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Fig. 13.4. Contour completion across edge inducers. After the retinal and thalamic preprocessing, the inputs received by the primary visual cortex are essentially edge-detected versions of the original images, as shown here for the Kanizsa triangle of Figure 13.3a. Each of the three sides (e.g. the one outlined with a dashed oval) has a gap (indicated by an arrow). The triangle boundary can be generated by filling in the gap through contour completion. Contour completion is therefore a possible mechanism underlying edge-induced illusory contours.