We saw how the Ponzo illusion looks from a standard point of view, but what if the Ponzo image is altered in some way? The graphics below and on the following pages are altered versions of the original Ponzo illusion. Look at each variation and determine whether the illusion still exists after the changes are made. How much is the illusion affected by a change in the graphic? Each variation seems to affect the illusion differently, and each person viewing the graphic may see the illusion differently. Think of other ways the illusion could be varied and how each of your ideas may affect the illusion.

Variation 1
 

By rotating the graphic onto its side, the illusion seems to become even more prominent. Notice how the lines are still converging—but from a different perspective, suggesting that, even with this change of perspective, the depth cues still influence the illusion. Rather than looking down upon the gray lines converging, we are more seemingly looking along the lines, as if they are to the right of us, converging at the end of a path, such as a hallway or tunnel. Even with that difference, the depth cues are strong enough to exploit our naturalsize constancy compensation, perhaps even more so than in the original. The leftmost bar appears much longer than the bar to the right, even though they are still the same length.

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