Comparing distortions induced by Luminance gradients

Abstract

An illusory display is presented in which a luminance gradient moves through the visual field. In front of this moving gradient, a grid of outlined, filled, squares is positioned. The movement of the gradient induces an illusory 'wavelike' motion of the superimposed squares, depending on the grey values of the squares and their borders. This effect of the moving gradient was decomposed into a dynamic part and a static part. The dynamic part was investigated by means of an experiment in which a temporal gradient induced a forward/backward pulsing of a single outlined square. The static distortions were investigated by means of two experiments. In the first one, an outlined square was positioned on a (stationary) spatial gradient, which caused the squares to look like trapezoid shapes. In the second one, the outlined squares were positioned on different luminance fields, which affected the apparent size of the squares. In all experiments the same ranges of luminance values for both the inner surface of the square and its borders were orthogonally combined. In addition, displays were also created in which the gradient was inside the square and the luminances of the outside surface and the contour were orthogonally combined. For each of the displays, observers were asked to indicate the direction and strength of the induced distortions. The overall results show large agreements between the dynamic distortion (forward/backward pulsing) and the aforementioned static distortion (trapezoid-like appearances). In contrast, different tendencies emerged between these distortions and the second static distortion dealing with the apparent size. The results are discussed and related to various phenomena reported in the literature.