154 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



purposes was first brought out bv the work of Erb and Westphal* 

 in 1875. 



Reinforcement of the Knee-jerk.— It was first shown by 

 Jendrassik (1S83) that the extent of the jerk may be greatly aug- 

 mented if, at the time the blow is struck upon the tendon, a strong 

 voluntary movement is made by the individual, such as squeezing 

 the hands together tightly or clenching the jaws. This phenomenon 

 was studied carefully in this country by Mitchell and Lewis, f who 

 ascertained that a similar augmentation may be produced by giving 

 the individual a simultaneous sensorv stimulation. Thev desis;- 



Fig. 70. — 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 hands (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 Idck is actually diminished, 

 while after an interval of 1 sec. 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 hands. — (Bowditch and Warren.) 



nated the phenomenon as a reinforcement. It is found that by a 

 reinforcement the knee-jerk may be demonstrated in some indi- 

 viduals in whom the ordinaiy 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 pre- 

 ceded the blow by too great an interval then, instead of an aug- 

 mentation of the jerk, there was a diminution which they desig- 

 nated as negative reinforcement or inhibition. This inhibiting 



* Erb and Westphal, "Archiv f. Psychiatrie," 1875, vol. v. 



|: Mitchell and Lewis, "American Journal of Med. Sciences," 92, 363, 1886. 



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



