Illusions: Two Eyes, Two Views; September / October 2009; Scientific American Mind; by Vilayanur S. Ramachandran & Diane Rogers-Ramachandran; 3 Page(s)

Humans enjoy stereoscopic vision (a). As we mentioned in our essay last issue, because our eyes are separated horizontally images we see in the two eyes are slightly different and the difference is proportional to the relative depth (b). The visual areas in the brain measure these differences, and we experience the result as stereo—what we all have enjoyed as children playing with View—Master toys.

Visual-image processing from the eye to the brain happens in stages. Rudimentary features such as the orientation of edges, direction of motion, color, and so on are extracted early on in areas called V1 and V2 before reaching the next stages in the visual processing hierarchy for a progressively more refined analysis. This stage-by-stage description is a caricature; many pathways go "back" from stage to stage—allowing the brain to play a kind of 20-questions game to arrive at a solution after successive iterations.