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Fig. 17.1. High-level influence on illusory contour perception. Low-level mechanisms such as contour completion can account for some illusory contour perception phenomena. However, surrounding context can affect how salient the illusory contours are, suggesting that higher levels influence the low-level mechanisms. The patterns in (a) and (b) have the same oval outline in the middle, but only in (b) is the oval seen as a floating illusory object. Similarly, (c) and (d) have the same square outline in the middle, but the illusory square is prominent only in (d). In (a) and (c), the boundary elements are perceived as individual objects at the high level, suppressing the illusory effect. Adapted from Hoffman (1998).