A Psychophysiological Investigation of the Watercolor Illusion

Abstract

In order to distinguish an object from its background, our visual system makes inferences about the brightness and color of the object's surface. The resulting percept of brightness and color of a surface depends not only on the illumination and reflectance of the surface itself, but also on cues in the surroundings of the surface such as edges, borders, and shadows. A striking example of this can be seen in a visual phenomenon known as the watercolor illusion. By juxtaposing two thin colored lines on a blank background and forming a boundary enclosing part of the background, the enclosed area will appear to contrast the unenclosed area with a hue similar to the inner bordering line, but less saturated. This effect is a combination of figure ground organization and illusory color spreading. Behavioral studies have previously found measurable differences in illusion magnitude when manipulating certain parameters of the illusion. The present thesis offers an alternate measure of the differences in illusion magnitude using electrophysiology, specifically the visual evoked potential (VEP). By utilizing electrophysiology, the present study developed a method to present the illusion in a way that creates a neural response that is reliably different from that generated by a matched control condition. This difference was furthermore correlated with behavioral responses to the illusion when certain parameters of the illusion were manipulated.