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Abstract:
We investigated the relationship between changes in contrast thresholds on illusory contours and the brightness changes perceived on these contours.
We measured the perceived brightness in the gaps of a Kanizsa square, where an illusory figure is perceived, and in the gaps of an outline square by determining the point of subjective equality with the background for a small line target presented in the gap. White inducers were displayed on a gray background. We also determined the decrement and increment thresholds for detecting the same target in these two configurations. While the gap appeared to be brighter than the background, there was no significant difference between increment and decrement thresholds, which means that the perceived brightness in the gap does not act like a simple contrast pedestal on the detection of the target. This implies that brightness and luminance are processed at different levels.
To investigate possible top - down influences of figural completion we compared the increment thresholds obtained in a closed configuration (same as above) to thresholds in an open configuration where only two sides of the square were outlined by the inducers. Contrast thresholds for both configurations were significantly lower than in the control configuration, where the target was displayed alone. We found no difference in increment thresholds between the open and the closed configuration, which suggests that later processing stages, in which the unity of the square is established, have no influence on contrast thresholds.
