When the rope is down, you see the board as a single, uniform surface. But when you lift the rope, you reveal a sharp edge, and the two sides of the board look quite different.
In fact, both sides of the board are identical: Each side gets lighter from left to right, creating a central boundary. The areas just on each side of the boundary are similar enough that covering the edge—where the contrast is greatest—causes you to see the board as one uniform surface.
Like our eyes, computer vision systems rely heavily on edge detection to locate distinct objects. Most often, computer vision systems find edges by detecting sudden changes in brightness or color; but these systems can be fooled when light is dappled or when the edges themselves are fuzzy or blocked.
Human edge detection is more sophisticated—sophisticated enough to see edges where they don’t exist. Some illusions—including the Exploratorium’s former logo—create what are called illusory contours with collections of shapes that merely suggest the presence of an edge.