The role of orientation and position in shape perception.

This study investigates the contributions of position versus orientation information in shape perception by putting the two in conflict. Sampling the orientation of, e.g., a rounded pentagon and positioning the samples on a circle creates a stimulus in which element positions are consistent with a circle but element orientations with a pentagon. Whether orientation or position dominates the percept depends on a number of factors. First, perceived shape shows a band-pass relationship with respect to number of samples. Element orientation captures element position unless elements are widely separated or very closely spaced. This effect is scale invariant. Second, increasing element envelope size or decreasing carrier wavelength strengthens the influence of element orientation, while other parameters such as the phase and polarity of the carrier or the scale of the Gabor are irrelevant. Third, the overall shape of the contour modulates the effect. The strength of the positional signal rises as the orientation difference between adjacent elements increases. Consequently, the computation underlying contour shape relies on a weighted combination of element orientation and position with weights, not fixed, but dependent on stimulus details. When orientation is dominant, its signal is strong enough to alter positional information, giving rise to the illusion of, e.g., a pentagon despite elements being on a circle.