J. Ninio et Jk. O'Regan, Characterisation of the misalignment and misangulation components in the Poggendorff and corner-Poggendorff illusions, PERCEPTION, 28(8), 1999, pp. 949-964
In the Poggendorff illusion, two colinear segments abutting obliquely on an
intervening configuration (often consisting of two long parallel lines) ap
pear misaligned. We report here the results of a component analysis of the
illusion and several of its variants, including in particular the 'corner-P
oggendorff' illusion, and variants with a single arm. Using a nulling metho
d, we determined an 'orientation profile' of each configuration, that is, h
ow the illusions varied as the configuration was rotated in the plane of th
e display. We were able to characterise a pure-misalignment component (havi
ng peaks and dips around the +/-22.5 degrees and +/-67.5 degrees orientatio
ns of the arms) and a pure misangulation component of constant sign, having
peaks at the +/-45 degrees orientations of the arms. Both these components
were present in both the classic and the corner-Poggendorff configurations
. Thus, the misangulation component appears clearly in the classic Poggendo
rff illusion, once the misalignment component is partitioned out. Similarly
, the corner-Poggendorff configuration, which essentially estimates a misan
gulation component, contains a misalignment component which becomes apparen
t once the misangulation is nulled. While our analysis accounts for much of
the variability in the shapes of the profiles, additional assumptions must
be made to explain the relatively small misangulation measured in the corn
er-Poggendorff configuration (1.5 degrees, on average, at peak value), and
the relatively large illusion measured in the configurations with a single
arm (above 6 degrees, on average, at peak values). We invoke the notion tha
t parallelism and colinearity detectors provide counteracting cues, the fir
st class reducing misangulation in the corner-Poggendorff configuration, an
d the second class reducing the illusion in the Poggendorff configurations
with two arms.