SENSORY DISCRIMINATION 



[467 



both the specific and unspecific afferent systems is the 

 discovery of evidence as to the functioning of cortico- 

 fugal connections from the primary projection areas 

 to subcortical centers of the specific afferent systems 

 (31, 101, 143, 158, 175, 176), to the reticular forma- 

 tion (31, 59, 92, 101), and from both the reticular 

 system and subcortical centers of the specific afferent 

 systems to peripheral end organs (67, 79, 81, 83, 93, 

 1 10, 173, 174). For many years there has been some 

 anatomical evidence for centrifugal fibers in the 

 primary sensory pathways but until recently the evi- 

 dence as to structural connections was rather sketchy 

 and there was little or no functional evidence to indi- 

 cate the possible physiological role of these recurrent 

 tracts. (This field is the subject of Chapter XXXI by 

 Livingston in this Handbook.) 



Electrophysiological experiments have provided 

 direct evidence that the input of the sensory svstcms 

 may be controlled at a number of levels, including 

 the peripheral end organ itself, by feedback through 

 the centrifugal pathways. For example, control ol 

 muscle spindle discharge has been demonstrated by 

 Granit & Kaada (83); of retinal discharge bv Granil 

 (81) and of cochlear discharge by Galambos (68). It 

 appears that the centrifugal activity may affect sen- 

 sory input at the end organ in several ways: a) b) 



influencing the level of spontaneous discharge, b) by 

 facilitating the peripheral excitatory process; or c) by 

 inhibiting the peripheral excitatory process. Similar 

 control may be exercised by higher centers upon those 

 lower in the main afferent systems; by specific afferent 

 centers on the reticular system; and by the reticular 

 system on centers of the specific afferent systems. 



The total picture of interacting specific and un- 

 specific afferent systems and of centrifugal control of 

 pathways within each system and between the two is 

 one of great complexity but one which must be under- 

 stood for a neurological theory of sensory discrimina- 

 tion. But this is still only a part of the total problem. 

 By sensory discrimination we mean a discriminatory 

 response to sensory stimulation. In the present chap- 

 ter we have confined our attention to the input or sen- 

 sory side of the picture. To account for motor re- 

 sponse, at least two other subsystems within the central 

 nervous system have to be considered: a) the centers 

 and pathways which are involved in motivation of 

 behavior and b) the motor system. Furthermore, the 

 manner in which a connection is established between 

 the sensory and motor systems is the most puzzling 

 and perhaps the most important problem of all. From 

 one viewpoint it can be regarded .is the central theme 

 of the subsequent chapters. 



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