Principal of Occlusion
This experiment is demonstrating the principal of occlusion.. While the squares seem to be moving back and forth between each other, the triangle should appear to be hiding (moving) behind the square at the top right. What is happening here is that the brain is trying to explain the disappearance of the triangle. This disappearance is an unnatural phenomenon that does not usually occur in nature. In nature, a moving object is more likely to temporarily hide behind an opaque object, and not simply disappear. "The continued existence of objects is accepted as a given by the visual system, even if the brain sometimes has to invent evidence to fulfill this expectation."