5/14 



DESIGN FOR A BRAIN 



Any inco-ordination would be shown by a divergence from the 



intended line. 



A second example is given by the record of Figure 5/13/2. 

 The subject, a patient with a tumour in 

 the left cerebellum, was asked to follow 

 the dotted lines with a pen. The left- 

 and right-hand curves were drawn with 

 the respective hands. The tracing shows 

 clearly that the co-ordination is poorer 

 in the left hand. What criterion reveals 

 the fact? The essential distinction is 

 that the deviations of the lines from the 

 dots are larger on the left than on the 

 right. 



The degree of motor co-ordination 

 achieved may therefore be measured by 

 the smallness of the deviations from 

 some standard line. Later it will be sug- 

 gested that there are mechanisms which 

 act to maintain variables within narrow 



limits. If the identification of this section is accepted, such 



mechanisms could be regarded as appropriate for the co-ordination 



of motor activity. 



Figure 5/13/2 : Record 

 of the attempts of a 

 patient to follow the 

 dotted lines with the left 

 and right hands. (By 

 the courtesy of Dr. W. T. 

 Grant of Los Angeles.) 



5/14. So far we have noticed in stable systems only their pro- 

 perty of keeping variables within limits. But such systems have 

 other properties of which we shall notice two. They are also 

 shown by animals, and are then sometimes considered to provide 

 evidence that the organism has some power of ' intelligence ' not 

 shared by non-living systems. In these two instances the assump- 

 tion is unnecessary. 



The first property is shown by a stable system when the lines 

 of behaviour do not return directly, by a straight line, to the 

 resting state (e.g. Figure 4/5/3). When this occurs, variables 

 may be observed to move away from their values in the resting 

 state, only to return to them later. Thus, suppose in Figure 

 5/14/1 that the field is stable and that at the resting state R 

 x and y have the values X and Y. For clarity, only one line 

 of behaviour is drawn. Let the system be displaced to A and 

 its subsequent behaviour observed. At first, while the repre- 



68 



