Holding two index fingers in front of the eyes can create the perception of a floating finger sausage. This best illustrates the effect of

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Retinal disparity is a binocular cue for perceiving depth by comparing images from the two eyeballs, and the brain computes distance. The "floating finger sausage" illusion occurs because each eye sees a slightly different image (the view of each index finger), and the brain tries to combine these into a single perception, creating the illusion of a floating object.

Holding Two Index Fingers in Front of the Eyes Can