SENSORY DISCRIMINATION 



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tions confined to cortical projection areas, but the 

 ability to make certain discriminations remains. 7 

 The effects of interrupting sensory pathways at 

 thalamic, tectal, bulbar or lower levels cannot be 

 summed up so readily. Evidence to date is quite 

 limited and will have to be considered in relation to 

 particular kinds of discrimination. 



Discriminable Dimensions of Sensation 



A primary sensory modality may be defined as a 

 sense organ or system of sense organs which has its 

 adequate stimulus and which is connected to higher 

 centers by its own separate nerve pathway. 8 Diffi- 

 culty arises in deciding upon the total number of 

 primary modalities. Vision, hearing, smell, taste, 

 kinesthetic and vestibular systems can be differ- 

 entiated readily. A question remains as to whether 

 or not the somesthetic sense should be divided into a 

 number of modalities, at least into three (touch, 

 temperature and pain) and perhaps into more (light 

 touch, deep pressure, warmth, cold and pain). This 

 problem may be resolved when sufficient evidence is 

 accumulated from experiments or, as seems more 

 likely, it may cease to be considered an important 

 problem. 



For the purposes of the present chapter, we shall 

 classify as separate sensory modalities the following: 

 a) vision, b) hearing, c) somesthesis (touch, tempera- 

 lure and pain), d) kincsthesis, e) vestibular sense, 

 f) taste and g) smell. 



Discovering the neurophysiological basis for dis- 

 crimination between primary sense modalities i^ 

 only the beginning of an understanding of sensory 

 discrimination. The next step is to look for mecha- 

 nisms to account for discrimination within each 

 sense since man and lower animals can make dis- 

 criminations based upon differences in the charac- 

 teristics of the stimuli affecting a single sensory 

 modality. 



On the basis of psychophysical experiments in 

 which the essential method was that of controlled 

 introspection, Wundt, Titchener and other psy- 



7 In discussing the effects of central nervous system lesions, 

 we shall be concerned in this chapter with sensory capacity 

 which can be demonstrated after the lesions, in many instances 

 there is a loss of a learned discriminatory habit following cen- 

 tral nervous system damage, but relearning occurs. See Chap- 

 ter LXI for review of studies on the neural basis of learning. 



8 This is, of course, an oversimplified definition in view of the 

 known interconnections among the many sub-systems within 

 the central nervous system: it is, nevertheless, a convenient 

 working definition for the present discussion. 



chologists of their time concluded that sensory 

 experience can be described in terms of basic at- 

 tributes such as intensity, quality, duration, ex- 

 tensity and clearness (attensity). 9 In the sections 

 which follow, we have chosen to discuss the neuro- 

 physiology of sensory discrimination under headings, 

 many of which are reminiscent of the categories set 

 up by psychologists of the introspection school. 



In attempting to bring together data from phys- 

 iological, psychophysical and psychophysiological ex- 

 periments, a complication arises which can lead to 

 confusion unless it is recognized in advance and kept 

 in mind during the subsequent discussion. In phys- 

 iological experiments, parameters of the physical 

 stimulus, such as intensity and frequency (or wave 

 length), are varied and physiological changes, such 

 as rate of impulses in a nerve fiber and number and 

 position of fibers excited, are measured. In psycho- 

 physical experiments, the same physical parameters 

 are varied and discriminations of the experimental 

 subject recorded. The results of psychophysical 

 studies have shown, however, that changes in a 

 single dimension of discrimination are not neces- 

 sarily a function of changes in a single dimension of 

 the physical stimulus. Thus, brightness discrimina- 

 tion in vision is a function not only of light intensity 

 but also ol wavelength, and pitch discrimination in 

 hearing is a function of both frequency and intensity 

 ol sound. The physiological events which are the 

 basis of brightness or of pitch discrimination must 

 then, in each case, be a function of at least two 

 parameters of the respective physical stimulus. 



In psychophysiological experiments, physiological 

 events are varied and changes in sensory discrimina- 

 tion measured under different conditions of periph- 

 eral stimulation, the usual parameters of physical 

 stimuli being varied, but in this case measurements 

 are made of changes in discrimination before and 

 after manipulation of a physiological condition. 

 Difficulty arises when the results of physiological 

 experiments, e.g. the variation in flow of nerve 

 impulses with change in intensity of peripheral 

 stimulation, are used in hypothesizing a neural 

 mechanism to account for a change observed in a 

 psychophysiological or psychophysical experiment. 

 If, for example, pitch discrimination is a function of 

 both intensity and frequency of sound, then the 

 neural correlates of both have to be considered in 

 formulating a neural mechanism to account for pitch 

 discrimination. 



9 For a review of the problem of defining dimensions of sen- 

 sation, see Boring t. - 7 ) • 



