Yerkes, InJiibition a)id Reinforcement. 135 



ity. One sound delays the time of reaction to another, 

 according to his findings, 45 '^, while a visual stimulus 

 causes a delay of 78 '^ in the auditory reaction. Apparently, 

 in the researches thus far made, complication of stimuli more 

 commonly causes inhibition than reinforcement. In the light of 

 the results which have been considered in this paper it is of in- 

 terest to inquire whether this may not be due to the fact that 

 the temporal relation has not been considered. Possibly any 

 two stimuli may be given in such relation that they will now in- 

 hibit, now reinforce one another. 



An investigation by Bowditch and Warren ('90) is of 

 special interest in this connection, since they studied the influence 

 on knee-jerk of various stimuli, given at different intervals with 

 respect to the tendon blow. As appears from the following 

 summary statement (p. 60-61) of their results there is striking 

 agreement between their findings for this reflex and mine for 

 the frog : 



"(i) In the majority of individuals experimented upon a 

 voluntary muscular contraction occurring simultaneously with 

 the blow upon the knee increases the extent of the knee-jerk, 

 but with the prolongation of the interval between the reinforce- 

 ment signal and the blow this effect is reversed ; the knee-jerk 

 becomes much reduced in extent and may even entirely disap- 

 pear. With a still further prolongation of the interval the knee- 

 jerk gradually returns to its normal value. The interval at 

 which the effect changes from positive to negative varies with 

 different individuals from 0.22" to 0.6". The interval at which 

 the knee-jerk returns to its normal value is \.']"-2.^". In two 

 individuals the effect of muscular contraction on the extent oi 

 the knee-jerk was wholly positive. 



"(2) The effect of a sudden auditory stimulus on the ex- 

 tent of the knee-jerk was, in the three subjects of experiment, 

 almost wholly positive, though great individual differences were 

 observed. The maximum effect was produced when the inter- 

 val between the sound and the blow was o. 2"-o. 3". 



"(3) The effect of a sudden visual stimulus upon the ex- 

 tent of the knee-jerk was with two of the three subjects of ex- 



