18/4 DESIGN FOR A BRAIN 



occur if the nervous system were multistable. Pavlov, for in- 

 stance, records that ' . . . the addition of new positive, and 

 especially of new negative, reflexes exercises, in the great majority 

 of cases, an immediate, though temporary, influence upon the 

 older reflexes '. And in experimental psychology ' retroactive 

 inhibition ' has long been recognised. The evidence is well 

 known and too extensive to be discussed here, so I will give 

 simply a typical example. Miiller and Pilzecker found that if 

 a lesson were learned and then tested after a half-hour interval, 

 those who passed the half-hour idle recalled 56 per cent of 

 what they had learned, while those who filled the half-hour with 

 new learning recalled only 26 per cent. Hilgard and Marquis, 

 in fact, after reviewing the evidence, consider that the phenomenon 

 is sufficiently ubiquitous to justify its elevation to a ' principle of 

 interference '. There can therefore be no doubt that the pheno- 

 menon is of common occurrence. New learning does tend to 

 destroy old. 



In this the nervous system resembles the multistable ; but the 

 resemblance is even closer. In a multistable system, the more the 

 stimuli used in new learning resemble those used in previous learn- 

 ing, the more will the new tend to upset the old ; for, by the 

 method of dispersion assumed here, the more similar are two 

 stimuli the greater is the chance that the dispersion will lead them 

 to common variables and to common step-functions. In psycho- 

 logical experiments it has repeatedly been found that the more the 

 new learning resembled the old the more marked was the inter- 

 ference. Thus Robinson made subjects learn four-figure numbers, 

 perform a second task, and then attempt to recall the numbers ; 

 he found that maximal interference occurred when the second task 

 consisted of learning more four-figure numbers. Similarly Skaggs 

 found that after learning five-men positions on the chessboard, the 

 maximal failure of memory was caused by learning other such 

 arrangements. The multistable system's tendency to be dis- 

 organised by new reactions is thus matched by a similar tendency 

 in the nervous system. 



18/4. One factor tending always to lessen the amount of inter- 

 action between subsystems is c habituation ', already shown in 

 Chapter 13 to be an inevitable accompaniment of an ultrastable 

 system's activities. There it was shown that an ultrastable 



194 



