  Although symmetry has been discussed in terms of a major law of perceptual
organization since the early conceptual efforts of the Gestalt school
(Wertheimer, Metzger, Koffka and others), the first quantitative measurements
testing for effects of symmetry on processes of Gestalt formation have seen the
day only recently. In this study, a psychophysical rating study and a
"foreground" versus "background" choice response time experiment were run with
human observers to test for effects of bilateral symmetry on the perceived
strength of figure against ground in triangular Kanizsa configurations.
Displays with and without bilateral symmetry, identical physically specified to
total contour ratio and constant local contrast intensity within and across
conditions, but variable local contrast polarity and variable orientation in
the plane were presented in a random order to human observers. Configurations
with bilateral symmetry produced significantly stronger figure against ground
percepts reflected by greater subjective magnitudes and consistently higher
percentages of "foreground" judgments accompanied by significantly shorter
response times. These effects of symmetry depend neither on the orientation of
the axis of symmetry, nor on the contrast polarity of the physical inducers. It
is concluded that bilateral symmetry, irrespective of orientation,
significantly contributes to the, largely sign invariant, visual mechanisms of
shape segregation that determine the salience of figure against ground in
perceptually ambiguous image configurations.

Neurons and Cognition