Informatioti Processing Theory 53 



distinguish forty-nine categories, can do better than in the one- 

 dimensional case. For example, a subject who can discriminate 

 wiiich of ten positions a point occupies on a line cannot place the 

 point in one of one hundred cells of a square, but can manage 

 only twenty-five. This is just what would be predicted if ten cells 

 of immediate memory were divided into two groups of five. In 

 other words, the compound discrimination reduces the accuracy 

 of discrimination for each dimension, but still allows independent 

 examination of each. 



The question arises as to the underlying neurological structure. 

 Is there a single set of pathways which performs this function for 

 all inputs including internal inputs? It seems unlikely, though not 

 impossible, that such a set of pathways is localized in one geo- 

 graphic position in the brain; but even if it is diffusely distributed, 

 as are other memory functions, one may still ask if one set serves 

 in common. Little work of the kind summarized by Miller has 

 been done on cross-modality studies, but one wonders if there is 

 a "final common path" for all sense modalities. 



Cllosely related to the notion of informationally ricii symbols 

 is the concept of a hierarchically organized memory. It is fairly 

 clear from both logical and psychological considerations that 

 nriemory organization is such that one trace can evoke a number 

 of others, each of which can in turn evoke a number of others, 

 and so forth; i.e., one trace is associated with several others, and 

 any one of them can be elicited without eliciting the otiiers. Such 

 structures liave largely been ignored in classical stimulus-response 

 models, where the theories have been concerned with the forming 

 of a single association between two traces. Neurophysiological 

 theories, perhaps reflecting the concern of the psychologist, have 

 concentrated on exploring the method of single associations. Some 

 meaningful questions might be asked as to the adecjuacy of linear 

 neurological models for explaining hierarchical structures. 



One such question is related to the concept of set, which has 

 been found extremely useful, if not necessary, in psychological 

 theories, and which has turned up under a variety of names with 

 only minor variations in meaning. It is recognized that a subject 

 can be "set," by instructions or by other experimental manipula- 

 tions, so as to give responses of a certain class, to perform operations 



