THE PATTERNING OF SKILLED MOVEMENTS 



1695 



other than speculative interpretations of their mecha- 

 nisms of functioning. 



A series of experimental arguments are, however, 

 available which seem to minimize greatly the 

 functional importance of these connections between 

 the various cortical areas for the integrative oper- 

 ations. Since 1924 Lashley (68) has emphasized the 

 unexpected fact that incisions distributed through 

 the cortex of a cat, in all possible vertical planes, 

 made in such a way as to disrupt the interareal 

 connections, are without marked consequences on the 

 comportment of the animal and on its capacity for 

 retention. 



Sperry (m), followed by Wade (125), made 

 analogous findings in monkeys b) isolating the motor 

 cortex from the neighboring areas, a procedure not 

 producing notable perturbations in the motor be- 

 havior of these animals. In man, Akelaitis (3) lias 

 shown that section of interhemispheric transcallosal 

 connections is without influence upon motor functions. 

 Agenesis of the corpus callosum is also without 

 apparent motor effect (32, 62). Finally, Penfield (93) 

 has shown that a nearly complete isolation of the 

 precentral gyrus from the neighboring areas leaves 

 intact the capacity of the individual to execute 

 skilled acts. No functional participation of the U- 

 shaped sensorimotor connections has been observed 

 (95). Thus, in the interior of the motor area the 

 neighboring regions seem to be connected vertically 

 by means of subcortical circuits. 



In view of the astonishing resistance of the cortical 

 elaborative functions to such mutilations, ii becomes 

 manifestly difficult to attribute to the horizontal 

 intracortical paths a functional role of the first order. 

 Penfield concludes, "Transcortical connection must 

 serve some useful function but, in all events, this 

 function is not essential for voluntary action" (1)4). 



The precentral regions are, therefore, under the 

 direct control of certain subcortical structures The 

 stream of impulses that produces the patterning of 

 skilled activities at the level of the precentral gyrus 

 most probably comes from such subcortical regions. 

 Thus it seems logical to search for some kind of 

 centralizing structure in regions which are in func- 

 tional connection with the different sensors and 

 associative sectors of the two hemispheres. 



CONCEPT OF A CENTRENCEPH ALIC SYSTEM OF INTE- 

 GRATION. Penfield has introduced the concept of a 

 subcortical sv stem of integrative coordination which 

 he designates 'centrencephalic' (93, 95). Without 

 assigning to this essentiallv functional system a 



CENTRENCEP 

 source of volitionc 



SUBCORTICAL 

 motor mechcnisms 



Infant like movements 

 and walking 



Adult skilled movements 



no. (>. Hypothetical diagram of the stream of volitional 

 nerve impulses producing adult sk illei I movements tin heavy 

 lines). The impulses come from the centrencephalic area to each 

 Rolandic motor Cortex ami from there descend to subcortical 

 motor mei nanisms and peripheral bulbospinal motoneurons. 

 In fine lines are shown the course of nerve impulses producing 

 voluntary action without involvement of the motor cortex. 

 \h ililied h .1111 I'enlielil '1 1 



precise anatomical location, Penfield places it in the 

 highest level ill the brain Stem, .is Merrick defined it 

 in [880. It comprises .1 group of mesencephalic and 

 diencephalic structures (including the thalamus) 

 which have direct fuiietioii.il connections with the 

 two 1 erebral hemispheres. 



I he importance of such a region is revealed l>v .1 

 series of clinical (93) and experimental findings 

 (58, 82, 95). I he) agree in establishing the ana- 

 tomical identification of the complex fields of recip- 

 rocal relations which unite the specific and associated 

 portions of the thalamus, as well as the diffuse sv sinus, 

 with the different sectors of the cortex. 



In the series of nervous events which underlie the 

 preparation for and the initiation of .1 voluntary 

 movement, Penfield emphasizes the special features 

 of this transactional operation which suggest that it 

 is responsible for the emission of 'voluntary' commands 

 (see fig. 6). This "final integration' must be assigned, 

 according to him, to the centrencephalic svstem. 

 "Only with a centrencephalic system of this sort 



