PERCEPTION I 6l I 



Training 

 figures 



Equivalent stimuli 



rain 



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fig. g. Equivalcnt-nonequivalent stimuli, designed according to Kliiver (259) and used by 

 Lashley (301) to establish range of perceived similarity in rats Trained to select, say, the solid 

 upright triangle shown in the In/) row of the training ligures, the animal will transfer its choice to the 

 corresponding outlines of an upright triangle and even to incomplete outlines. Thus it will select, 

 on successive presentations of pairs of test stimuli (in lop row), those shown to the left in each pair. 

 The same applies to the training ligures shown in the second, third and fourth rows, and to the corre- 

 sponding arrays of test stimuli ( )n actual presentation of training or test ligures, lateral positions 

 of rewarded and unrewarded patterns are randomized, so that the rat cannot solve these problems 

 by acquiring simple left- or right-going tendencies. 



There must be some differences. Although positive 

 evidence is hard to find, one might search for these 

 differences in two directions, a) There are some indi- 

 cations that lower animals may show narrower limits 

 in transfer from one learned discrimination to other 

 discriminanda, and b) conversely, under certain 

 natural conditions lower animals mav show nondiffer- 

 ential reactions to stimuli that for man are obviously 

 different. To take the first possibility first: Lashley's 

 rats failed to transfer reaction from a white triangle 

 (on a black ground) to a black triangle (on a white 

 ground), a transfer possible for monkey, ape and man. 

 (Note, however, our difficulties with the identification 

 of faces in photographic negatives. ) Similarly, rats are 



less tolerant of rotation of figures — an isosceles triangle 

 is not equivalent to the same triangle rotated 90°; 

 there are similar though lesser difficulties on such 

 transfer tests for apes and for human children (147). 

 The other possible source of data for species differ- 

 ences lies in observations such as Buytendijk's (83) 

 who showed for toads how the range of perceptual 

 equivalence (perceived similarity) expands or con- 

 tracts in particular directions depending on the ani- 

 mal's internal state. A toad is placed into a cage with 

 inanimate objects, such as matches and strands of 

 dried moss. As long as the hungry animal has not re- 

 ceived any food, it does not react to these objects. 

 After it has eaten a worm, it begins seeking the broken 



