Figure 2.

Altered perception of the form of a transparent cube in response to different interpretations of the same retinal stimulus. When the stimulus is interpreted in its actual (top-down) orientation, the observer perceives a cube. Notice that the perceived cube is drawn without perspective because this is the two-dimensional representation that best depicts the subjective experience. When, however, the stimulus is interpreted in its illusory orientation, a truncated pyramid is seen. Two faces have been opacified to render the illustration unambiguous, as in Fig. 1. Although monocular viewing facilitates their appreciation, the phenomena described here can also be seen in binocular view and at any distance.