TOUCH AND KINESTHESIS 



423 



thalamus) is the chief cortical representative of the 

 medial lemniscal system. As such it must be in some 

 way critically involved in the elaboration of tactile 

 and kinesthetic sensations, and we have already con- 

 sidered some functional properties of this field. Never- 

 theless, there is evidence available that this region is 

 not the e.xclusive recipient of all the discharges trans- 

 mitted through the medial lemniscal and spino- 

 thalamic relays. While the evidence to this effect is 

 fragmentary and much more work is needed to eluci- 

 date the details the general picture which emerges 

 seems to be as follows. The postcentral family of fields 

 (areas i to 3 of primates) appears to represent a focal 

 region for tactile and kinesthetic activity. This 

 activity is undoubtedly based primarily on the inflow 

 from the ventrobasal complex. .Surrounding this core 

 region is a belt of cortical fields which receive, apart 

 from any connections they may have with the post- 

 central region itself, some sensory somatic inflow 

 directly from the thalamus. It is convenient to con- 

 sider the evidence under several headings. 



.^NATOMic^L EVIDENCE. Most of the cclls of the ventro- 

 basal complex are definitely known to project exclu- 

 sively upon the postcentral region, and it seems likely 

 that all do so. Assuming this to be true, it follows that 

 if impulses evoked by tactile stimuli are transmitted 

 directly from the thalamus to some fields surrounding 

 the postcentral region, they must be relayed through 

 thalamic nuclei other than the ventrobasal complex. 

 In fact evidence is available that a thalamic element 

 intercalated between the ventrobasal complex and the 

 medial geniculate body projects upon the second 

 somatic field (see p. 418). It seems probable further 

 that this element may be a terminal station for some 

 spinothalamic fibers. Whether other thalamic ele- 

 ments of the ventral or posterior nuclear groups which 

 partly surround the ventrobasal complex receive some 

 medial lemniscal or spinothalamic fibers is not clear. 

 It is tempting to speculate that for some this may be 

 true since, if this were so, a number of electrophysio- 

 logical observations would be readily understandable. 



ELECTROPHYSIOLOGICAL EVIDENCE. It has been known 

 since the early days of the evoked potential technique 

 that the extent of the cortical areas activated by sen- 

 sory somatic stimuli may vary with the anesthetic 

 state. In deeply pentobarbitalized animals it is usually 

 the classical first and second somatic fields which are 

 activated by natural tactile stimuli. Under different 

 anesthetic conditions, with no anesthesia at all, or 

 when such drugs as chloralose are used, potentials 



may appear in other regions as well, sometimes only 

 when nerve volleys are used as stimuli (i, 8, 9, 14, 38, 

 39, 84, 92, 142, 165, 167, 276). [Other references are 

 given by Buser (37).] 



In the cat, which was most extensively studied, 

 such additional regions in which potentials evoked by 

 sensory somatic stimuli are likely to appear most 

 consistently are the precentral homologue, the an- 

 terior portion of the lateral gyrus and the suprasylvian 

 gyrus. It is known that a removal of the first somatic 

 area (38, 278) does not abolish e\oked potentials in 

 the second .somatic field. Recently it has been shown 

 by Albe-Fessard & Rougeul (8) that responses in the 

 lateral and the suprasylvian gyri are likewise not 

 abolished by such a removal. Similar evidence has 

 been offered (84, 142, 165) in respect to potentials 

 evoked in the precentral region of the monkey by 

 stimulation of the afferent nerves. 



The fact that such potentials tend to appear under 

 special conditions of stimulation or when the excita- 

 bility of neurons is enhanced by drugs hardly di- 

 minishes the significance of the phenomenon if 

 transcortical spread from the postcentral area can be 

 excluded. Since this was done for several regions sur- 

 rounding this area the conclusion seems warranted 

 that, under certain conditions at least, some sensory 

 somatic discharges may relay to other regions of the 

 neocortex without mediation of the primary receiving 

 field. 



EXPERi.MENT.\L psvcHOLOGic.vL EVIDENCE. The elec- 

 trophysiological evidence suggests then rather strongly 

 that not only the postcentral region but also a belt of 

 fields around it may be of significance for the capacity 

 of the animal to appreciate and handle sensory somatic 

 information. Nevertheless, it is reasonable to expect 

 somatic area I to be the central region of the somes- 

 thetic system. The problem is to determine the exact 

 role played by the different fields in the somesthetic 

 capacity of the animal since it is obvious that these 

 fields cannot by any means be functionally equivalent. 

 Unfortunately only a small number of studies is avail- 

 able on the ability of an animal to perform somesthe- 

 tic discrimination tasks after ablations of the post- 

 central field or other cortical regions. A systematic 

 analysis has not proceeded very far perhaps because 

 of the confusion which was created by the finding that 

 simple somesthetic discriminations are still possible or 

 can be relearned after removal of the first somatic 

 field. The studies of Ruch & Fulton (210), who re- 

 viewed the older literature on the subject, although 

 severely handicapped by lack of anatomical controls, 



