Abstract
Purpose: To study a new Illusion of misalignment. Given a two-dimensional configuration made up of a white vertical rectangle partially occluded by a black horizontal one, so that the two extreme components of the occluded rectangle remain visible, when the figure is monocularly viewed, the two components, geometrically aligned, appear misaligned. The direction of the effect depends on the viewing eye: when the figure is observed with the right eye, the upper component appears displaced leftwards relative to the lower one; observing with the left eye, the upper component is displaced rightwards relative to the lower one. These results can be better perceived by rapidly closing and opening the viewing eye. Trained observers reported that the apparent misalignment can be perceived even when the figure is binocularly viewed. Under these conditions the direction of the illusion changes during prolonged inspections. When the figure is 90 deg rotated with the partially occluded rectangle horizontally oriented, its visible components can again be perceived misaligned, but this effect depends on the viewing angle, not on the viewing eye. When the figure is viewed front parallel with the centre at the same high of the observer's eyes, the two components appear aligned; when the figure is viewed higher than the observer's eyes, the right component appears displaced upwards relative to the left one; when the figure is below the line of the observer's eyes, the right component is displaced downwards relative to the left one. The results are essentially the same for monocular and binocular viewing, since in this case the misalignment has the same direction for both eyes. The roles of the geometrical properties of the stimuli, of their spatial location relative to the viewing eye and of the depth segregation in stereoscopic conditions were studied in five experiments. An explanation based on depth processing is proposed, and some implications for picture perception are discussed.
Supported by Fondazione Banco di Sardegna, ERSU and Fondo d'Ateneo ex 60% (to BP). We thank Francesco Micella and Franca Bua for assistance in testing the subjects.