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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



intact extrinsic sectors is invariant under much more 

 restricted ranges of transformations of the input — such 

 as diflTerentiation in the case of contrast and contour 

 (80), texture and acuity (39); continuous (orthogonal) 

 projective in the case of position, distance, form and 

 rigid motion (40, 41, 43). 



The effects of lesions of the posterior intrinsic 

 sectors can also be characterized usefully in this way. 

 Differentiative behavior which remains invariant 

 under still fewer transformations of the input is inter- 

 fered with by such lesions. In the extreme, unique re- 

 sponses, i.e. 'absolute" differentiations, would be most 

 affected. 



Unique responses can occur only wlicn both an 

 'absolute' unit and an 'absolute' reference point have 

 been fixed. As indicated in the discussion of the results 

 of the multiple object experiment, the mathematical 

 learning theory provides an approach to the specifica- 

 tion of these units and their referents. The fact that 

 this mathematical device has proved so powerful a 

 tool in the analysis of some completely unexpected 

 effects of posterior intrinsic sector lesions lends sup- 

 port to its usefulness in the development of the model. 



MODEL OF POSTERIOR INTRINSIC MECH.-^NISM 



Deficiencies of Transcortical Reflex 



Models of cerebral organization relevant to com- 

 plex psychological processes have been based to a 

 large extent on clinical neurological data and have 

 been formulated with the 'reflex' as prototype. Such 

 models, implicitly or explicitly, assume that the 

 efTects of receptor activity are transmitted to receiving 

 or sensory areas; from these, neural activity converges 

 upon the association cortex where 'elaboration' takes 

 place; the "elaborated' or "associated' neural events 

 are then relayed to the "motor cortex which is con- 

 sidered the final common path for all cerebral ac- 

 tivity. These models fail to take into account the 

 finding that extratelencephalic afferents reach the por- 

 tions of the cortex usually referred to as 'motor' as 

 well as those known to be 'sensory.' Nor do they con- 

 sider the extent of the origin of efTerents from the 

 cerebral mantle, an extent which includes the 're- 

 ceiving' as well as the 'motor' areas. 



Electrophysiological and neuroanatomical experi- 

 ments demonstrate that somatic afferents are dis- 

 tributed to both sides of the central fissure of primates 

 (i, 38, 66, 82, 126, 152). A recent monograph (74) 

 documents thoroughly the evidence for a more ex- 



tensive origin of the pyramidal tract from the entire 

 extent of the postcentral as well as from the precentral 

 cortex of primates. This marked afferent-efferent over- 

 lap is not limited to the somatic system. With respect 

 to vision, eye movements can be elicited from stimula- 

 tion of practically all of the striate cortex (145); these 

 eye movements can be elicited after ablation of the 

 other cortical areas from which eye movements are 

 ojjtained. With respect to audition, ear movements 

 have been elicited from the auditory system (137). 

 From the portion of the cortex implicated in gustation, 

 tongue and chewing movements may be elicited (5, 

 1 36) ; respiratory effects follow stimulation of the 

 olfactory 'receiving' areas {58, 114). Thus, an overlap 

 of afferents and efferents is evident not only in the 

 neural mechanisms related to somatic function but 

 also in those related to the special senses. The over- 

 generalization to the brain of the law of Bell and 

 Magendie (81) which defines 'sensory' in terms of 

 afferents in the dorsal spinal and "motor' in terms of 

 efferents in the ventral spinal roots must, therefore, 

 give way to more precise investigation of the differ- 

 ences in internal organization of the afferent-efferent 

 relationship between periphery and cortex in order 

 to explain differences such as those between "sensory' 

 and "motor' mechanisms. As yet, only a few experi- 

 ments toward this end have been undertaken (4, 16, 

 121). 



The afferent-efferent overlap in these projections, 

 or to use a term that takes account of this afferent- 

 efferent overlap, these 'extrinsic' systems, suggests the 

 possibility that the intrinsic systems need not be con- 

 sidered as association centers upon which pathways 

 from the sensory sectors converge to bring together 

 neural events before these can determine movement 

 via the motor pathways. A series of neurobehavioral 

 studies (11, 26, 70, 131, 132, 143), in which the ex- 

 trinsic sectors were surgically crosshatched, circum- 

 sected or isolated by large resections of their surround 

 with little apparent effects on behavior, has cast 

 further doubt on the usefulness of a 'transcortical' 

 reflex model. Additional difficulties are posed by the 

 negative electrophysiological and anatomical findings 

 whenever direct connections are sought between the 

 extrinsic and intrinsic sectors (115, 138). Experi- 

 inentalists who followed Flourens in dealing with this 

 problem, including Munk (97), von Monakov (139), 

 Goldstein (45), Loeb (79) and Lashley (68), have in- 

 variably come to emphasize the importance of the 

 extrinsic sectors not only in 'sensory-motor' behavior 

 but also in the more complex psychological processes. 

 Each in\estis;ator has had a slightly different approach 



