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 o.I 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.

Preliminary Series. Visual Stimulus Moving Finger. Averages for 20 reactions.

Aver. 148

128

13

27

O

150 178 25 14 Visual Stimulus Moving Red Disc.

Visual o.1" before electric. Visual 0.5" before electric.

I

но

155 259 24
132 227 17 7

I

O

I

O

143 230 74 24

2

Aver. 147 126 29 71

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 o.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. I have found it de

sirable, 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 o 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.

The auditory stimulus used was either the sound of a quick hammer blow (momentary stimulus of Series I), or the ringing of an electric bell for a certain length of time (prolonged stimulus of Series II). In Fig. 1 the bell is shown. It was placed eighty cm. from the frog, and in order that the influence of vibration of the experiment table 1 Arch. f. d. ges. Physiol., vol. 81, p. 227, 1900.

Figure 1. Auditory-tactual Reënforcement-Inhibition apparatus. P, pendulum; p, contact point of P; b, attachment for electro-magnet, a; m, key for circuit of electric bell, B; n, key for magnet circuit of tactual apparatus; K, hand-key for release of pendulum and temporary closing of electric bell circuit; k,, k,, k, keys in circuits; e, f, g, magnetic release for tactual apparatus; 1, pivoted lever, bearing rubber cone, T, and weights, w. (Drawn by Dr. Wm. E. Hocking.)

might be avoided it was suspended from the pendulum frame. When the hammer was used it was placed sixty cm. from the frog, on the pendulum table. The holder for the frog and the tactual apparatus occupied a separate table which was not disturbed by the jars of the pendulum table.

The tactual stimulus was given by a rubber cone, T, two mm. in diameter at its apex. This rubber point, after the electric release of the lever to which it was attached, struck the frog at the middle point of a line drawn between the posterior margins of the tympana. The intensity of the stimulus could be varied by weighting the lever, l, at w.

All experiments were made with the green frog, Rana clamata Daudin. The reactions were taken regularly at half-minute intervals in pairs: first, a tactual stimulus reaction, then an auditory-tactual reaction. Ten, fifty, or one hundred pairs constituted a series. So far as the condition of the frog is concerned there seems to be nothing undesirable in long series, for fatigue does not appear, and so long as the animal is kept moist and in an unconstrained position, it continues to react normally, and without frequent struggles to escape. The advantage for the purposes of this investigation of taking the reactions in pairs, rather than taking separate series of reactions for each stimulus or combination of stimuli, is obvious. It enables us to compare directly the reactions of each pair, in other words those reactions which took place under most nearly identical conditions, and to note at once whether the auditory stimulus reënforced or inhibited the tactual reaction.

During a series the intensity of the tactual stimulus was changed as conditions demanded, but for any one pair of reactions it was always the same. It not infrequently happened that an intensity which at first caused merely a slight movement of the leg, later in the series uniformly brought about a maximal contraction, or the reverse might be true, and inasmuch as a maximal reaction to the tactual stimulus alone left no opportunity for judging of the influence of the auditory stimulus, when it was given in addition to the tactual, it was always necessary in such cases so to alter the intensity of the tactual stimulus that a medium reaction resulted.

The frogs, after being placed in the saddle-like holder and held firmly for a few seconds, seldom struggled very much, but if bound tightly they became irresponsive to the stimuli.1 It was, therefore, necessary after they had quieted down to loosen the bands which held them in position. For the purpose of excluding the influence of visual 1 A case of inhibition.

stimuli a wire screen cap covered with black cloth was put over the head; this served to keep the animal in position as well as to exclude visual stimulation.

a. Momentary auditory stimulation. Four frogs were used for a study of the influence of the momentary sound produced by a hammer blow, and for each of these animals fifty pairs of reactions were recorded in series each day. The temporal relation of the stimuli was changed daily during a week of experimentation: the results therefore consist of fifty pairs of reactions with each frog for each of the following seven intervals: (1) Auditory and tactual stimuli simultaneous, (2) auditory .25" before tactual, (3) auditory .45′′ before, (4) auditory .15" before, (5) auditory .65" before, (6) auditory .35" before, (7) auditory .90" before. The intervals were used in the experiments in the above order to avoid the formation of the definite habits of reaction which regular increase in the interval would have favored.

Typical of the results with all the animals are the following (Table 3) which were obtained with No. 1, a male. The figures in cach case indicate the average of fifty reactions. Reënforcement and inhibition are expressed in terms of the tactual reaction, i. e., the auditorytactual reaction is so many per cent greater (reënforcement) or less (inhibition) than the tactual. In the tables reënforcement is indicated by the sign; inhibition by the - sign. In the last column of the table is given the number of reactions that were reënforced or inhibited. This was determined by comparing directly the reactions of each pair. Cases in which the two reactions were the same were distributed equally between the two classes: tactual reactions rëenforced by auditory stimulus, and tactual reactions inhibited by auditory stimulus. Assuming that the auditory stimulus was without effect upon the tactual reaction, the number of reactions in these two classes would be approximately the same, hence all auditory-tactual reactions over half in a series, i. e., over twenty-five, which are greater than the corresponding tactual reactions, are reënforced reactions, and can be taken as a measure of the reënforcing influence of the auditory stimulus. In the same manner all reactions over half which show inhibition can be taken as a measure of the inhibitory value of the auditory stimulus.

As preliminary tests described in an earlier paper1 furnished evidence of sex-differences, it is worth while to compare the results given by the males and females in these experiments with momentary auditory stimulation. For purposes of comparison I have presented 1 Arch. f. d. ges. Physiol., vol. 107, p. 213, 1905.