Nova Mind Games: http://www.pbs.org/wgbh/nova/body/visual-mind-games.html
“Like a tiny, ever-alert judge seated inside your head, your brain is constantly making decisions about evidence brought to it by way of your senses, to give you as clear an understanding as possible of what you’re seeing, feeling, tasting, etc. Sometimes you can catch your brain trying to, well, make up its mind about how to present such information. Here we offer a series of optical brainteasers culled from Phantoms in the Brain, by Dr. V.S. Ramachandran and Sandra Blakeslee.”
Click Launch interactive
Notice that this skeleton drawing of a cube can be seen in one of two different ways—either pointing upward and to the left, or downward and to the right. The perception can change even when the image on your retina is constant.
Eggs and cavities
In this graphic, the shaded disks are all identical except that half of them are light on top and the rest are dark on top. The ones that are light on top are seen as eggs bulging out from the screen, whereas the ones that are dark on top are seen as cavities. This is because the visual areas in your brain have a built-in sense that the sun is shining from above, which makes bulges (eggs) light on top and concavities light below.
Size contrast illusion
The two central, medium-sized disks in this image are physically identical in size. Yet the one surrounded by the large disks looks smaller than the one surrounded by the little ones. When a normal person reaches out to grab the central disk, his/her fingers move exactly the same distance apart for either of them—even though they look different in size.
Blind spot demonstration #1
Shut your right eye and look at the black dot on the right with your left eye. From about one and a half feet away, move your face slowly towards the screen. At a critical distance the circular hatched disk on the left will fall entirely on your blind spot and disappear completely. If you move your face closer still to the screen, the disk will reappear. You may need to “hunt” for the blind spot by moving your face to and fro several times until the disk disappears. Notice that when the disk disappears you don’t see a dark void or hole in its place. The region is seen as being covered with the same light gray color as the background.
Blind spot demonstration #2
Repeat the procedure described in blind-spot demonstration #1. Shut your right eye, look at the small black dot on the right with your left eye, and move your face towards and away from the screen until the hatched square on the left falls on your blind spot and disappears. Does the vertical line look continuous, or does it have a gap in the middle? There is a lot of variation from person to person, but most people “complete” the line. If the illusion doesn’t work for you, try aiming your blind spot at a single black-white edge (such as the edge of a black book on a white background) and you will see it complete itself.
Blind spot demonstration #3
Repeat the blind-spot procedure. The upper half of the line in this illustration is white and the lower half black. Does your brain complete the vertical line in spite of this internally contradictory evidence?
Blind spot demonstration #4
Repeat the experiment in blind-spot demonstration #3, “aiming” your blind spot at a pattern that resembles a swastika—an ancient Indo-European peace symbol. The lines are deliberately misaligned, one on either side of the blind spot. Many people find that when the central hatched disk disappears, the two vertical lines get “lined up,” whereas the two horizontal lines do not line up—there is a slight bend or kink in the middle.
Blind spot demonstration #5
Close your right eye and train your left eye on the white dot. Amazingly, when the blind spot is aimed at the center of a “bicycle wheel,” no gap is seen. People usually report that the spokes converge toward a vortex.