Contrast dependency in perceptual filling-in

Two experiments were conducted to investigate how stimulus contrast affected the time required for perceptual filling-in. The stimuli consisted of a Gabor patch (target) and a circular grating region (surround). In Experiment 1, the target contrast was manipulated, and the surround contrast was fixed. Filling-in was significantly delayed with higher target contrast, but this delay was observed only when the target contrast exceeded the surround contrast. In Experiment 2, however, a much smaller effect of changing the surround contrast occurred. Possible reasons for this asymmetric effect are discussed.