i6 9 8 



MWWlnoK ill PHYSIOLOGY >-~ NEUROPHYSIOLOGY III 



net and its specificity of organization (inherited or 

 acquired); and c) the postulated 'field of extraneous 



influence' exerted by "will' or •mind' capable of 

 controlling, orienting and modifying the stream of 

 influence converging toward the effector structures ol 

 the system. 



Such formulations, as stressed by their author, only 

 push back the difficulty of solving a problem which 

 remains posed in all its hermetic complexity how- 

 to conceive, indeed, the nature, the origin and the 

 maintenance of such a 'field of influence,' the action 

 of which is so powerful and the spatiotemporal 

 organization of which appears as an ineluctable 

 necessity which is still to be explained. 



Endeavors are numerous in this field but, as Sperry 

 so well stated it, "it is not a solution we aspire to, but 

 only a basis on which to begin" (113). Unless the 

 scientist can soon have at his disposal a new technique 

 suitable for attacking such problems experimentally, 

 the "enchanted loom' as poeticallv imagined l>\ 

 Sherrington (107), weaving its ""shifting harmony of 

 dissolving but always meaningful patterns," will long 

 keep its secret. 



\I)\lll\l; PLASTICITY OF THE SYSTEM OF ACTION; 

 ITS CONDITIONS AND ITS LIMITS 



The formerlv classic concept of a motor projection 

 containing all the detailed elements of the patterning 

 of command, like the perforated music rolls of a 

 mechanical piano, mm appears clearly untenable. 

 It would necessitate on the part of originating struc- 

 tures and mnemonic functions a gigantic task ven 

 difficull to conceive in terms ol nervous mechanisms. 

 I he most perfect pre-existing plan could not account 

 for the astonishing capacity of the nervous system 

 to adjust our movements to the ever-changing and 

 most unforeseen circumstances of their achievement. 



I lie aphorism of Claude Bernard, ""We will, then a 

 function is achieved on its own,'" is more in accord- 

 ance with our intuitive apprehension of the realm of 

 such phenomena with .ill tli.it lliev imply concerning 

 prestructured arrangements and automatically 

 achieved regulations. The structures originating 

 voluntary movement intervene to foresee, to star! 



off, lo direct and to slop the working of Complex 

 machinery which contains in itself the regulating 

 elements ol its <>w n activity . 



It must be borne in mind that the fundamental 

 framework ol this machinery is made up oi an as- 

 sembl) ol neuron- anatomically ordered into ap- 



propriate patterns of interconnections. Among these 

 elaborate and intricate patterns of synaptic linkages, 

 some are preformed and organized directly in the 

 growth process itself, others are adjusted by functional 

 regulation through learning processes (112). Both 

 constitute a kind of 'motor repertoire' upon which the 

 animal must draw for most of its performances. 



The extent to which and the means by which such 

 basic neuronal architecture provides the structural 

 basis for the plasticity of its functional effectiveness 

 must now be considered. In approaching the problem 

 of plasticity it appear-- useful to distinguish, with 

 Weiss (130), two quite different aspects of its ex- 

 pression in behavior. 



On the one hand, we may consider the range of 

 adaptability which in some degree characterizes all, 

 even the most stereotyped, modes of motor per- 

 formance. As a rule they present, within given limits, 

 some degree of variability in their formulation In- 

 central commands and in their adjustment to environ- 

 mental changes. Plasticity is thus understood as 

 "elasticity within a given qualitative performance 

 admitting of quantitative adaptation to what, for the 

 given species, is a normal range of variability of the 

 environment" (130). We shall refer to this first aspect 

 as the flexibility of the usual or inborn forms of action. 



On the other hand, we must take into account a 

 faculty of the organism to adapt its motor comport- 

 ment to new situations bv inventing novel coordi- 

 nating patterns and in fixing them in the structure. 

 It adds new elements to its own motor repertoire. 

 Plasticity is defined there as the "ability of an organ- 

 ism to cope with emergency situations lying beyond 

 the normal range of elasticity, by creating new 

 performances previously not even latently in 

 existence" ii 30). This second aspect is to be referred 

 to as the adaptative learning of new tonus 01 action. 

 Let us examine first the conditions and the limits of 

 the 'flexibility' of motor performances 



Flexibility of Motot Performances 



To adapt performance in accordance with external 

 changes requires that the motor centers be, somehow 

 or other, informed of these changes I hen-lore, we 

 are immediately faced with the problem of sensor) 

 control of the adjustment of motor performance. 



kli.l 1 viivk RiH I 01 SENSORY INFORMATION Since 



Charles Bell 1 to) recognized the fact that motor action 



is impaired bv breaking the "circle of nerves' which 



conveys the command from the brain to the muscle 



