PHYSIOLOGICAL CHANGES IN THE CENTRAL SYSTEM. 29! 



different from the same line on a white one, and so on 

 throughout all contrasts. 



This relation between the stimulus and the sensation 

 is expressed in the psycho-physic law by the formula 

 that sensation increases in intensity according to the 

 logarithm of the stimulus. The limits within which this 

 law is applicable do not immediately concern us, its use 

 here being merely to express the fact that at any moment 



30- 



1.0" 



1.7' 



FIG. 68. 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 o. The time 

 intervals which elapsed 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" it amounts to nothing, during the next 0.6" the height 

 of the kick is actually diminished the longer the interval, after 

 which the negative reinforcement tends to disappear, and 

 when 1.7" is allowed to elapse, the height of the kick ceases to 

 be affected by the clenching of the hand. (Bowditch and 

 Warren.) 



the activities of the nervous system under the influence 

 of existing stimuli form a background against which a 

 new stimulus according to its intensity and character 



