THE CHARACTERISTICS OF STIMULI 45 



dependent upon the absolute intensity of the stimulus, that is, 

 upon the extent of the alterations in the external vital conditions, 

 but the intensity of the response that can be observed. One 

 refers frequently to threshold stimulation, to stimulation beneath 

 the threshold, to submaximal, maximal and supermaximal stimu- 

 lation. Such a classification is in many ways very valuable. It 

 is not only of practical value for the establishment of definite 

 intensities of stimulation, but also for the study of the state of 

 irritability in the living organisms. 



The threshold of stimulation furnishes roughly a standard for 

 the degree of irritability of a living system. The threshold value 

 of a stimulus is then that degree of intensity which is just suffi- 

 cient to bring about a perceptible response. The threshold of 

 stimulation is low, that is, the irritability is great, when the inten- 

 sity of the threshold stimulus is small; the threshold is high, 

 that is, the irritability of a system is small, if the intensity of the 

 threshold stimulus is great. All intensities of stimuli beneath the 

 threshold are sub-threshold stimuli. Here a point must not be 

 overlooked, which in older physiology did not generally meet 

 with sufficient attention. From the fact that the sub-threshold 

 stimuli produce no apparent effects, the wrong deduction must 

 not be made, that they have no effect whatsoever. The concep- 

 tion of the threshold of stimulation originated in the field of 

 muscle physiology and that of the special senses. Here the indi- 

 cator of the response is, on the one hand, contraction of the 

 muscles, and on the other, conscious sensation. There was a 

 great temptation to consider the stimulus altogether ineffectual, 

 if it produced no conscious sensation or no contraction of the 

 muscle. Today with our finer and more sensitive indicators for 

 the study of the alterations in the living substance, we know in 

 reality that sub-threshold stimuli, which produce no apparent 

 effect in the living substance, can have an effect in reality. 



I will call your attention later to the fact that these sub- 

 threshold stimuli play a very important role under certain condi- 

 tions in the activities of the central nervous system. It only 

 depends upon the sensitivity of our special senses, or the indi- 

 cators used for this purpose, as to whether the alterations can 



