Yerkes and Dodson, Habit Formation. 481 



2. The relation of the strength of electrical stimulus to rapidity 

 of learning or habit-formation depends upon the difficultness of 

 the habit, or, in the case of our experiments, upon the conditions 

 of visual discrimination. 



3. When the boxes which are to be discriminated between differ 

 very greatly in brightness, and discrimination is easy, the rapidity 

 of learning increases as the strength of the electrical stimulus is 

 increased from the threshold of stimulation to the point of harmful 

 intensity. This is indicated by curve II. Our results do not 

 represent, in this instance, the point at which the rapidity of learn- 

 ing begins to decrease, for we did not care to subject our animals 

 to injurious stimulation. We therefore present this conclusion 

 tentatively, subject to correction in the light of future research. 

 Of its correctness we feel confident because of the results w^hich 

 the other sets of experiments gave. The irregularity of curve II, 

 in that it rises slightly for the strength 375, is due, doubtless, to 

 the small numbers of animals used in the experiments. Had we 

 trained ten mice with each strength of stimulus instead of four the 

 curve probably would have fallen regularly. 



4. When the boxes differ only slightly in brightness and dis- 

 crimination is extremely difficult the rapidity of learning at first 

 rapidly increases as the strength of the stimulus is increased from 

 the threshold, but, beyond an intensity of stimulation which is 

 soon reached, it begins to decrease. Both weak stimuli and strong 

 stimuli result in slow habit-formation. A stimulus whose strength 

 is nearer to the threshold than to the point of harmful stimulation 

 is most favorable to the acquisition of a habit. Curve III verifies 

 these statements. Itshows thatwhendiscriminationwasextremely 

 difficult a stimulus of 195 units was more favorable than the weaker 

 or the stronger stimuli which were used in this set of experiments. 



5. As the difficultness of discrimination is increased the 

 strength of that stimulus which is most favorable to habit-forma- 

 tion approaches the threshold. Curve II, curve I, curve III is the 

 order of increasing difficultness of discrimination for our results, 

 for it will be remembered that the experiments of set III were 

 given under difficult conditions of discrimination; those of set I 

 under medium conditions; and those of set II under easy condi- 

 tions. As thus arranged the most favorable stimuli, so far as we 

 may judge from our results, are 420, 300, and 195. This leads us 

 to infer that an easily acquired habit, that is one which does not 



