PATTERN-DISCRIMINATION IN VERTEBRATES 327 



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correspondingly fewer, than those of the members of the other 

 system: How great must this difference be in order to effect 

 discrimination ? To what extent does the difference-threshold 

 depend on the absolute size of the members of the two systems ? 



3. The difference-threshold for direction of visible striae. 

 Given two systems of striae the members of the two systems being 

 respectively equal in width but lying in different directions: 

 How great a difference of direction is necessary to effect dis- 

 crimination ? To what extent does the difference-threshold for 

 direction depend on the absolute width of the members of the 

 two systems ? 



4. The threshold for contrast. Given two systems of visible 

 striae, the members of which are respectively equal in width 

 and direction, the juxtaposed striae on one field being of equal 

 brightness, and those on the other field being of unequal bright- 

 ness: How great must the brightness difference on the second 

 field be in order to effect discrimination between it and the 

 sensibly uniform field ? 



The fourth problem might be called that of brightness- 

 discrimination. This term is now used in behavior literature, 

 however, to designate a particular form of brightness-discrimina- 

 tion in animals, which is studied by the Yerkes-Watson method. 

 The two problems are of course quite different. In the Yerkes- 

 Watson problem, the two screens, having the same area and differ- 

 ing in brightness, differ also in luminous intensity; each field is 

 of sensibly uniform brightness ; and the ' ' contrast-areas ' ' are 

 widely separated. The present problem is one of discrimination 

 between two fields of equal luminous intensity, one of which has 

 a uniform surface brightness, while the other presents a system 

 of alternate bright and dark bands in juxtaposition. The degree 

 of contrast between the juxtaposed bands on the one field requisite 

 to discrimination being the factor sought for, it is evident that 

 the results obtained by this method are not necessarily com- 

 parable with those obtained by the Yerkes-Watson method. I 

 therefore prefer to treat this problem as one in pattern-dis- 

 crimination. 



A large number of changes may be rung on the four problems 

 suggested above. The results would depend on the range of 

 wave-lengths used, the luminous intensities of the fields, the bright- 

 ness of the surroundings, etc. Further combination of two or 



