RESULTS OF EXPERIMENTS.

1. Electric Stimulation and Light.

The following specimen records leave no room for doubt as to the inhibitory influence of increase in light intensity on the electric reaction. In these tests the visual stimulus was given from 1 to 2 seconds before the electric by the turning on of a 16-candle power electric light which was placed 30 cm. in front of the animal in one series, and 15 cm. above it in another. The laboratory records appended are self-explanatory.

Red light, 16 c. p., 2 seconds before and until electric stimulation.

Table I indicates the lack of response to a 1 cell electric stimulus when accompanied by an increase in light intensity, and Table II proves conclusively that the light is the cause of the inhibition of reaction.

All reaction-times are given in thousandths of a second, indicated by 6.

The inhibitory influence of light in this case depends upon the intensity of the electric stimulus. Even a very strong light will not cause much retardation of reaction to a 3 or 4 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 ('00, p. 253) 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.'

1 "Blendung oder blosse Lichtentziehung erhöht die Erregbarkeit für mechanische Reize." (p. 253-)

2. Electric Stimulation and Sound.

Inasmuch as the experiments here described were conducted in a laboratory where noise and jar are unavoidable, it is worth while at this place to offer reasons for the belief that sounds did not to any considerable extent affect the time of reaction to other stimuli.

As tests of the influence of loud sounds on the electric reaction-time, an apparatus was arranged whereby an electric bell rang for a certain interval before the electric stimulation. The bell was placed 40 cm. from the frog, and for one series of 300 reactions it rang o. 1 second before the electric stimulation, for another 1.0 second before. The reactions were taken in pairs, first a reaction to the electric stimulus alone, then one to the electric stimulus preceded by the auditory, at the rate of one a minute. The results may be summarized, without mention of other values than the means, thus:

Series I.

Series II.

Average of 300 reactions to 2 Cell Electric Stimulus Alone, 172.0 6 Average of 300 reactions to 2 Cell Electric Stimulus, when preceded for 0.1 second by Auditory Stimulus, 176.5 6

Average of 300 reactions to 2 Cell Electric Stimulus Alone, 144.7 61 Average of 300 reactions to 2 Cell Electric Stimulus, when preceded for 1.0 second by Auditory Stimulus, 150.2 6

In each of these series there is evidence that the sound caused slight inhibition or delay of reaction, but when we consider, as will be made clear later, that the probable error of the averages is greater than the apparent delay, it is at once evident that we can not safely argue from these results to the inhibitory influence of sound. Indeed most observations on record tend to show that audition is not very important in the frog, at least when it is out of water.

3. Electric Stimulation and Moving Object.

ments.

Preliminary Experi

For the purpose of determining the effect upon reactiontime to an electric stimulus of stimulation of the eye by a

1 The conditions were not precisely the same for the two series, as the frogs had become inactive.

rapidly moving object, experiments were made in which, as in the case of sound and electricity, reactions to the electric stimulus alone and to visual and electric were alternated. Thus in case of each pair of reactions it was possible to note whether the visual stimulus lengthened or shortened the reaction-time. The visual stimulus was given by quickly bringing a finger before a window in the reaction-box.

As a preliminary test two series of 20 pairs of reactions each were taken with two frogs (Nos. 5 and 6). In the first series the finger was suddenly moved over the window, and the electric stimulus was given either simultaneously or a small fraction of a second later. It was of course impossible to arrange for an accurate measurement of the temporal relations of the stimuli in this case. In case of the second series the finger was moved back and forth before the opening for an interval of not less than a second before the electric stimulus was given.

These experiments yielded results which were surprising in view of the previous work. When the stimuli were given almost simultaneously the visual reinforced the electric, i. e., shortened the time of reaction. As appears in the upper part of Table III, the average time of 40 reactions (20 for each frog) to the electric stimulus was 1486, and to the electric when it followed upon the visual, 1286. Furthermore, an examination of the pairs of reactions shows, as the table indicates, that there were 27 cases in which the visual stimulus caused reinforcement to 13 in which it caused inhibition. When the visual stimulus preceded the electric by a considerable interval (1 second, see the right side of the table) exactly the reverse effect appeared, there was marked inhibition of reaction. The averages are 150 6 for electric stimulation alone, and 1786 when it was preceded by the visual stimulus. In this series there were 25 cases of inhibition to 14 of reinforcement.

TABLE III.

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

Preliminary Series.

Visual Stimulus Moving Finger. Averages for 20 reactions.

In Table IV I have given the probable errors, standard deviations and coefficients of variation of the means, except in case of Series II.

4. Electric Stimulation and 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 me to undertake a more accurate study of the subject. To this end a revolving disc, which moved at the rate of one revolution per