The effect of skew symmetry on figure-ground assignment

Symmetry is a salient cue for figure-ground (FG) assignment which was described by the Gestalt psychologists in the early 1900s. Perfect bilateral symmetry, however, is quite rare. If a viewer looks at a symmetrical region and is not perfectly aligned with its vertical midline axis, the image projected onto the retina is a skewed version of the symmetry. This skew symmetry, therefore, is likely to more often be experienced. Does skew symmetry also affect FG assignment? In Experiment 1 observers either viewed stimuli frontally or with the monitor swiveled and were asked to make FG judgments. In order to obtain a more objective measurement, Experiment 2 employed a short-term memory matching task in which observers matched contours to previously viewed FG displays. Experiments 3a and 3b addressed the possibility that observers may have used the orientation of the monitor as a cue as to the orientation of the stimuli. Skewed stimuli were presented on a frontally facing monitor. Observers performed the explicit FG report task from Experiment 1. Experiment 4 dealt with the possible confound of parallelism by making the edges of the stimuli nonparallel. All four experiments suggest that skew symmetry is used as a FG assignment cue.