A further difficulty for the 
angular-displacement theory 
becomes evident when we try to 
explain why the Poggendorff 
illusion occurs in a figure 
displaying an effect that was 
discussed in Chapter 5—–that of 
illusory contour. If the vertical 
inducing lines of the 
Poggendorff illusion are 
replaced by irregularly spaced 
semicircles, for example, 
vertical contours appear to be 
present although none 
physically exists between the 
semicircles. The oblique lines 
can "intersect" the vertically 
perceived boundaries at regions 
where there is no actual 
vertical contour to form acute 
or obtuse angles, and the 
illusion occurs. Thus, although 
this illusory effect does not 
rule out the hypothesis that the 
Poggendorff illusion is based on 
misperception of angle, such 
an effect cannot readily be 
attributed to lateral inhibition 
of contour-orientation detector 
cells. No such inhibiting 
contour would be present in 
such a figure.