24 BRAIN MECHANISMS AND LEARNING 



system knows the equation for tt, it can grind out an unlimited number of 

 digits, and yet carry none of this as bits of information.) It is its function, 

 also, to decipher how existing engrams form the scaffolding for new ones. 

 Here is the crux of the problem of learning. It depends upon the evolved 

 structures — the improved units, patterns, and numbers in the more 

 advanced nervous systems, but it depends no less on the improved 

 physiology — lowered thresholds, faster conduction, greater spontaneity, 

 easier fixation, and a host of other attributes, which arc just coming to be 

 recognized as important characteristics of the 'higher' animals as compared 

 with the 'lower' ones. But before coming to grips with the problem of 

 the neural mechanisms of learning, a few more general considerations 

 involving the fixation of expierience will be helpful. 



Several questions must be faced for all systems that fix experience. The 

 first, of course, is: How does a system at a given time, with an enduring 

 architecture, contributed by the past experience of the race or itself, 

 interact with the environment to give a new enduring architecture? As 

 alreaciy indicated, this applies at each level and over all time spans. 

 Second, when does a reversible change — a homeostatic response to stress, 

 or a behavioural change in response to a stimulus — become irreversible? 

 What is the limit of homeostatic tolerance, the Rubicon that is crossed, 

 when the transient response becomes either an adaptive change or a non- 

 adaptive (pathic) breakdown? When does a dynamic memory become a 

 structural one, much as the spoken word becomes fixed in writing? At 

 what stage does the totipotent embryonic cell become irreversibly differen- 

 tiateci and specialized for a particular function? When does the individual 

 growing up in his society acquire the set of values, customs, skills that 

 characterize it? When can he no longer learn a new language without an 

 accent, or face a different culture with no sense of xenophobia ?Third, 

 what is the mechanism, or the carrier, of the operation? How is the 

 change in the system given an adaptive (or a non-adaptive) direction? 

 Fourth, turning from the individual to the race, how is the adapted 

 individual selected or, to the extent that individual change is passed on, 

 how is this achieved? Last, what is the mechanism of cumulative racial 

 change? 



Such questions are not merely disembodied abstractions, even as regards 

 the nervous system and behaviour. Learning must utimately be at the 

 molecular level, as well as at cellular, organ, and individual levels. The 

 material record of experience must be found in some change in the 

 number, kind, or position of particles; in the pattern of ions or molecules 

 in neurones or at their junctions. As acquired racial information is passed 



