646 COMPARATIVE ELECTRO-PHYSIOLOGY 



the responsive sensation and the character of the molecular 

 change that induces it has been regarded as unascertainable. 

 1 That many of our feelings depend immediately upon the 

 condition of the nervous elements is beyond doubt. . . . 

 What is the peculiar nature of the excitation upon which 

 the different feelings depend for their differences of 

 quality ? What is the characteristic change in the excita- 

 tion that gives rise to two kinds of tone which the feelings 

 possess, to pleasure and to pain ? Physiological psycho- 

 logy can answer none of these questions with much con- 

 fidence.' ' 



The fundamental contrast of tone in question raises the 

 inquiry, therefore, whether it may be possible to discover any 

 antecedent nervous changes of opposed character. Taking 

 an instance of response by some simple form of sensation, it 

 is well known that while moderate stimulus produces a feeling 

 which may be described in general as not unpleasurable, or 

 even distinctly pleasurable, an intense stimulus of the same 

 nature will cause a displeasurable or even painful, sensation. 

 These fundamental differences of quality are classified as 

 ' positive and negative tones ' of sensation, the term ' positive ' 

 being here associated with perceptions which are not un- 

 pleasant, or even actually pleasant, while ' negative ' refers to 

 the reverse. While the sensations ensuing under moderate 

 stimulus, then, such as moderate pressure or moderate light, 

 are of ' positive ' tone, those brought about by more intense 

 stimulus are apt to become converted into negative. The 

 positive sensation grows to a maximum, according to the rise 

 of stimulus-intensity within a certain limit. Beyond this 

 point, sensation becomes, first, less and less positive, and then 

 increasingly negative, as the intensity of stimulus continues 

 to be augmented. Or a simple stimulus, such as a light blow, 

 which evokes a positive sensation, will, when often repeated 

 that is to say, when employed tetanically induce a negative 

 or painful sensation. It is thus seen that the tone of sensation 

 is in some way associated with the intensity or duration of 



1 Ladd, Outlines of Physiological Psychology (1891), p. 387. 



