|MadSci Network: Neuroscience|
It's not that our eyes and associate neurons are seeing anything unusual. In fact, to the eyes, there's nothing mysterious about an optical image. The deception arises when the image is interpreted by the brain. It's in the brain's interpretation that an image appears to be something else. Often this is based on the brain's merging of both the left and right images from the eyes into one image. This merging allows many ways to fool the brain into thinking it is seeming something it is not.
An example would be 3D glasses. There's nothing 3D about it until you use a red filter for one eye and a blue filter on the other -- then your brain tries to combine it into one image. It's this combination that makes the image appear to be 3-dimensional.
I hope this answers your question.
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