i6 5 8 



II WUHOOK OK PliYSIOl OOY 



XErmiiMiVMi >i i k.y hi 



bing one's band along a curved edge, the apparent 

 curvature gradually diminishes. If one then rubs 

 one's hand along an objectively straight edge, the 

 latter seems curved in the opposite sense. 



Kohler (271) believed that all these effects might 

 be closely related to some central process of "satiation" 

 which he felt caused the apparent reversals on pro- 

 longed inspection of an ambiguous pattern (e.g. the 

 Necker cube; see fig. 40). Subsequently, Kohler & 

 Wallach (281) subjected the visual forms of figural 

 aftereffects to a detailed investigation. A paradigmatic 

 experiment is illustrated in figure ^8. Prolonged 

 fixation of an inspection pattern, / (which may be 

 an) kind ul stable contour in the visual field), induces 

 predictable changes in the appearance of a test pat- 

 tern, T, that is subsequently viewed. In essence, any 

 contour presented at a short distance from the retinal 



APPEARANCE OF 

 TEST FIGURE: 



;(H. Figures used by Kohlet & Wallach 1 duce 



figural aftereff© Cs I ollowing inspection of the pattern marked / 

 (the solid black rectangles), .1 subsequently exposed test 

 pattern, 7 (the foui outline squares) appears temporarily 

 alteri d, the two squares on the left seem to have moved further 

 apart, tin two on the right, close! togethet Fixation is main- 

 tained ii ighoui .11 \ I Kohlei & Wallach 



o 



o 



fig. 39. Aftereffects induced by active scanning of various 

 patterned fields .1 Scanning the set of curved lines for i min. 

 induces apparent curvature in an objectively straight line, 

 the curved line shown at righl is now called straight. Analogous 

 transformations are shown under B-E. Scanning the set ol 

 vertical lines for I min. induces horizontal elongation and 

 distortion of circles \Bi) and converts squares into rectangles 

 (B2). C and I) Checkered fields convert circles into polygons. 

 /■ Scanning for 1 min. within the circle converts .1 horizontal 

 line as shown. In a sense, the bias induced by the special ex- 

 posure fields sets up new equivalence classes, polygons are now 

 circles, etc. For similar interpretations see Taylor & I'apcrt 

 (463) 'Adapted from Held (unpublished observations 



region of the previously inspected contour appears n> 



be displaced away from ih.n region. With increasing 

 separation ol / and '/" lines, these apparent displ.u . 

 incuts increase In .1 maximum and then decrease 

 [the 'distance paradox' of Kohler & Wallach (281)]. 

 In their physiologic theory of these figural after- 

 effects, Kohler and Wallach assumed that satiation 

 builds up during prolonged inspection of contours as 

 .1 slow polarization in primary projection fields (271, 

 273, j8i ). Actual measurements of d.C. potential 



changes from occipital (277, 278, 280) and other 



regions (279, 282) are adduced in support of the 

 theory in which cortical potential gradients and theii 

 configurations in a volume conductor form the repre- 

 sentations of perceived distance and shape. As 

 l.aslilrv has pointed OUl ' [o8), the theorv is unique 



