A New Optical-Geometrical Illusion

We describe and attempt to explain a new and unusual optical-geometrical illusion with three levels of distortion. The illusory figure is made up of three juxtaposed bands of the same width, which, when appropriately juxtaposed, appear to be of different widths. We hypothesised that the effect would depend on the combined action of various factors: (i) the band shapes and their reciprocal spatial position; (ii) the degree of coincidence of the sides of the juxtaposed bands; and (iii) the inability of the perceptual system to account for all the projective transformations. An experiment was conducted in which the shape of three stimuli was manipulated through affine transformation as well as variation of side lengths. The participants' task was to evaluate the width of the bands. The results revealed a robust and stable illusory effect; the factors that seem to influence the illusion most are band shape and conjoining side lengths.