in response to a given cutaneous stimulus in darkness than in daylight.[151]
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.