646 COMPARATIVE ELECTRO-PHYSIOLOGY 



the responsive sensation and the character of the molecular 

 change that induces it has been regarded as unascertainable. 

 ' 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.' l 



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 l^sychoJogy (1891), p. 387. 



