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Go to the west (downhill) side of the Men’s Faculty Club. There’s a sign next to the path with a wheelchair logo saying “Accessible Entrance”.

Stand on the other side of the path from that sign. Look downhill at the dead, grayish tree. It’s important that you don’t move your head while viewing the tree. The main trunk is bifurcated.

Look at the left half of the tree and make a judgment (while not moving the head!) of the 3d shape of the cluster of branches. Does it appear spherical in shape (that is as deep as it is wide) or ellipsoidal (less deep than it is wide). Most people report that it looks ellipsoidal.

Now walk downhill to the left of the tree and notice that the branch cluster is really spherical, not ellipsoidal.

This illusion is probably the consequence of impoverished depth information. It’s relatively easy to estimate the width of the branch cluster, but it’s difficult to estimate the depth. Because you weren’t moving your head, motion parallax doesn’t provide a useful cue to the depth. Because you were standing pretty far away, binocular disparity doesn’t provide a useful cue to the depth either.

The branches are all different sizes, so the relative sizes they create on your retina are also not a very useful cue to the depth. The brain appears to have what statisticians call a Bayesian prior (basically an expectation) that objects we see are more likely to be flat than they are to be spherical. When we make the depth information weaker, the prior affects our perception of the 3D shape.

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