A number of well-known motion illusions arise when luminance modulates next to a stationary edge (e.g., Anstis and Rogers, 1975; Gregory and Heard, 1983). Here, we reduce these phenomena to four novel elemental conditions and show how these conditions can be combined (like building blocks) to generate an infinite number of new illusory configurations.
Click on the “Elemental Conditions” button in the accompanying movie . In the top two panels, the luminance of the edge modulates next to stationary black or white center fields; in the bottom two panels , the luminance of the center modulates next to black or white stationary edges (Figure 1A shows one frame of the movie). In all four conditions, the fields appear to move even though they maintain a fixed spatial position. The apparent direction of motion may seem counter-intuitive: when the luminance of a modulating edge is similar to the luminance of the center, the motion is outward, whereas when the luminance of a modulating center is similar to the luminance of the edge, the motion is inward.

Read more about the illusion and possible explanations

Visual illusions based on single-field contrast asynchronies Arthur G. Shapiro, Justin P. Charles & Mallory Shear-Heyman Journal of Vision. 2005. 5:764-82