TOUCH AND KINESTHESIS 



401 



the functional significance of the ipsilateral inflow 

 diflPers materially from that of the contralateral one. 



PATTERNS IN TACTILE THALAMIC AREA. Detailed in- 

 formation regarding the pattern of projection of the 

 lemniscal system upon its thalamic rela\' nucleus is 

 provided by studies using the e\oked potential tech- 

 nique. This method, as applied to study of the thala- 

 mus, involves passing a recording electrode down 

 through the thalamus in successive rows of penetra- 

 tions so placed as to explore the thalamic areas acti- 

 vated. At successive intervals during its downward 

 passage the electrode is held stationary and the area 

 of the body surface in which stimulation evokes elec- 

 trical activitv at a given point is determined. The 

 figurine drawings which can be constructed from the 

 data for each point are placed in proper relation to 

 one another and to the thalamic nuclear outlines, as 

 determined by study of the serial sections of the ex- 

 perimental brains (182, 184, 185, 207). The pattern 

 of representation in the monkey is shown in figure 3. 



Analysis of this figure reveals that the body surface 

 of the monkey is represented as a distorted image of 

 the animal. The face and head are represented within 

 the arcuate portion of the ventrobasal complex, the 

 liody in its external element. The middorsal line of the 

 body from nose to tail is represented across the top 

 of the complex, the trunk and girdle regions, proxi- 

 mal and then distal parts of the extremities in suc- 

 cessively more ventral positions. The only ipsilateral 

 projection is that of the peri- and intraoral regions. 



Perusal of such a figurine map makes it clear that 

 a given small area of the body surface is not repre- 

 sented at a thalamic ' point' and only there. Stimula- 

 tion of a small spot on the skin evokes intense activity 

 at a limited thalamic locus and less intense activity 

 over a considerable surround. It follows that a given 



thalamic locus can be activated to some degree from 

 a considerable area of skin, which is smaller for some 

 and larger for other parts of the topographical pat- 

 tern. As the peripheral spot stimulated is shifted across 

 the skin the peak activity shifts across the thalamic 

 pattern, its submaximal and liminal fringes shifting 

 with it. The problem is to understand the precision 

 of spatial discrimination of which the organism is 

 capable, which depends upon an anatomical sub- 

 strate of ' point to area' and reciprocallv, 'area to 

 point' projection of the receptor sheet upon the central 

 configurations. Some physiological mechanisms which 

 appear of importance in this regard will be considered 

 later. 



When the representation pattern shown in cross- 

 section in figure 3 is analyzed in three dimensions, it 

 results that any given dermatomal (segmental) region 

 of the body is represented in the ventrobasal complex 

 in a narrow curving lamella of tissue, concave medi- 

 all\". Within such a narrow sheet the proximal skin 

 areas of the dermatome are represented dorsallv, the 

 distal ventrally. 



Extension of such studies to a series of mammals 

 allows some estimate of the phyletic trends in thalamic 

 tactile representation. The sequence of that repre- 

 sentation is in principle the same in the rabi)it, cat 

 and monkey (fig. 4). The entire body surface is repre- 

 .sented in each case, but striking differences in em- 

 phasis exist. In the rabbit, the bulk of the available 

 tissue is given to the projection of head and face, 

 while the cat possesses a ijalanced spinal and tri- 

 geminal projection. In the monkey the increased de- 

 velopment of the hand and foot as tactile organs is 

 indicated by an increased share of the pattern de- 

 voted to their representation. 



This general pattern depicted by electrophysio- 

 logical studies is a confirmation and extension of that 



FIG. 4. Schematic outlines of body representation in the ventrobasal thalamic complex in rabbit, 

 cat and monkey. The figures do not intend to depict with accuracy the actual relationships but aim 

 to emphasize the dominance of the trigeminal representation in the rabbit, and the relative increase 

 of the representation of the limbs in cat and monkey. The representation of the ti'unk and extremities 

 is located quite anteriorly in the ventrobasal complex of the rabbit. In the cat and monkey this 

 representation reaches progi'essively very much farther caudally. 



