Yerkes, InJiibition and Reinforcement. 129 



rapidly moving object, experiments were made in which, as in 

 the case of sound and electricity, reactions to the electric stim- 

 ulus 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 be- 

 fore 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 frac- 

 tion 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 ivere given 

 almost simultaneously the visual reinforced the electric, i. e. , short- 

 ened the time 0/ reaction. As appears in the upper part of Ta- 

 ble III, the average time of 40 reactions (20 for each frog) to 

 the electric stimulus was hS^', and to the electric when it fol- 

 lowed upon the visual, 1280. 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 elect} ic by a considerable interval (/ second, see the 

 right side of the table') exactly the reverse effect appeared, there 

 was marked inhibition of reaction. The averages are 150^ for 

 electric stimulation alone, and 178(3 when it was preceded by 

 the visual stimulus. In this series there were 25 cases of in- 

 hibition to 14 of reinforcement. 



