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(From the Harvard Psychological Laboratory, Hugo MÜNSTERBERG, Director.)


An investigation of the time relations of neural processes in the frog, which began with the determination of simple reaction-time (YERKES, '03. p. 598 et seq.), has now been extended to a study of the influence of complication of stimuli on time of reaction. In this report an attempt will be made to present, in summary, certain results which contribute somewhat to our knowledge of inhibition (Hemmung) and reinforcement (Bahnung).

Attention was called, in the paper referred to above (p. 627 et seq.), to the inhibition, by visual stimuli, of visible motor reactions to auditory stimuli, as well as to the apparent reinforcement, by an auditory stimulus (tuning-fork sound), of reactions to visual stimulation by a moving red disc, These observations led to a more detailed and systematic study of the influence of complication of stimuli, so far as reaction-time is concerned.

The work thus far done includes studies of (1) the effect of stimulation by increase in light intensity upon reaction-time to electric stimulation of the skin, (2) the effect of an auditory stimulus upon electric reaction-time, (3) the effect of visual stimulation by the appearance of a moving finger, (a) when shown almost simultaneously with the giving of the electric stimulus, and (b) when shown a considerable interval (at least

1 This work will be published in detail, in connection with other results, in Volume 2 of the Harvard Psychological Studies.

one second) before the giving of the electric stimulus, (4) the effect of visual stimulation by a moving red disc, shown in one series of experiments 0.1", and in another 0.5" before and until 'the electric stimulus was given.

In all cases the 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. The general method of the investigation was the same as that described in my earlier paper (p. 601). 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 batteries. 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



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.


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Title of investigation.

Electric-Visual Series (Red Light). Experimented on.

Green Frog No. 4. Harvard Psychological Laboratory. 9.15 A. M., Feb. 28, 1902. Chronoscope control average 1896. Electric stimulus, 1 Cell. Red light, 16 c. p., 2 seconds before and until electric stimulation. Number of Experiment.



144 0 1




I 2

192 3

587 4

No reaction. 5

No reaction. 6

Reaction to second stim. 7

No reaction. 8

No reaction.

No reaction.
No reaction.

No reaction.

No reaction. 14

1190 15

No reaction. 16

No reaction. 17

No reaction. 18

No reaction. 19

No reaction.

No reaction. Table I indicates the lack of response to a i 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.


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, 189 6.

Electric stimulus, 1 Cell.

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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 ('oo, 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.)

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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 0. i 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 :

Average of 300 reactions to 2 Cell Electric Stimulus Alone, 172,0 6 Series I. 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 Series II. 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. Preliminary Experi

ments. 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.

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