The relationship between your brain and your eyes

How can people understand the amount of processing their brain does on image information sent to it by the eyes (in other words, what they think they are seeing vs what their eyes are actually seeing)?

People typically do not have much understanding of what seeing really is. The common impression, often promoted by eye doctors is that the two eyes are like cameras that run their video images to the brain where they are seen and analyzed by some sort of processor looking at two monitor screens.

While this explanation has the attractive quality of simplicity, it obscures some very important features of human vision that are decidedly not like a pair of cameras. The images on the retina are already being separated into their component parts as the optic nerve exits the back part of the eye. These "packets" of data are then reassembled in the occipital cortex, the part of the brain responsible for processing vision. What we see "out there" is really a "virtual reality" simulation of what is actually there, projected by the occipital cortex onto the world so as to make maximal sense of it.

Thus, we don't see a person up close as being truly bigger than one across the room despite the difference in image size on our retinas: we interpret this as an indication of relative difference in distance. We don't see two overlapping images from the two eyes, but rather a single, optimized image to which we now can attribute 3-D qualities. It sounds nonsensical at first, but Seeing is not passive, but very much an active process that needed to be learned in our childhood -much like a language!

Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic

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