Experiments on new contrast patterns

The appearance of two identical gray patches on black or white surround can vary according to the spatial distribution of the patches. This illusion is common to well-known contrast effects and any theory of visual perception has to deal with them. We present some new contrast patterns that show unexpected effects with respect to other known ones and discuss possible explanations of these visual effects. We consider three main explanation hypotheses that reduce the interpretation to different visual tasks: centre - surround low-level visual tasks, higher-level visual tasks, and multi-resolution processing. The new configuration proposed, a checkerboard configuration with a gray square surrounded by black or white vertical and horizontal squares, cannot be explained by the first two hypotheses. Moreover, it is not clear if multi-resolution processing alone can explain this effect or if other low-level tasks should also be taken into account. Multi-resolution hypotheses, proposed among others by John McCann, show that low-pass filtering of the patch configuration exhibits the same behaviour as that observed at normal resolution. We applied a multi-resolution analysis to the basic configuration and to some new variants, with contradictory results. The variants introduce a variable-thickness white frame around the gray patch. We also performed an observation test with ten students, to check the effectiveness of the appearance shift, and found that the test data agree.