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Serendip is an independent site partnering with faculty at multiple colleges and universities around the world. Happy exploring!

Most people (even many who work on the brain) assume that what you see is pretty much what your eye sees and reports to your brain. In fact, your brain adds very substantially to the report it gets from your eye, so that a lot of what you see is actually "made up" by the brain. Some special features of the anatomy of the eyeball make it possible to demonstrate this to yourself. The front of the eye acts like a camera lens, differently directing light rays from each point in space so as to create on the back of the eye a picture of the world. The picture falls on a sheet of photoreceptors (red in the diagram), specialized brain cells (neurons) which are excited by light.

The sheet of photoreceptors is much like a sheet of film at the back of a camera. But it has a hole in it. At one location, called the optic nerve head, processes of neurons collect together and pass as a bundle through the photoreceptor sheet to form the optic nerve (the thick black line extending up and to the left in the diagram), which carries information from the eye to the rest of the brain. At this location, there are no photoreceptors, and hence the brain gets no information from the eye about this particular part of the picture of the world. Because of this, you should have a "blind spot" (actually two, one for each eye), a place pretty much in the middle of what you can see where you can't see.

Look around. Do you see a blind spot anywhere? Maybe the blind spot for one eye is at a different place than the blind spot for the other (this is actually true), so you don't notice it because each eye sees what the other doesn't. Close one eye and look around again. Now do you see a blind spot? Hmm. Maybe its just a little TINY blind spot, so small that you (and your brain) just ignore it. Nope, its actually a pretty BIG blind spot, as you'll see if you look at the diagram below and follow the instructions.

Close your left eye and stare at the cross mark in the diagram with your right eye. Off to the right you should be able to see the spot. Don't LOOK at it; just notice that it is there off to the right (if its not, move farther away from the computer screen; you should be able to see the dot if you're a couple of feet away). Now slowly move toward the computer screen. Keep looking at the cross mark while you move. At a particular distance (probably a foot or so), the spot will disappear (it will reappear again if you move even closer). The spot disappears because it falls on the optic nerve head, the hole in the photoreceptor sheet.

So, as you can see, you have a pretty big blind spot, at least as big as the spot in the diagram. What's particularly interesting though is that you don't SEE it. When the spot disappears you still don't SEE a hole. What you see instead is a continuous white field (remember not to LOOK at it; if you do you'll see the spot instead). What you see is something the brain is making up, since the eye isn't actually telling the brain anything at all about that particular part of the picture.

Alright, you say, that's kind of neat, but maybe the brain isn't "making it up." It just knows to put white where the blind spot is. Let's try another situation and see what happens.
Resources Elsewhere:
MD Support, a website serving the Macular Degeneration Community, adapted this exhibit for diagramming vision: http://www.mdsupport.org/map/map.html.

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Serendip is an independent site partnering with faculty at multiple colleges and universities around the world. Happy exploring! Most people (even many who work on the brain) assume that what you see is pretty much what your eye sees and reports to your brain.

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IT'S ALL ABOUT THE FOVEA
Our synchronized eyeballs shift in their sockets relentlessly. Stimuli of interest are captured by peripheral vision and processed at lightning speed to determine the precise trajectory of saccadic shifts. Saccadic movements place selected points of interest precisely in line with the fovea, the tiny portion of each retina which provide visual acuity. The irises optimize the amount of incoming light by adjusting the pupil apertures. The ciliary muscles stretch the lenses (which in their relaxed states resemble transparent jelly-filled bags) to refract incoming light patterns to the foveae. Focusing by adjusting the shape of the lenses is known as accommodation.

The spherical shape of the eyeballs provides maximum maneuverability. The protruding corneas provide the correct focal lengths without compromising the spherical structure of the embedded portions of the eyes. (The corneas are responsible for about 70% of focusing.) Again, mostly in service of optimizing fixation at the foveae, the antagonistic muscles of the irises and the ciliary muscles of the lenses also make constant fine adjustments.
ENCOUNTER WITH YARBUS

Russian psychologist Alfred L. Yarbus investigated the nature of eye movements during the fifties and sixties. His famous eye tracking experiments revealed that subjects who viewed complex scenes, scanned them differently when asked to perform specific tasks. Students should be reminded that the saccadic jumps, of the eye are happening about three times a second; too rapid to be under direct conscious control.

Yarbus used Repin's painting They did not expect him in his most influential study. Repin is revered for his realist painting in Russia, as Tolstoy is for literature.

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Active sense perception - TOK RESOURCE.ORG

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Our synchronized eyeballs shift in their sockets relentlessly. Stimuli of interest are captured by peripheral vision and processed at lightning speed to determine the precise trajectory of saccadic shifts. Saccadic movements place selected points of interest precisely in line with the fovea, the tiny portion of each retina which provide visual acuity.

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Crazy

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Can You Trust Your Ears? (Audio Illusions)

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Yanny or Laurel video: which name do you hear? – audio

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gaze

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vision

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ojos que no ven corazon que no siente

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creer para ver

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O que os olhos não vem, o coração não sente.

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O que os olhos não vem, o coração não sente.

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witness

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ojos que mo ven corazon que no siente

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Why Your Brain Thinks These Strawberries Are Red | Science Of Illusions | WIRED