REFLEX ACTIONS. 



147 



jerk may be demonstrated in some individuals in whom the ordi- 

 nary blow upon the tendon fails to elicit a response. Bowditch and 

 Warren* studied the phenomenon of reinforcement and brought out 

 a fact of very great interest. They studied especially the time 

 interval between the blow upon the tendon and the reinforcing act 

 and found that if the latter preceded the blow by too great an inter- 

 val then, instead of an augmentation of the jerk, there was a dimi- 

 nution which they designated as negative reinforcement or inhi- 

 bition. This inhibiting effect began to appear when the reinforcing 

 act (hand-squeeze) preceded the blow by an interval of from 0.22 

 to 0.6 sec, and the maximum inhibiting effect was obtained at an 



mm 



Fig. 65. Showing in millimeters the amount by which the "reinforced" knee-kick 

 varied from the normal, the level of which is represented by the horizontal line at 0, "nor- 

 mal." The time intervals elapsing between the clenching of the hand (which constituted 

 the reinforcement) and the tap on the tendon are marked below. The reinforcement is 

 greatest when the two events are nearly simultaneous. At an interval of 0.4 sec. it 

 amounts to nothing: during the next 0.6 sec. the height of the kick is actually dimin- 

 ished the longer the interval, after which the negative reinforcement tends to disappear; 

 and when 1.7 sec. is allowed to elapse the height of the kick ceases to be affected by the 

 clenching of the hand. {Bowditch and Warren.) 



interval of from 0.6 to 0.9 sec. Beyond this point the effect became 

 less noticeable, and at an interval of 1.7 to 2.5 sec. the reinforcing 

 act had no influence at all upon the jerk. These relations are 

 shown in the accompanying curve (Fig. 65). These authors con- 

 firmed also the fact that a sensory stimulus, such as a gentle blast 

 of air on the conjunctiva or the knee, may reinforce the jerk. The 

 physiological explanation of the reinforcement, negative and posi- 

 tive, is a matter of inference only, but the view usually held is that 

 it is due to "overflow." That is, many facts, such as strychnin 

 tetanus, indicate that the neuromuscular machinery of the entire 

 central nervous system is more or less directly connected and that 



* Bowditch and Warren. "Journal of Physiology," 2, 25, 1890. 



