52 Information Storage and Neural Control 



Of course, such procedures are dangerous, and I hesitate to 

 make any very strong suggestions. The danger lies in the fact that 

 a theory which embodies an hypothesized mechanism, Hke any 

 otlier theory involving an assumption, can only prove the suf- 

 ficiency of the hypothesis, not its necessity. Anyone who accepts 

 directions from a psychologist runs the risk of getting" lost. None- 

 theless, I will indicate a few possibilities based on mechanisms 

 which have been found useful in psychological and computer 

 theory. 



One observation which has proved highly important to psy- 

 chological process models has appeared in the preceding discussion 

 of Yngve's hypothesis. I refer to the concept of a limited scratch 

 pad, or immediate memory, as it is called. It has often been recog- 

 nized that permanent memory can persist even after severe dis- 

 turbance of the ongoing cerebral activity, such as that brought 

 about by freezing or electroshock. Since any form of persistent 

 trace must undoubtedly require periods of time, at least on the 

 order of seconds, for establishment, then some temporary form of 

 storage, basically different from the permanent form, must be 

 utilized to maintain the information until it can be permanently 

 stored. Miller (21) has shown that the capacity of this immediate 

 memory, as inferred from a variety of psychological studies, is 

 remarkably constant. This capacity is not measured in bits of 

 information, however, but in terms of the number of symbols 

 which can be temporarily remembered; i.e., a subject may retain 

 about seven binary digits, about seven decimal digits, or about 

 seven monosyllabic adjectives, all of which differ in amount of 

 information as defined by Shannon. Thus, the hunian is capable 

 of conceptually complex activity largely because he is capable of 

 dealing with informationally rich symbols, and he is provided 

 with a capacity which is largely independent of the richness of 

 his thoughts. 



By measuring a subject's success at discriminating various 

 numbers of stimuli which differ along one diniension, one finds 

 that the capacity of the human communication channel is rela- 

 tively constant at about seven discriminations. If one then gives 

 the subject the task of discriminating stimuli which vary on two 

 dimensions, one discovers that the subject, although unable to 



