

11 \M)K()(IK <>r l'HYSIdl.c H.Y 



NEUROPHYSIOLOGY III 



other systems, the evidence was, and to some extent 

 still is, controversial. 



Studies of cortical cytoarchitecture and, more im- 

 portant, tracing of anatomical pathways by selective 



staining techniques provided further evidence of the 

 basic structural organization of the sensory systems. 5 

 The most important advances, however, were made 

 when electrophysiological methods were used to 

 trace the sensory pathways in the central nervous 

 system. The use of the evoked potential method has 

 been of special value in mapping the areas of the 

 cerebral cortex to which the primary sensory systems 

 project. (Sec particularly Chapters XYII through 

 XXI, XXIY and XXX of this Handbook for studies 

 of individual projection areas.) 



The increased knowledge gained through electro- 

 physiology has not led to a sudden clarification of the 

 significance of topographical organization. Rather, it 

 has brought to light a complexity of organization, 

 the significance of which remains to be explained. 

 For example, at the cortical level, the primary pro- 

 jection areas as defined by cytoarchitecture, myelo- 

 architecture or tracing of degenerating pathways to 

 or from thalamic nuclei were once thought to be 

 unitary regions confined to relative!) small portions 

 of the main cerebral lobes. Evoked potential maps 

 have shown thai for the visual, somesthetic and 

 auditor) systems, at least, and for most mammals 

 ili. 11 have been siudied, the regions of primary pro- 

 jc< linn are more extensive than was formerly thought 

 and the) arc not unitary, that is there may be two 

 or more projection areas or two or more subdivisions 

 ol the projection region for each system, [n all mam- 

 mals (man included) thai have been examined, two 

 or more somatic areas ha\i- been found, evidence foi 

 dualit) of projection being that the different parts 

 oi the bod) project in an orderly fashion in two 

 separate but adjacent areas. Likewise, in most mam- 

 mals two visual areas and two auditor) areas can be 

 defined (see < hapters XXIV and XXX). Evidence 

 for dual projection of visual and auditor) systems in 

 pi mi. itcs is not conclusive, but this lack of evidence 



ma) be due to difficulties encountered in exposing 



and in adequate!) exploring the relevant regions. 



The dual projection areas discovered b\ the evoked 



potential method in man) instances have also been 



shown to be different with respect to their thalamic 



connections. Foi the visual, auditor) and somatii 



■ in- there i- some evidence thai there ma) be 



three or mon projei areas which must, in terms 



ol in ii organization, be considered as distinct, 



aaming o) multiple projection anas has no1 



been simply the subdividing of the regions classified 

 as projection cortex by the older anatomical methods; 

 parts of the cortex formerly called 'association' areas 

 have now been shown to have direct afferent con- 

 nections with the thalamic nuclei of the main sensor) 

 systems. Consequently, the sensory projection areas, 

 as now defined, occupy a larger portion of the total 

 cortex than was once thought to be the case. In 

 mammals below the primates, there are only small 

 strips of 'silent' cortex lying between the visual, 

 auditory and somatic projection regions. Even in 

 primates, the extent of so-called association cortex 

 is reduced in cortical maps based upon the latest 

 electrophysiological studies. 



While it was recognized that many reflex re- 

 sponses to sensory stimulation can be carried out v ia 

 connections in the spinal cord or in brain-stem 

 centers, it has been the common view, until relatively 

 recent times, that conscious discriminations or 

 learned discriminations depend upon the intactness 

 of the appropriate regions of the cerebral cortex. 

 Despite convincing evidence from experiments on 

 lower animals and the lack of critical evidence from 

 clinical investigations of man, this view is still ex- 

 pressed or implied bv man) writers of textbooks and 

 even by some authors of current research publica- 

 tions. 



Evidence bearing on the role of the cerebral 

 cortex in sensory discrimination will be dealt with in 

 more detail below when the neural mechanisms of 

 different kinds of discrimination are discussed. In 

 conjunction with the present discussion of the topo- 

 graphical organization of the primary sensor) 

 modalities, it is appropriate to note that, with the 

 possible exception of vision in man, evidence from 

 both clinical and experimental investigations in- 

 dicates that for each ol the sensor) systems that has 

 been studied carefully, some kinds of learned dis- 

 crimination can be made after complete destruction 

 of the cortical projection anas of that system. 6 In 

 Subprimate mammals, it h.is been shown that a 



learned discriminatory response to a sensory cue 

 such as the flashing ol a light, onset of a fairl) loud 

 ionc or tactual stimulation of the skin can be estab- 

 lished after complete or near!) complete ablation of 



all cerebral cortex ( 32, 75). 



As we shall see, the capacit) foi sensor) dis- 

 crimination may be altered bv more limited abla- 



' It iniisi be kept in mind, however, thai we do not have 

 clinical cases with lesions suitable foi comparison with lower 

 .mini. ils in which the sensory projection areas of a single 

 system have been complete!) ablated bilaterallj 



