THE PATTERNING OF SKILLED MOVEMENTS 



I 70 I 



to attain a given goal (such as attainment of main- 

 tenance of a given equilibrium or pursuit of a moving 

 goal) by their operation despite unexpected changes 

 occurring (within a certain range) in the field of 

 external forces. They present a type of 'flexibility' of 

 their performance. They give a clue to understanding 

 how a simple physical system, the organization of 

 which rests on unmodified rigidly connected working 

 parts, can, thanks lo the feed-back action, present a 

 certain range of freedom in the adjustment of its 

 performance. Homeostatic processes and more general 

 neural activities have been analyzed in this w,iv 



(54, 83). 



Attempts also have been made to approach certain 

 aspects of human sensorimotor behavior in the same 

 way. Although too schematic and too simple to 

 account for the complex total process involved in 

 human behavior, the analogies with the physical 

 systems just described provide a suggestive model foi 

 the dynamic aspects of human controller tasks (23, 

 24). Figure 8 shows some aspects of these analogies 



The analogies between the mechanism, oi volun- 

 tary movements and those of servomechanism are 

 also found to be close, although not complete (103). 

 The stream of volitional impulses which initiates 

 skilled movements may be seen as a programmed 

 input which puis to work the cortical motor mecha- 

 nisms considered as part of a complex servomech- 

 anism. 



Several closed loops have been identified which 

 modulate l>\ feed-back control the emission of 

 corticofugal impulses. Some are long loops including 

 either various proprioceptive or exteroceptive feed 

 back circuits, more or less directly coupled with the 

 organ of movement or with the outcomes of [In- 

 action; thrv constitute, therefore, 'output-informed' 

 feed-back circuits. Others are shorter loops connecting 

 the motor cortex to the cerebellum or to the other 

 subcortical vvav stations. They do not include the 

 peripheral output of the system, and hence constitute 

 what Ruch calls 'input-informed' circuits (103). 

 Organized close-loop controls have been found at all 

 levels of the nervous system (see fig. 9). 



The spinal machinery presents the closest com- 

 parison with the servomechanisms (85). Thanks to its 

 many self-regulating circuits, it gives to its output, 

 the contraction of muscle, smoothness and precision. 

 Despite the classic view of reflexes as stereotyped 

 reactions of a rigid prearranged apparatus, this 

 reflex machinery taken as a functional whole appears 

 as a self-adjusting mechanism of high flexibility. 

 Such systems adapted to local regulations are in- 



Thai 



S m.C 



A 



C.St 



N.Cb. 



P.Cb. 



4 



Ext. A 



I 



I 



F.b.m 



— ► 



Pyr. 



-4/ Mr, 



1 Action^ ' 



fig. q. Simplilied diagram showing some examples of out- 

 put- and input-informed circuits playing part in the control of 

 motor command. S.m.C, sensory-motor cortex; Thai., thala- 

 mus, C. St., corpus striatum; Pyr., pyramidal tract; X. Cb., 

 neocerebellum; P. Cb., paleocerebellum; F.b.m., bulbomesen- 

 cephalic tin niations, Mn., motoneuron; /■u\., intrafusal recep- 

 tors. Art . articular receptors; and Ext . exteroceptive receptors. 



eluded in larger functional units likewise organized 

 as self-regulatory devices of a higher order of com- 

 plexity. We know, for instance, the astonishing 

 flexibility of postural regulatory systems. Taken as a 

 whole, such systems act like time-continuous error- 

 detecting devices that position the body in space bv 

 varying the output of the muscle to counteract 

 changes in gravitational force (103 I. 



According to Ruch (103), the corticocerebello- 

 cortical circuit may also represent a part of a mech- 

 anism bv which an instantaneous order of cortical 

 origin may be •"amplified and extended forward in 

 time." It should be efficient in starting and in stopping 



