72 John B. Calhoun 



Although I have treated Broadbent's theory only sketchily here, its 

 importance in the context of the present discussion is his documentation 

 of the necessity for postulating the existence of some neural mechanism 

 having the properties he attributes to the "filter." His thesis evolved 

 primarily from studies with human subjects, and to a much lesser extent 

 from studies of rats and dogs. He accepts the existence of a filter, and even 

 that there are intraindividual differences in the effectiveness of its matura- 

 tion. His concern with the evolution of the filter extends only to his belief 

 that animals with a smaller cortex probably also have a less well developed 

 filter. He does not discuss how a less well developed filter would differ 

 from a more highly evolved one. We might anticipate the four attributes 

 of filters listed above to be less well developed. That is, signals sequentially 

 passing the filter represent a class of stimuli having a wider range of varia- 

 bility; shifts from one sensory channel to another occur with greater fre- 

 ciuency; a stimulus need be only slightly more intense or novel to generate 

 signals capable of having priority over other signals in passing through the 

 filter, and two stimuli must be associated in time much more frequently 

 for their signals reaching the temporary store to have a higher conditional 

 probability of passing in sequence through the filter. 



Let us turn to a consideration of how the social use of space may have 

 encouraged the evolution of animals with more efficient neural filters. At 

 the dawn of mammalian evolution, we can visualize a type having close 

 equivalence both morphologically and physiologically to contemporary 

 shrews of the genus Sorex, with the exception that they had developed 

 essentially no neural filter mechanism for screening signals passing from 

 the temporary store to the limited capacity channel. All stimuli arriving 

 separately would get through the limited capacity channel and it was 

 purely a matter of chance which of two simultaneously arriving stimuli 

 might find passage. Under these circumstances, no discrimination may be 

 made between aversive stimuli emitted by neighbors at a distance. This 

 would lead to a uniform distribution of home range centers at approximately 

 2.0 sigma distance between centers. 



Paleontological evidence suggests that these early diminutive mammals 

 preyed upon insects and other small invertebrates. Furthermore, since 

 among present day shrews Blarina is dominant to Sorex in the hierarchy 

 of use of space, it follows that evolution permitting such differential con- 

 trol of space must have proceeded prior to further marked alteration with 

 reference to feeding habits. Therefore, it appears that early in the history 

 of shrew-like mammals there was sufficient evolution to permit one species 

 to inhibit the extent of home range of another. For simplicity's sake let 

 us designate the earlier form as A, and the later derived one as B. Two 

 characteristics gradually become fixed in B. It evolved an altered vocaliza- 



