Table 1.

Summary of observations

Retinal image	Interpretation	Behavior of object
Stationary; proximal and distal elements differ in size	actual	appears as cube
	illusory	appears as truncated pyramid
Changes as observer moves head from side to side	actual	cube appears to move in direction opposite to head movement against background (motion parallax)
	illusory	cube appears to rotate in the same direction as head movement
Changes as object rotates	actual	cube appears to rest on surface and rotates in same direction as support
	illusory	cube appears to tumble in opposite direction as support