Zach is looking at a reversible figure, which first appears to be a vase, and then appears to be two faces. His perception of the figure keeps switching between these two interpretations. This switching perception is caused by the fact that

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Quizplus · Accepted Answer

The figure-ground distinction in reversible figures is often ambiguous, which causes the viewer's perception to switch between different interpretations. This is because the viewer's brain cannot decisively determine which elements of the figure are the "figure" and which are the "ground." The Gestalt principles of proximity and closure may also be at work in reversible figures, but they do not directly cause the switching perception. The phi phenomenon is a different type of illusion that involves perceiving movement where none actually exists. The principle of simplicity applies to all Gestalt principles, including those that apply to reversible figures.

Zach Is Looking at a Reversible Figure, Which First Appears