SENSORY DISCRIMINATION 



'457 



thalamic tract; again, there is a deficit in differential 

 sensitivity. 



Results of experimental investigations using animals 

 as subjects can be supplemented by observations 

 made in careful clinical studies of man. In the latter, 

 of course, it is usually impossible to obtain measures 

 of discriminatory capacity in the same subject both 

 before and after central nervous system damage 

 caused by injury or disease. Moreover, the exact 

 extent of the lesion cannot be controlled nor, in 

 most instances, accurately determined post-mortem. 

 Nevertheless, what evidence there is tends to sup- 

 port the conclusions which may be drawn from ani- 

 mal studies, namely that intensity discriminations 

 can be made after severe damage to cortical projec- 

 tion areas. As noted earlier (p. 1452), there is some 

 question as to the degree of blindness which follows 

 ablation of the visual cortex in man. The defect 

 following damage to or removal of parts of the visual 

 cortex is not one of complete blindness in a circum- 

 scribed part of the visual field with normal vision in 

 all other parts. Some residual sensitivity to strong 

 stimuli remains in the center of most scotomatous 

 areas (112); while in those parts which appear 

 normal on routine perimetry and in which acuity is 

 normal, more subtle defects, such as increased local 

 adaptation and reduced critical flicker frequency, 

 may be observed (16, 207). In cases of scotomas 

 resulting from cortical damage, it is difficult to con- 

 trol completely for the possibility of stimulating 

 innervated parts of the retina by stray light; ii is 

 difficult to determine that all projection to a given 

 cortical sector has been destroyed; and it is difficult 

 to show that all projection fibers from a given retinal 

 area have been destroyed by a cortical lesion. 



Because of the inadequacy of post-mortem ex- 

 amination of extent of brain damage and, in most 

 instances, the lack of carefully controlled visual 

 tests, it is impossible lo evaluate the results of studies 

 in which total blindness has been reported after 

 complete bilateral destruction of visual cortex or of 

 radiations to the cortex. Retraining methods such as 

 used in experimental animals have not been used. It 

 still remains a possibility that human subjects with 

 complete bilateral ablation of the visual cortex 

 might, witli the proper procedures, be trained to 

 make discriminations to gross changes of intensity of 

 a visual stimulus. The assumption is that more subtle 

 kinds of visual cues must be attended to. 



quality discrimination. Summarizing the evidence 

 bearing upon the neural mechanisms of discrimina- 



tion of different qualities within each of the senses is 

 less straightforward than the similar review for in- 

 tensity discrimination. Since it is difficult to arrive 

 at a satisfactory definition of sensory quality, we 

 shall accept the qualities which were listed by intro- 

 spective psychology and which have been accepted 

 quite generally by investigators studying problems 

 of sensation. For vision, qualities of sensory dis- 

 crimination refer to colors; for hearing, to tones of 

 different pitch; for touch, to light touch, deep pres- 

 sure, warmth, cold and pain; for taste, to charac- 

 teristics such as sweet, sour, bitter and salty; for 

 smell, to odors such as burnt, fragrant and putrid. 



As we have seen, the extension of the specific 

 nerve energy theory to account for discrimination 

 of qualities of sensation within a given sense modality 

 was an obvious step and was quickly taken by other 

 investigators of Johannes Miiller's time. The specific 

 nerve energy doctrine implied that each sensory 

 modality had its end organ and connecting neural 

 pathways and centers. In extending the doctrine to 

 account fur sensory qualities, a search was made for 

 hi eptors which were selectively excited by the physi- 

 cal stimulus conditions known from psychophysical 

 studies to give rise to the qualitative attributes of 

 sensation. 



HelmholtZ initiated a search (which lias continued 

 until the present) lor color receptors in the retina 

 and for separate neural paths and centers serving 

 these receptors. Details of the consequences of this 

 search are given in Chapters XXIX and XXX. To 

 summarize briefly: two classes of receptors in the 

 retina have been identified, rods and cones. These 

 two classes of receptors are related to scotopic and 

 photopic vision, but only the cones appear to func- 

 tion in color vision. There is no direct evidence that 

 types of cones having different sensitivities to dif- 

 ferent parts of the visible color spectrum can be 

 identified. 



In recent years, the peripheral mechanisms of 

 wavelength analvsis have been Studied carefully by 

 Granit ami his co-workers (53, 77-80, 82). (They are 

 discussed by him in Chapter XXIX of this Handbook.) 

 The results of single unit analysis of optic nerve fibers 

 have been incorporated into the dominator-modulator 

 theory of retinal physiology. The dominators are ele- 

 ments with a strength of discharge to light which 

 simply reflects either the scotopic or photopic luminos- 

 ity curve. They are the carriers of the Purkinje shift. 

 The modulators, on the other hand, exhibit sensi- 

 tivity to the various wavelength ranges not predicted 

 by the luminosity functions. A major conclusion to be 



