Quantifying the Watercolor Effect with Cortical Responses

Introduction: The watercolor effect is a visual phenomenon that occurs when bright colored lines are juxtaposed with darker lines on an achromatic background, forming patterns that suggest figure and ground. The colors of the lines appear to seep into the surrounding regions and cause a change in the percept of the colors of these regions. The present study investigated the cortical activity invoked by this illusion by measuring physiological responses as reflected in the visual evoked potential (VEP). Methods: VEPs were recorded from participants in response to stimuli presented on a CRT. Stimuli comprised patterns of colored lines that induced the watercolor illusion as well as control patterns wherein the identical lines were repositioned such that the illusion was eliminated or greatly reduced. Presentations of the two patterns were randomly intermixed. A psychophysical color matching paradigm was used to quantify the perceptual strength of the illusion independent of the VEP results. In further experiments, spatial, temporal, and chromatic parameters of the stimuli were systematically modified to quantify their respective effects on the strength of the illusion and the resultant VEP. Results: Some but not all of the VEP components reflected changes in the perceived illusion arising from systematic changes in the characteristics of the stimulus. Conclusion: Our findings suggest that changes in the strength of the watercolor illusion and perhaps other color induction illusions can be objectively quantified using components of the VEP. Meeting abstract presented at VSS 2012