Having now presented the results of my own investigation I wish to call attention to certain of their relationships to the work of other investigators, and to discuss briefly their significance.
TABLE 5. PROLONGED AUDITORY STIMULATION (ELECTRIC BELL)
| Frog No. 2. Female. Weight usually 25 grams. | ||||||
| Interval. | Tactual Stimulation. | Auditory and Tactual Stimulation. | Amount of Reënforcement or Inhibition. | Number of Reactions, Reënforced or Inhibited. | ||
| 0″ | 9.20 | mm. | 12.12 | mm. | + 31.7% | +10.0 |
| .25 | 4.56 | 11.88 | +160.5 | +22.5 | ||
| .45 | 8.94 | 16.94 | + 89.5 | +18.5 | ||
| .65 | 17.18 | 22.50 | + 31.0 | +15.5 | ||
| .90 | 9.42 | 13.32 | + 41.4 | +14.5 | ||
| 1.20 | 10.54 | 9.64 | - 8.5 | - 2.5 | ||
| 1.40 | 24.00 | 20.64 | - 14.0 | - 6.0 | ||
| 1.58 | 19.16 | 17.80 | - 7.1 | - 7.0 | ||
| 1.95 | 14.50 | 15.40 | + 6.2 | + 5.5 | ||
| Frog No. 3. Male. Weight usually 5 or 10 grams. | ||||||
| 0″ | 14.92 | mm. | 25.20 | mm. | + 68.9% | +11.0 |
| .25 | 15.88 | 38.54 | +142.7 | +24.0 | ||
| .45 | 13.48 | 26.02 | + 92.9 | +13.5 | ||
| .65 | 18.30 | 27.94 | + 52.6 | +13.0 | ||
| .90 | 20.94 | 29.06 | + 38.8 | + 9.5 | ||
| 1.20 | 21.90 | 30.58 | + 39.6 | + 1.0 | ||
| 1.40 | 19.18 | 18.34 | - 4.4 | - 5.5 | ||
| 1.58 | 32.24 | 26.30 | - 18.1 | - 3.0 | ||
| 1.95 | 13.86 | 14.14 | + 2.0 | + 2.0 | ||
VI. DISCUSSION OF LITERATURE AND RESULTS
The literature on reënforcement and inhibition is large, and even that portion of it which deals especially with the importance of the temporal relations of stimuli in connection with reënforcement and inhibition is so extensive that it does not seem worth while to attempt to give a systematic résumé of it for the purposes of this paper. I shall therefore call attention merely to those investigations which have contributed directly to the solution of the problems with which we are now concerned.
Bowditch and Warren[156] discovered that knee-jerk in the human subject is reënforced when an auditory, a visual, or a tactual stimulus precedes the tendon blow by .1˝ to .5˝, whereas the same stimuli have an inhibitory influence when they are given from .5˝ to 1.0˝ before the tendon blow.
At the suggestion of Bowditch, Cleghorn[157] undertook to investigate the influence of complication of stimuli upon voluntary movements. In this research graphic records taken in connection with an ergograph indicated (1) that "a sensory stimulus" applied just as the muscle was beginning to contract (voluntarily) caused an increase in the height of the contraction, and (2) that the relaxation following a contraction with intercalated sensory stimulus is quicker and more complete than when no stimulus is given (p. 344). Cleghorn did not give special attention to the significance of the temporal relations of the stimuli which he employed, and his work was limited to the phenomenon of reënforcement of voluntary action by reason of the appearance, during the progress of his research, of an excellent paper on the interference of stimuli by Hofbauer.[158]
Hofbauer covered thoroughly the ground which Cleghorn had planned to work over. The ergographic method was employed also by Hofbauer in his very careful study of the interference of impulses in the central nervous system of man. It was noticed that while the subject was rhythmically contracting a certain group of muscles in response to some prearranged signal (e.g., the sound of a metronome) the report of a pistol caused the contraction which immediately followed it to be much greater than the average of the rhythmic series, while the next contraction was correspondingly less than the average. It thus appeared that the sudden sound caused, first, reënforcement of the voluntary movement, then, inhibition. The reënforcement is greatest, according to Hofbauer, when the voluntary movement occurs immediately after the pistol report. When the report precedes the metronome signal by .2˝ reënforcement is still marked, but thereafter it decreases rapidly in amount, until finally at .5˝ inhibition appears. When the interval between the two stimuli is 1.0˝ the first stimulus has practically no effect upon the voluntary movement in response to the second. (Hofbauer, p. 558.)
What Bowditch and Warren, not to mention other students of the subject, have described for reflex action in man, Hofbauer, Cleghorn, and others have shown to hold true also of voluntary movements. Unfortunately my own investigation was completed up to the point of the writing of this paper before I read Hofbauer's work, so I have not followed methods of dealing with my data which would make our results directly and easily comparable. But, whatever may be the relations of our results in detail, there can be no doubt that what he has demonstrated for man is true in its important aspect of the reënforcement-inhibition phenomena for the frog.